ADDITIONS AND CORRECTIONS

OBSERVATIONS OF A NATURALIST

IN THE PACIFIC

CHAPTER I
INTRODUCTION

The study of insular floras.—Their investigation in this work from the standpoint of dispersal.—The significance of plant-distribution in the Pacific.—The problems connected with the mountain-flora of Hawaii.—The persistence of dispersing agencies at the coast, their partial suspension on the mountain-top, their more or less complete suspension in the forest, and the effect on the endemic character of plants.—The connection between the endemism of birds and plants.—The relative antiquity of plants of the coast, forest, and mountain-top.—The genetic relation between coast and inland species of the same genus.—The ethics of plant-dispersal.—Evolution takes no heed of modes of dispersal.—The seed-stage is the price of Adaptation.

To proceed from the general to the special is the only method of dealing with insular floras. A broad and comprehensive grasp of plant-distribution, such as is only acquired by a life-time of research aided by travel and the handling of large collections, is a necessary foundation for the study; yet in the nature of things such qualifications can be possessed by but a few. To direct an inquiry in the opposite direction, and endeavour to attack the problem of continental floras through the insular floras would result merely in the investigation of a few of the many questions connected with plant-distribution.

The panoramic sketch of the surveyor on the mountain-top aids him in a thousand ways when after months of tedious labour he plots the details in his chart. Without such a panoramic view of the plant-world in his mind’s eye, an observer like myself can only look for guidance to the writings of those who have generalised on the foundations of a far broader experience, such as those of Bentham, De Candolle, Gray, Hooker, Schimper, and others.

It would be quite possible for a botanist possessing a profound general acquaintance with the plant-world to dispense altogether with actual observation and experiment on modes of dispersal. It would be quite possible for him to arrive at conclusions, which, even if they did not always come into line with results of observation and experiment, we should be compelled to prefer. It is only from his more elevated position that a general can follow the course of a battle; whilst the private with his experience confined to a limited area of the field of conflict may form the most erroneous ideas of the progress of the fight. So it is with observers whose employment it is to struggle with the details and secondary principles of plant-distribution, and so it is with the generaliser who has already roughly mapped out the principal features of the main problem.

When Mr. Bentham in 1869, remarking on the paucity of species common to tropical Asia and America, characterised them either as plants wholly or partially maritime and spread by the currents, or as weeds dispersed by cultivation over the warm regions of the globe, he mentioned amongst the plants in the former category, Gyrocarpus jacquini. This tree presents one of the mysteries connected with the Pacific islands; and I don’t imagine that this eminent botanist could have known anything except inferentially as regards the mode of dispersal of its fruits. Yet experiment shows how well founded the inference was, whilst behind it lay a life-time of botanical research.

The author thus approaches the subject of the floras of the Pacific islands rather as a plotter of detail than as a delineator of great designs. However much we may study the means of dispersal, we have behind them the great facts of distribution, serving like the main stations of a trigonometrical survey, and with these we have to make our lesser facts and observations square. One is conscious all the time that much of what seems new in one’s researches has already been foreseen by the generaliser, and that one can do little else than assist in confirming some of his results. This is all that I can lay claim to in this work.

The floras of the islands and coasts of the tropical Pacific are here regarded entirely from the standpoint of plant-dispersal. The fruits and seeds rather than the flowers have been the subject of my investigations; and although there is much to please the eye in the flora of a Pacific island, it was always with a sense of disappointment that I turned away from some pretty flowering plant that failed to present me with its seed. Amongst the wonders of the plant-world rank the Tree Lobelias of the Hawaiian Islands; yet their greatest charm to me lay not so much in their giant-flowers and their arborescent habit as in the mystery surrounding the home of their birth and their mode of arrival in these islands. When I first stood under the shade of the lofty Dammara vitiensis, the Kauri Pine of Fiji, all my interest lay in its cones lying on the ground; and I remember how eagerly I handled my first specimen, and how anxiously I watched its behaviour when experimenting on its capacity for different modes of transport. When a strange plant presented itself on a beach, my first care was to ascertain the fitness of its fruits or seeds for transport by the currents; and all inland plants with fruits likely to attract frugivorous birds were at once invested with a special interest for me.

The mangrove swamps were always great places of interest, and months of my sojourn in the Pacific must have been passed in exploring their creeks and in examining their vegetation. Botanists usually avoid these regions; but the observation of the germination of the Rhizophora fruits on the trees and the inquiries connected with their methods of distribution over the oceans were pursuits so engrossing that I ignored the numerous discomforts connected with the exploration of these gloomy regions. The magnificent mangrove forests of the Ecuador coast of the Pacific will live longest in my memory, though the risks were considerably greater and the discomfort of existence extreme. But the mangrove swamps present us with glimpses into the conditions of plant life during the warmer epochs of the earth’s history, when perhaps the seed-stage was largely dispensed with, whilst an atmosphere, laden with moisture and screening off much of the sun’s light, enveloped most of the circumference of the globe.

The plant world viewed only from the standpoint of dispersal may lack much that is pleasing to the eye, though it abounds with small and great problems fascinating to the reason. Matters of great moment are here involved, and in the case of the Pacific islands they concern not only the source of the oceanic floras, but the story of the islands themselves; whilst behind these there rise up questions of yet deeper import, questions that are bound up with the beginnings of genera and species, and with other mysteries of life on the earth. The distribution of plants presents something more than a problem of means of dispersal, or a problem of station, or a problem of plant migration connected with climatic changes. It is something a great deal more than all three, since it is indissolubly connected with a past, of which unfortunately we know very little. Let us take it to be a question of means of dispersal, and then in imagination transporting ourselves to the Scandinavian coast, let us gather up the stranded West Indian beans of Cæsalpinia, Mucuna, and Entada, that have been drifted there for ages by the Gulf Stream, and lie in some cases semifossilised in the adjacent peat-bog. Was ever dispersal so utterly purposeless as this? Yet here lies a principle of plant-dispersal that is fundamental. We see it in the thistle-seed floating seaward in the wind. Nature never intended its pappus for such an end. It was formed for quite another purpose, yet it aids largely the dispersion of the plant. What can be more significant than that?

Or let us take it to be a matter of station. Given time and the recurrence of the same conditions, with others I once imagined that we could explain most things in plant-distribution, whether of plants at the coast or of plants inland, whether of plants of the alpine peaks or of plants of the plains, or of plants of the river or of the pond. Time, it was held, had long since discounted the means of dispersal, and distribution became merely an affair of station. But the supplanting of many indigenous species of a flora by introduced species is a common story in the plant-world; and such a view needs no further discussion here. Nor is distribution only concerned with plant-migration. Any theory of the origin of alpine floras on tropical mountains will have to explain the presence of the temperate genera, Geranium and Sanicula, not alone on the summits of the mountains of Equatorial Africa and Madagascar, but on the uplands of Hawaii in mid-Pacific, where also are found Ranunculus, Vaccinium, Fragaria chilensis (the Chilian strawberry), and Drosera longifolia.

Taking genera of different stations each in their turn, and following up the clues thus afforded, it would be possible to find support for all the reputable views relating to plant-distribution. The wide range of aquatic plants under conditions that completely change the character of the terrestrial vegetation, such, for instance, as Myriophyllum and Ceratophyllum, might be plausibly attributed to the relative uniformity of the conditions of aquatic life both in time as well as space. The occurrence of Vaccinium on mountain-tops over most of the world, even on the highlands of Samoa, Tahiti, and Hawaii in the Pacific Ocean, would be rightly regarded as evidence of active dispersal of the seeds through the agency of birds from one mountain-summit to another, whether in mid-ocean or in the centre of a continent. The prevalence of the same beach-plants over most of the globe in the same climatic zones would point unmistakably to the predominant agency of currents. But with many plant-genera, some of which range the world, whilst others again may be restricted to a single group of islands in the Pacific, there is often no question either of means of dispersal, or of station, or of plant-migration, and problems of a very different nature are opened up.

When we leave the beach and the mountain-top, the river and the pond, all the troubles of distribution begin; and since but a small proportion of plants in a typical flora belong to these stations, it follows that difficulties will dog our steps with the large majority of the plants. The agencies of dispersal now working around us, the current, the wind, the insect, the bird, and the bat, will explain many of the features of littoral and alpine floras and of the vegetation of ponds and rivers. Here we have in so many cases wide-ranging genera with the means of dispersal ready to hand. We can connect the wide range of Vaccinium with the wide range of birds of the grouse and other families that feed on the berries. We can associate the great areas of aquatic or sub-aquatic genera, like Potamogeton and Sparganium, with the migratory habits of the ducks in the stomachs of which we find their seeds. We can connect the great ranges of beach plants like Ipomœa pes capræ in the tropics, and Convolvulus soldanella in the temperate regions with the currents, and the almost cosmopolitan range of many ferns and lycopods with the winds and other agencies.

When, however, we enter the forests we find genera that are often much more restricted in their areas, and species that are yet more limited in their range. There is very little dispersal going on here. The birds are strange. Their distribution is usually very local. They look lazily down at us from the branches, as they disgorge the seeds and stones of the fruits they have eaten, which cover the ground around. We can almost fancy that they say:—“Our work is done. We rest from the toil of our ancestors. They carried seeds to far-distant Hawaii, Tahiti, and Savaii. Our work is done.” And as we walk through those noiseless forests, where the machinery of species-making is ever in silent motion, we become aware that we are treading one of Nature’s great workshops for the manufacture of species and genera. Outside the forest all is bustle and hurry. We are in the streets, or rather in the distributing areas of the plant-world. We hear the noise of the breaker, the roar of the gale, the cry of the sea-gull, the flapping of a myriad pairs of wings of some migrating host overhead, and we know that the current, the wind, and the bird are actively at work; but their operations are confined mainly to the beach, the mountain-top, the river, and the pond.

Let us take a well-wooded Pacific island several thousand feet in height. We find on its beaches the same littoral plants that we have seen before on the tropical shores of Malaya, of Asia, of Africa, and of America. We find in its ponds and rivers the same species of water-plants, such as Ceratophyllum demersum, Ruppia maritima, and Naias marina, that are familiar to us in the cool and tepid waters of much of the globe. On its level summit, if it remains within the clouds we find in the boggy ground, where Sphagnum thrives, genera that are represented in Fuegia, New Zealand, and the Antarctic islands, such as Acæna, Lagenophora, and Astelia, and the world-ranging Drosera longifolia. In other elevated localities we find Ranunculus, Geranium, Sanicula, Artemisia, Vaccinium, and Plantago, chiefly genera of the temperate regions of the northern hemisphere; whilst there are also found Gunnera, Nertera, and Uncinia, all hailing from the south and belonging to the Antarctic flora characterising all the land-area around the globe in the latitude of New Zealand and Fuegia. The Hawaiian species of Nertera and of Uncinia occur also in New Zealand, and the first-named is found also in Tristan da Cunha and in South America. In the Hawaiian uplands there is also to be seen Deyeuxia, a genus of grasses found in the Tibetan highlands and in the Bolivian Andes at elevations of 16,000 to 19,000 feet; and the same species that exists in Australia may be found in the mountains of Hawaii. Here also, both in Hawaii and Tahiti, occurs Luzula campestris.

In making the foregoing remarks on the alpine plants of a Pacific island, I have had Hawaii in my mind, but we find the elements of a similar widely-distributed mountain-flora in the less lofty peaks of Tahiti and Samoa, and traces even in Fiji, where the mountains, however, have only a moderate elevation. But the point I wish to lay stress on is the cosmopolitan yet temperate character of the mountain-flora of an island lying in the midst of the tropical Pacific. As he shifts his station on this mountain-summit, the observer might at different times imagine himself in the Sierra Nevada of California, on a Mexican tableland, on a peak of the Andes, or in the lowlands of Fuegia. Other plants that I have not mentioned, such as Coprosma, would bring back to him New Zealand. He might even be on a mountain-top in Central Africa, or on a Madagascar plateau; whilst in the boggy region of an elevated Hawaiian tableland he would meet with not only the physical conditions, but also several of the plants found on the higher levels of Tristan da Cunha.

It is, however, to be noted that although these mountain-tops in the mid-Pacific have been stocked with genera from the four quarters of the compass, the species as a rule are restricted to that particular archipelago. Whilst the beach and the river in most cases possess plants that have very wide ranges over the earth, a good proportion of the species on the mountain-summit are not found elsewhere. This implies a partial suspension of the means of dispersal on the mountain-top, whilst the currents and waterfowl are still actively distributing the seeds of the littoral tree and of the aquatic plant. We here get a foreshadowing of another great principle, or of another line along which Nature has worked in stocking these islands of the Pacific with their plants, a subject concerning which much will be said in later pages.

Hitherto, we have dealt only with a small proportion of the flora, and with but a small portion of the area of the island. We have yet to deal with the intermediate region between the sea-border and the summit of the island, or, in other words, with the forested mountain slopes. This is the home of many of the peculiar species and peculiar genera, both of plants and birds; and it is with this zone that we shall be mainly concerned when we come to contrast the floras of the several archipelagoes of the tropical Pacific. Here the agencies of dispersal have, to a large extent, ceased to act; and the question will arise as to the connection between the endemic character of the plants and the endemic character of the birds. We shall have to ask why this island, after receiving so many plants, ceased to be centres of dispersal to other regions. It is possible that these seeds or fruits have lost their capacity for dispersal; but only a few instances of this change present themselves. Rather it may be supposed that the birds that originally brought the seeds to the island came to stay; and this at once suggests another query as to the cause of the change of habit. I am alluding here not to the plants with minute seeds, such as Sagina and Orchis, which Mr. Wallace, in his Darwinism, regards as capable of being transported by strong winds over a thousand miles of sea; but to those numerous plants found in the Fijian, Tahitian, and Hawaiian forests, where the seeds and “stones” are large and heavy, measuring often as much as a quarter of an inch (6 mm.), and sometimes nearly an inch (25 mm.) in size. The reader will be surprised to learn how little “size” has determined the distribution of seeds and fruits in the Pacific. He will have to appeal to the habits of pebble-swallowing of the Dodo, the Solitaire, the Goura pigeon, the Nicobar pigeon, &c., if he desires to find a parallel in the habits of birds.

It is here assumed that the reader is already acquainted with the principles involved in a discussion of island-floras, principles clearly laid down in the writings of Hooker, Wallace, Hemsley, and others. As a general rule in an island or in a group of islands where there are a large number of plants not found elsewhere, there is also a large endemic element in the avifauna, and where none of the plants are peculiar, endemic birds are either few or wanting. As an example of the first we may mention Hawaii, and Iceland affords an instance of the second. But there is no hard and fast rule connecting the endemic character of the plants and birds of an island with its distance from other regions. Even the small group of Fernando Noronha, lying only some 200 miles off the coast of Brazil, possesses its peculiar birds and its peculiar plants; and we can there witness the singular spectacle, as described by Mr. Ridley, of an endemic bird, a frugivorous dove, engaged in scattering the seeds of endemic plants over the little group. This is the only fruit-eating bird in the islands, remarks the same botanist in the Journal of the Linnean Society (vol. 27, 1891); and “when one sees the number of endemic species with edible fruits, one is tempted to wonder if it were possible that they were all introduced by this single species of dove, or whether other frugivorous birds may not at times have wandered to these shores.” This inter-island dispersal in a particular group of peculiar plants by peculiar birds is a common spectacle in the Pacific. The contrast between the large number of plant-genera possessing fruits that would be dispersed by frugivorous birds and the poverty of fruit-eating birds in the avifauna is well displayed in Hawaii.

The island of St. Helena would seem to offer an exception to the rule that endemic birds and endemic plants go together, since, though its flora possesses a very large endemic element, there are scarcely any endemic or even indigenous birds recorded from the island. We can never know, however, how much of the original fauna disappeared with the destruction of the forests. It would nevertheless appear that but few of the genera possessing peculiar species of plants were adapted for dispersal by frugivorous birds. The lesson to be learned from this island concerns the Compositæ, often arboreous, that constitute the principal feature of its flora. St. Helena retains almost more than any other island evidence of the age of Compositæ which has left its impress on many insular floras; and when we discuss the original modes of dispersal of the endemic Hawaiian genera of the same order we shall look to the flora of this Atlantic island for assistance in the matter. To the age of Compositæ belong the beginnings of several insular floras.

To return to the main line of our argument, it would seem that in a Pacific island there is a constant relation between free means of dispersal and the preservation of specific characters. The ocean-current and the aquatic bird are in our own time actively engaged in dispersing the seeds of shore-plants and water-plants, and we see the same species ranging over the world. On the other hand on the mountain-top the agencies of dispersal are beginning to fail, and as a result many a mountain has some of its species restricted to its higher regions. In the forest zone there has been a more or less complete suspension of the activity of the dispersing agencies, and new genera are formed whilst peculiar species abound. Free means of communication with other regions restrains but does not arrest the differentiating process that is ever in progress throughout the organic world. Isolation within certain limits gives it play.

It is in this connection interesting to reflect that during the differentiation of the inland flora the littoral plants have lagged behind or have remained relatively unchanged. The currents have been working without a break throughout the ages; and the cosmopolitan Ipomœa, that now creeps over the sand of the beach, or the wide-ranging Rhizophora, that forms the mangroves of the coast-swamp, must have witnessed the arrival of the ancestors of several of the endemic inland genera. The swamp-plants of the littoral flora are probably older, however, than the beach-plants which have been recruited from time to time in one region or another of the tropics from the inland flora. Yet as a body the littoral plants have lagged far behind the inland flora. We might thus expect that in a Pacific island, excluding the wind-distributed plants, such as the ferns and the lycopods, the most ancient types of the plants would be found at the coast, the most modern in the forests, whilst the plants of the mountain-summit would represent an intermediate age.

But true as this may be, the composition of a strand-flora is a very complex one. Although, as Prof. Schimper remarks, the mangrove formation is more isolated than the beach formation, and affords evidence of a much earlier separation, the beach-plants as a body are anything but homogeneous in their character. Their physiognomy varies to some extent with the alteration in the characters of the inland flora, changes to which the mangrove formation makes a very slow response. Yet amongst the plants of the beach we find strangely assorted forms that are as ancient denizens of the coast as the mangroves themselves. Take, for instance, Salsola Kali, that thrives alike on a beach in Chile, on the sea-shore in Devonshire, and in the salt-marshes of the interior of Tibet. Then, again, there is a type of littoral plant, of which Armeria vulgaris and Plantago maritima may be taken as examples, which is equally at home on the beach and on the tops of inland mountains. We might in a sense apply the wrecker’s motto,

“What the sea sends and the land lends,”

to the history of a littoral flora. Yet on the other hand the inland flora in its turn receives a few recruits from the littoral flora; and it is the relation between the inland and coast species of the same genus that offers one of the most fascinating studies in the botany of the Pacific Islands.

This introductory chapter may be concluded with a few remarks on what may be termed “the ethics of plant-dispersal.” Not that this is in any way a suitable phrase, but it best expresses my sense of the lack of propriety in some things connected with this subject. It is odd, for instance, that we speak of the dispersal of plants and animals in the same breath, as if the process was in both cases identical. Seeing that from this point of view we judge a plant only by its seeds and fruits, it is apparent that we are following quite a different method than that which we employ in the study of the dispersal of animals. Whilst the zoologist classifies the units of dispersal, the botanist does nothing of the kind; and the two systems of classification are at the outset fundamentally distinct. The student of plant-dispersal thus often finds himself placed in an awkward dilemma. For him a family is a collection of allied genera having similar seeds or fruits and fitted often for the same mode of dispersal. A family like Sterculiaceæ, possessing such a variety of seeds and fruits suitable for very different modes of dispersal, is from his standpoint a collection of dissimilar units. Genera like Commersonia, Waltheria, Kleinhovia, Sterculia, and Heritiera, that he so often meets with in the Pacific Islands, have in these respects frequently very little in common; and yet one of the earliest determining influences in plant-life must have lain in the capacity for dispersal.

Yet chance seems to reign in the processes of plant-dispersal ever going on around us. In the floating seed, in the achene with its light pappus blown before the gale, in the prickly mericarp entangled in the plumage of a bird, in the “stone” of the drupe disgorged or ejected by the pigeon, in the small grain that becomes adhesive in the rain, in the tiny rush-seed enclosed in the dried pond-mud on the legs of some migratory bird, in all these we see the agencies of dispersal making use of qualities and of structures that were developed in quite another connection and for quite another purpose. That such characters have been so to speak appropriated by these agencies is a pure accident in a plant’s life-history. If the evolutionary force had been in operation here, it would have selected some common ground to work on. There would have been some uniformity in its methods, whereas the modes of dispersal are infinite. The qualities and characters that happen to be connected with dispersal belong to a plant’s development in a particular environment. They can never have been adapted to another set of conditions that lie quite outside that environment. There is a relation of a kind between the specific weight of wood and the density of water, and this, in a sense, sums up the connection between a seed and its distributing agencies.

Evolution has never concerned itself directly with means of dispersal. Evolution and Adaptation represent the dual forces that rule the organic world, the first an intruding force, the last a passive power representing the laws governing the inorganic world. To these laws the intruding power has often been compelled to bend, and it has had to pay its price, and sometimes it has succumbed, and sometimes it has turned its defeat into a victory. Nature, so watchful over the young plant, as represented by the seed, is finally compelled to let it go, and dispersal begins where evolution ends, or rather when the evolutionary power fails. The seed-stage itself is the price of adaptation. The death of the individual may also be regarded from the same standpoint. It represents a defeat of the evolutionary force, which, however, has been retrieved by the gift of reproductive power.

CHAPTER II
THE FLORAS OF THE PACIFIC ISLANDS FROM THE STANDPOINT
OF DISPERSAL BY CURRENTS

The initial experiment.—The proportion of littoral plants.—The two great principles of buoyancy.—The investigations of Professor Schimper.—The investigations of the author.—The great sorting process of the ages.—Preliminary results of the inquiry into the buoyancy of seeds and fruits.

In the previous introductory chapter some of the numerous questions affecting insular floras were briefly referred to. I will now ask my reader, if he has had the patience to read it, to consign that chapter for the time at least into oblivion, and to proceed with me to our Pacific island with the intention of investigating its flora from the standpoint of dispersal. We will together take up the subject de novo, after banishing from our minds all preconceptions that we may have possessed.

After having been over the island gathering specimens of all the seeds and fruits, we return to our abode on the beach. But we are puzzled where to begin. The problem presents itself as a tangled skein, and our difficulty is to find an “end” that we can follow along with some chances of success. In our trouble we look around us; and at that moment we see a number of floating seeds and fruits carried by the current past the beach. This presents us with a clue and our investigation begins.

OCEANIA
John Bartholomew & Co., Edinr.

We place all our seeds and fruits in a bucket of sea-water and notice that many of them sink at once. In a few days we look again and observe that many more are at the bottom of the bucket, only a small percentage remaining afloat. We then remark to our surprise that nearly all of the floating seeds and fruits belong to coast plants, those of the inland plants, which indeed make up the great bulk of the flora, having, as a rule, little or no buoyancy. After a lapse of weeks and months the seeds and fruits of the coast plants are found to be still afloat. In the results of this experiment we see the work of the ages. There has been, in fact, a great sorting process, during which Nature has “located” the plants with buoyant seeds or seed-vessels at the sea-coast, placing the others inland. This is the clue that we shall follow up during many chapters of this book; and having in this manner introduced the reader to the subject, I will now refer to the general results of my investigations in this direction in the Pacific Islands.

In Fiji there are about eighty littoral plants out of a total of at least 900 species of indigenous flowering plants, that is to say about nine or ten per cent. ([Note 1]), the littoral grasses and the sedges being with one or two exceptions excluded. These shore plants belong to the sandy beach and to the coast swamp, and most of them are distributed over the tropical shores of the Indian and Pacific Oceans, whilst not a few occur on the coasts of tropical America. They form the characteristic plants of the coral atoll, and many of them have long been known to be dispersed by the currents. From the list given in [Note 2] it will be seen that these eighty species belong to about seventy genera. Nearly all of them (95 per cent.) possess seeds or seed-vessels that float at first in sea-water; whilst three-fourths of them (75 per cent.) will float unharmed for two months and usually much more, and several of them will be found afloat after a year or more, being still capable of reproducing the plant ([Note 3]).

The prevalence in the Fijian strand-flora of Leguminosæ, which are included in my list under the divisions Papilionaceæ, Cæsalpinieæ, and Mimoseæ, is very significant. They make up about 29 per cent. of the total. Excluding weeds and a few other introduced plants, there are some fifty species known from the Fijian Islands, and of these almost half belong to the littoral flora, which as we have seen constitutes only a fraction (one-tenth) of the whole flora. If we regard the genera, we find that out of some thirty Leguminous genera twenty are littoral and in most cases exclusively so. This conspicuous feature in the constitution of the strand-flora is of prime importance as concerns the question of adaptation to dispersal by currents, since nearly all the Leguminosæ with buoyant seeds offer themselves as defiant exceptions to any such law.

I will now contrast the Fijian inland flora with that of the coast from the point of view of the buoyancy of the seed or fruit, according as it presented itself for possible dispersal by currents. Rather over a hundred plants were experimented upon ([Note 4]). After excluding some introduced plants there remain some ninety species belonging to about sixty genera, and of these quite 75 per cent. sank at once or in a few days. I may add that all kinds of fruits are here represented, the capsule, the achene, the coccus, the berry, the drupe, &c. Of the buoyant residue few possess seeds or fruits that will float uninjured for any length of time. Not many gave indications directly in opposition to the principle that whilst the seeds or fruits of shore-plants generally float, those of inland plants usually sink, since as pointed out in [Note 5] most of the difficulties are removed during the subsequent developments of the principle discussed in the later pages of this work or are to be explained on other grounds stated in the note.

We pass now from Fiji as typical in its flora of the Western Pacific to Tahiti as representing in its flora the more strictly oceanic groups of Eastern Polynesia. In the Tahitian region, which is taken as including in a general sense the Society Islands, the Marquesas, and the Paumotus, there are only between 50 and 60 littoral plants, excluding the occasional additions from the inland flora. As indicated by the letter T preceding the species in the list of Fijian shore plants, nearly all are to be found in Fiji, and the few not yet recorded from that group, which I have referred to in the remarks following the list, will probably be found there by some subsequent investigator. In Tahiti also between 75 and 80 per cent. of the strand plants have seeds or seedvessels that float for months; and here also Leguminosæ predominate, forming about 30 per cent. of the total. A conspicuous negative feature in the Tahitian strand-flora is concerned with the absence of the mangroves and their numerous associated plants, which together form the mangrove formation in Fiji. This remarkable character in the distribution of shore plants in the Pacific is discussed in [Chapter VI].

Not having visited Tahiti, I can only deal inferentially with the inland plants, as in the case of the strand-flora. Here also the plants are in the mass Fijian in a generic and often in a specific sense, and there is no reason to believe that the principle involving the non-buoyancy of the seeds or fruits of inland plants does not as a rule apply to Tahiti as well as to Fiji.

The Hawaiian Islands, standing alone in the North Pacific, form a floral region in themselves, a region that is the equivalent not of one group in the South Pacific, such as that of Fiji or of Tahiti, but of the whole area comprising all the groups extending from Fiji to the Paumotu Archipelago. Lying as it does mainly outside the zone of influence of the regular currents that would bring the seeds of tropical plants to its shores, Hawaii possesses a strand-flora that is meagre in the extreme. Not only does it lack the mangrove formation so characteristic of Fiji, but it lacks also many of the plants of the beach formation that are found both in Fiji and in Tahiti, plants that give a peculiar beauty to the reef-girt beaches all over the South Pacific. Its poverty is sufficiently indicated in the number of its species, thirty in all, barely more than half of the number found in Tahiti, and not much over a third of those occurring in Fiji. Though coral reefs with their accompanying beaches of calcareous sand are relatively scanty, the characteristic littoral plants have not been numerous enough to hold their own against intruders from the inland flora, and endemic species have taken a permanent place amongst the strand plants. The Hawaiian strand-flora has thus quite a facies of its own, and it will be found discussed in [Chapter VII.], whilst a list of the plants is given in [Note 28]. It will thus not be a matter for surprise that the littoral flora of Hawaii follows the principle of buoyancy only in a modified degree. It is true that about two-thirds of the species of the present beach flora possess seeds or seed-vessels that float for months; but since there are reasons for believing that several of them are of aboriginal introduction, this proportion is reduced to a third. In the list of the Fijian shore plants given in [Note 2], those occurring also in Hawaii are preceded by H.

When we look to the Hawaiian inland flora for indications respecting the principle of the non-buoyancy of the seeds or seed-vessels of inland plants, we find that so far as it has been there tested this principle receives fresh support from the plants growing on the slopes of the Hawaiian mountains. Although the author was only able to sample the inland flora, we have in the list given in [Note 6] all kinds of plants, from the forest-tree to the herb, and most varieties of fruits. Excluding a few introduced plants, there are in this list about fifty species of indigenous plants belonging to about forty genera. Of these plants quite 80 per cent. possess seeds or fruits that sink either at once or in a week or two. Of the “buoyant” residue very few have seeds or fruits that will float for months. These apparent exceptions to the principle are in great part capable of being explained on the grounds referred to in [Note 5] in connection with the Fijian inland plants; and I have alluded to them in [Note 7].

The littoral flora of Fiji is essentially Malayan and Asiatic, and for our purpose is eminently typical. Its plants are found far and wide on the tropical coasts of the Old World, and sometimes also in the New World. In more than half the species we are concerned with the dispersal by currents of more or less dry indehiscent fruits that range usually in size from a marble to a cricket-ball, as illustrated by those of Hernandia peltata and Barringtonia speciosa, whilst with most of the rest the currents distribute large seeds, several of which are Leguminous, as in the case of Mucuna, Cæsalpinia, and Entada, with others of the Convolvulus type, as in the instance of Ipomœa pes capræ. It is remarkable that in selecting plants with buoyant seeds or seed-vessels for a station at the coast Nature has generally ignored those with very small seeds. When such small seeded plants, as Sesuvium portulacastrum, occur on the beach, the seeds have as a rule no buoyancy. Pemphis acidula is, however, an exception; but its case is a very rare one. It will be established in the next chapter that the non-buoyancy of small seeds is generally true also of plants growing by the river or by the pond.

The point at which we have arrived in our inquiry concerning the general collection of seeds and seed-vessels that we placed in sea-water is that the plants with buoyant seeds or seed-vessels have been for the most part “located” at the coast. But if we look a little more closely at the sunken and floating seeds, we find that in the same genus there are species with seeds or seed-vessels that sink and species with those that float. We look again and then perceive that the same general principle is true of different species of the same genus growing inland and at the coast. We learn now that as a rule when a genus possesses both littoral and inland species, the seeds or fruits of the former float in sea-water for a long time, whilst those of the latter have little or no floating power. But we have yet to examine the structure of the coverings of the buoyant seed or fruit; and we shall then discover that the different behaviour in water is often associated with corresponding structural differences of a striking character. The structural causes of buoyancy are dealt with in [Chapter XII.]; and we will now content ourselves with enunciating the second principle that in a genus comprising both coast and inland species, only the coast species possess buoyant seeds or seed-vessels.

The important principle above indicated was not altogether new to me, as is shown in the next chapter. But it was new in the case of the floras of the Pacific Islands. When it first presented itself in Hawaii I was engaged in trying to find a connection between the inland and littoral species of Scævola; and its discovery led me to form a plan worthy almost of Don Quixote, namely, to cultivate the beach species of Ipomœa, Scævola, and Vitex in the interior with the hope of finding them converted into inland species when I returned to Hawaii after a lapse of years. Little matters often determine a career, and for a while my future movements and probably the remainder of my life were largely centred around my interests in the well-being of Scævola Kœnigii. The scheme was actually undertaken, and I had fixed on a little plot of land at the foot of the mountains rising behind Punaluu in Kau. The transaction was on the point of completion when the owner changed his mind and the plan fell through. Subsequent observation and reflection have led me to believe that in most cases no connection exists between the littoral and inland species of a genus; and I have dwelt on this incident merely to show the importance that I rightly attached to this distinction, whilst misinterpreting its meaning.

But to return to my own investigations. Had I indeed read more carefully Professor Schimper’s work on the Malayan strand-flora, this subject would have been found discussed by an observer far abler than myself, though from a very different standpoint, that of Adaptation and Natural Selection. He points out (pp. 179-182) that with a number of these tropical genera possessing both littoral and inland species, such as Barringtonia, Calophyllum, Clerodendron, Cordia, Guettarda, and Terminalia, greater buoyancy of the fruits of the shore species is associated with certain structural characters in the fruit-coverings, whilst with the inland species, where the floating power of the fruits is either much diminished or entirely absent, these structural characters are either less developed or lacking altogether.

The question of structure and the debateable matters concerned with it are treated at some length in Chapters [XII.] and [XIII.], and Professor Schimper’s views are there given. I will content myself with remarking that the genus Terminalia was especially studied by him in this respect. He tested the buoyancy of the fruits of ten species, and found that the flotation period varied from nothing to 126 days and more. By far the best “floaters” were the fruits of Terminalia Katappa, the only littoral species tested, all the others being inland species with less buoyant fruits, and diminished ranges, some of the fruits sinking at once, whilst the others sank usually in a few days or in a few weeks. It was also ascertained that, although the buoyant tissue in the fruit-coats varied in amount generally with the floating-powers, it was rarely absent altogether in the inland species, a very significant conclusion, as will subsequently be pointed out.

Several other striking examples of this principle came under my notice in the Pacific, and perhaps the most significant is that of Scævola, a genus of the Goodeniaceæ, confined mainly to Australia and the Pacific islands, but possessing also a littoral species, S. Kœnigii, that is found on tropical beaches all round the globe. It is associated in both Hawaii and Fiji with inland species, none of which are common to the two archipelagoes, and in the case of the Hawaiian species not found outside the group. All the species have fleshy drupes, both coast and inland plants, the “stone” in the littoral species possessing a thick covering of buoyant tissue, which is absent or but slightly developed in the inland species. The fruits of the shore species float for many months; whilst those of the inland species experimented on by me (S. Chamissoniana and S. Gaudichaudii in Hawaii, and S. floribunda in Fiji) sank at once or within a few hours. Here we are only concerned with the difference of buoyancy between inland and littoral species. The several other questions involved concerning this genus will be dealt with later on in this work.

The genus Morinda offers another good example of this principle. It includes one widely-spread littoral species (M. citrifolia), found not only in all the Pacific archipelagoes, but also over much of the tropics. It is associated in all the large groups with one or more inland species, some of which are endemic and others more generally distributed. The littoral species displays in its pyrenes a singular air-cavity, the nature of which is discussed in [Chapter XII.], which endows them with great floating powers. This cavity is not found in inland species, and the pyrenes have in consequence no floating power (see [Note 8]).

Calophyllum Inophyllum, an Old-World littoral tree, spread far and wide over the Pacific islands, has very buoyant fruits. In the groups of the South Pacific it is associated with inland species that are commonly found in the forests, namely, C. spectabile and C. Burmanni, the fruits of both of which, according to my observations in Fiji, have limited floating powers, sinking after periods varying from a few days to four weeks, and lacking in great part the buoyant coverings of the littoral species. Professor Schimper obtained similar results with inland species from other regions ([Note 9]).

The fruits of the two Fijian coast trees, Barringtonia speciosa and B. racemosa, possess great floating powers; whilst those of an undescribed species that I found in the mountains of Vanua Levu sink at once. Another Fijian inland species (B. edulis, Seem.) that is often planted, has fruits that float heavily for about a month. This difference in buoyant powers is also associated with characteristic differences in the structure of the fruits. It would be interesting to learn what floating capacity belongs to those of the Samoan endemic species (B. samoensis, Gray). Professor Schimper’s observations on the genus in the Malayan region point in the same direction, but more than one difficulty awaits its solution in the re-examination of the genus. He says, however, that B. excelsa, Bl., a Malayan species, sometimes cultivated and growing both inland and at the coast, has fruits that floated for one hundred days after drying (p. 173).

A striking instance of this principle is afforded in the case of the two Fijian species of Tacca, the wide-ranging littoral species, T. pinnatifida, where the seeds float for several months, and the inland species, T. maculata, Seem., found also in Australia and Samoa, where the seeds sink at once or in a few days. The seeds of the shore plant owe their buoyancy to the spongy tissue in their coverings, which is either absent or much less developed in those of the inland species. This point might also be determined for the new Samoan inland species described by Reinecke, the German botanist, as T. samoensis.

Another good illustration is afforded by the two species of Premna of the South Pacific, though here the buoyancy of the “stone” of a drupe is concerned. With P. taitensis or P. integrifolia, a small littoral tree or shrub, these stones possess great floating-power, and are often found in the floating seed-drift of the Fijian estuaries and in the stranded drift of the beaches. In the case of Premna serratifolia, an inland tree of moderate size, the stones have as a rule little or no buoyancy. As shown in [Note 32], where this genus is discussed in detail, the buoyancy is mainly due to empty seed-cavities.

Other instances might be given in illustration of this principle; but it will have been noticed that already many of the familiar trees and shrubs of a tropical beach have been mentioned in this connection either by Professor Schimper or by myself. There are other genera that afford similar indications but in a less direct fashion.

For instance, there are three widely spread Leguminous beach plants of the Pacific, Erythrina indica, Canavalia obtusifolia, and Sophora tomentosa, none of which are found in Hawaii; but in that group the genus is represented in each case by an inland species, Erythrina monosperma, Canavalia galeata, and Sophora chrysophylla, the last two species being peculiar to those islands. The seeds of the three littoral species will float for a long time in sea-water, whilst those of the three Hawaiian inland species have no buoyancy. I may say that some very interesting questions relating to the origin of these inland species are here raised. They will be discussed in a later chapter ([Chap. XV.]).

There are a number of plants belonging to the Convolvulaceæ in these islands that behave in an irregular way in flotation experiments ; but their inconstant behaviour can in most cases be explained in accordance with the principle that in the same genus the shore species have buoyant seeds and the inland species non-buoyant seeds. Thus, whilst the seeds of the littoral species, Ipomœa pes capræ, I. grandiflora (Lam.), and I. glaberrima (Boj.), can float for long periods, and those of the inland species, I. pentaphylla, I. tuberculata, and I. Batatas (Sweet Potato), have no buoyancy, the seeds of other inland species, I. insularis (Steud.), I. bona nox (L.), and I. turpethum (R. Br.), are inconstant in their behaviour. The three last-named species are, however, to be found also flourishing at times at and near the coast, and the varying floating powers of their seeds may probably be connected with their varying stations. This is indeed suggested by the case of Argyreia tiliæfolia in Hawaii, in which in my experiments the seeds of plants growing at the coast floated, sometimes for months, whilst those from inland plants sank.

This behaviour of the Convolvulaceæ becomes yet more intelligible, and more in accordance with the principle, when we reflect that the cause of buoyancy is not concerned with the seed-coats or with the nucleus, neither of which are able to float, but with the air-spaces left by the incomplete filling-up of the seed-cavity by the crumpled embryo. The extent to which the seed-cavity is filled up varies not only between different genera and between different species of the same genus, but also amongst individuals of the same species. Even the seeds of Ipomœa pes capræ, amongst the most typical of floating seeds, display this variation, and they show it also in their floating power, since about a third of the seeds usually sink during the first month or two of the flotation experiments. We can thus explain also why in the case of Ipomœa insularis seeds from Fiji floated for months, whilst those from Hawaii had no floating power.

The seeds of the different species of Hibiscus also appear to behave very irregularly; but even here most of the difficulties can be removed, when we come to consider a further extension of the principle. Thus, whilst the seeds of Hibiscus tiliaceus, a wide-ranging littoral tree known to be dispersed by the currents, float for a long time, those of H. Youngianus (Gaud.), an endemic Hawaiian species, and of two wide-ranging species, H. diversifolius (Jacq.) and H. Abelmoschus (L.), also float for some time. The Hawaiian plant, however, grows in wet places; and this applies also to H. diversifolius which grows in swamps at and near the coast. The extension of the principle to water-side plants generally, which is discussed in the next chapter, will explain the difficulties connected with these two species. But we have in H. Abelmoschus a remarkable exception to any rule of buoyancy, since it grows in dry situations, is often cultivated, and yet possesses a special layer of buoyant tissue in the seed-coats to which the floating power is due. The seeds of Hibiscus esculentus (L.), the widely spread cultivated plant of the tropics, have no buoyancy.

Some curious indications are supplied by Cæsalpinia, a Leguminous genus, containing two wide-ranging shore species. Speaking generally the rule applies; and I found in Fiji that whilst the seeds of the two littoral plants (C. Bonducella and C. Bonduc) were as a rule buoyant, those of an inland mountain species sank. But it is very remarkable that although the seeds of C. Bonducella have long been known to be transported by the currents, and are often stranded by the Gulf Stream on the coast of Scandinavia, when it grows in Hawaii, where it is as a rule an inland plant, the seeds lose their buoyancy. This is quite in accordance with the general principle; but I must refer the reader for a general treatment of this genus to [Chapter XVII.] There also will be found the instance of another Fijian littoral plant, Afzelia bijuga, a common littoral tree with buoyant seeds which also lose their buoyancy when the tree grows inland. A similar instance is afforded by Kleinhovia Hospita, the seeds of which seem to lose their buoyancy in inland stations. Not all littoral plants, however, lose the floating power of the seeds when grown away from the coast. The seeds of Ipomœa pes capræ retain it in spite of the change of station. This point is dealt with in [Chapter XIII] and in [Note 44].

In concluding this general sketch of the first results obtained by testing the buoyancy in sea-water of a collection of seeds and fruits from a mountainous Pacific island, such as we find in Fiji, I must remind the reader that the subject has only been lightly treated. Enough, however, has been said to illustrate the character of the sorting-process by which in the course of ages the plants with buoyant seeds or seedvessels have been gathered at the coast. This is indicated:—

(1) By the far greater proportion of species with buoyant seeds and seedvessels amongst the shore plants than among the inland plants.

(2) By the circumstance that almost all the seeds or fruits that float unharmed for long periods belong to shore plants.

(3) By the fact that when a genus has both inland and littoral species, the seeds or fruits of the coast species as a rule float for a long time, whilst those of the inland species either sink at once or float only for a short period.

These results, therefore, justify our dividing the flora of our island into two groups, the one including the plants with buoyant seeds or fruits and comprising most of the littoral plants, the other including the plants with non-buoyant seeds or fruits, a group which contains almost all the inland plants and indeed nine-tenths of the flora. This classification is a very crude one; but it enables us at once to assign a value to the agency of currents in stocking a Pacific island with its plants. Yet this is but the initial step in an inquiry that branches off in a thousand different ways, even if restricted to the littoral plants. There are a host of difficulties connected with the history of the strand-flora of such an island which can only be properly gauged when viewed from various standpoints.

CHAPTER III
THE LESSON OF THE BRITISH FLORA

Results of observations on the buoyancy of over 300 British plants.—The small proportion of plants with buoyant seeds or seedvessels.—Their station by the water-side.—The great sifting experiment of the ages.—Summary.

The singular relation between station and seed-buoyancy that exists in an island of the tropical Pacific, such for instance as Vanua Levu, Tahiti, or Hawaii, would lose much of its significance if it stood alone in the economy of plant-life. It must be true not only of tropical floras generally, but of those of the temperate regions; and there can be little doubt that it prevails all over the world. Displayed to us at first in a Pacific island, it acquires a new significance when we study it in the light of numerous observations made in Europe. It exhibits itself then as part of a far wider method pursued by Nature in determining the stations of plants. It is not only at the coast, but also at the river-bank and at the lake-side that Nature “locates” the plant with the buoyant seed or seedvessel. This relation is indeed as well exhibited in inland districts as it is at the coast.

In this connection I have the results of my own investigations on the buoyancy of the seeds and fruits of British plants and on the composition of the seed-drift of ponds and rivers, which were carried on in the years 1890-96. Some of them were published in a short paper on the seed-drift of the Thames, read before the Linnean Society of London in June, 1892, and in the columns of Science Gossip for April, May, and October, 1895; but the mass of the observations remain in my notebooks. Nor do my observations of the period since elapsed lead me to alter the position then adopted. I have since pursued the same line of inquiry in Hawaii, Fiji, on the Pacific coast of South America, and in Sicily, and with the same results.

Since the elaboration of my notes was begun in 1900, Dr. Sernander, the Swedish botanist, has published (1901) his work in Swedish on the Dispersal-biology of the Scandinavian plant-world, in which the seed-drift of river, pond, and sea is exhaustively treated. Although this author has dealt with plant-dispersal from a somewhat different standpoint, I have perused his pages with the keenest interest and with great profit, having gone over much of the same ground with respect to the seed-drift of ponds and rivers. Yet the introductory remarks to my paper in Science Gossip in 1895 are as apposite now as they were then, and the reader will, I trust, pardon my reproducing them.

“By following up the path of inquiry that is concerned with the flotation of seeds and seedvessels, we are guided into other fields of research that give promise of interesting discoveries in connection with plant-life. We are led in the first place to consider the question of utility, and to ask whether the buoyancy of the seed or fruit has been a matter of moment in the history of the species. Nature is ever engaged in telling off the plants to their various stations. She places the yellow iris at the river’s side and assigns to the blue iris its home in a shady wood. Under her direction the common alder thrives at the water’s edge, whilst its fellow species live on the mountain slope. These and similar operations are carried on daily around us, and we know but little of the wherefore and the how. We are induced, therefore, to inquire whether by pursuing the line of investigation above indicated we may be able to get a glimpse at the methods adopted by Nature in selecting stations for plants.”

I possess the results, which are given in [Note 10], of buoyancy experiments and observations on the seeds and seedvessels of about 320 British flowering plants belonging to about 65 families. Of these about 260 are included in my own results, the data for the rest being obtained from the writings of Darwin, Martins, Thuret, Kolpin Ravn, and Sernander. In the great proportion of cases, 240, or 75 per cent., sinking took place at once or within a week; whilst 80, or 25 per cent., floated for a longer period, usually a month or more; and about 60, or nearly 20 per cent., floated for several months. It is to this last small group that belong the seeds or seedvessels that float through the winter in our ponds and rivers.

If the grasses had been properly represented, the grains of which possess as a rule but little buoyancy, except through air-bubbles temporarily entangled in the glumes, the proportion of seeds and fruits that sink at once or in a few days would probably have been about 80 per cent. Then again, since the plants from stations where buoyant seeds and seedvessels are most frequently found—that is at the river-side, the pond-margin, and the sea-coast—are much more completely represented in these experiments than those from other stations, it would seem that even 80 per cent. is too low a figure. Even if the 80 plants with the buoyant seeds or seedvessels included all the species thus characterised, which they certainly do not, we should obtain an estimate for the British flora (rather over 1,200 species of flowering plants) of about 93 per cent. with non-buoyant seeds or fruits. This is, of course, too high. It is, however, very probable that the proportion of plants with non-buoyant seeds or seedvessels for the whole British flora is about 90 per cent.

This proportion of plants with non-buoyant seeds or seedvessels, that is to say, of those that sink at once or within a week, is also approximately correct for the flora of one of the larger islands of the tropical Pacific. The data at my disposal only enable me in the cases of Fiji and Hawaii to fix it at between 95 and 85 per cent., or on an average 90 per cent. With the floras of continental regions the proportion would doubtless be markedly higher. That seeds and seedvessels as a rule possess but little buoyancy was a sound conclusion of Darwin, and one, as he remarked, that is in accordance with the common experience of gardeners. Thuret, after experimenting on the buoyancy in sea-water of the seed or seedvessels of 251 species of plants, belonging to 77 families and to various regions, found that scarcely two per cent. had any powers of flotation, all the rest sinking at once or in a few days, a result that led De Candolle in a note to this memoir to reiterate his opinion regarding the inefficacy of currents as plant distributors. Thuret, however, did not select many of his plants from stations where buoyancy is most frequently exhibited, and his estimate errs, therefore, in imputing too little buoyancy to seeds in general. The power of seeds and fruits to germinate after prolonged flotation in sea-water has long been well established, and it is often illustrated in this work, so that there is no need to dwell upon it here. (See [Note 11].)

Of the 240 species of British plants where sinking took place at once or within a week, in about 50 per cent. the plants had dry indehiscent fruits, such as we find in the genus Ranunculus and in the Umbelliferæ, the Compositæ, and the Labiatæ; whilst in about a third the plants had dehiscent fruits with small seeds, such as are characteristic of the Cruciferæ, the Caryophyllaceæ, and the Juncaceæ. Plants with large seeds, such as those of Nuphar luteum and Convolvulus arvensis, make up only six per cent. of those of the non-buoyant group, the remainder comprising plants with berries, such as Solanum, and others with miscellaneous fruits.

Of the 80 plants where the seeds or fruits floated more than a week, usually for several weeks, and often for months, 70 per cent. possessed dry, indehiscent fruits, such as those of Hydrocotyle vulgaris, Bidens cernua, Lycopus europæus, Carex, &c., whilst only 6 or 7 per cent. had dehiscent fruits with small seeds, such as we find in Lysimachia and Menyanthes, the remainder being generally characterised by large seeds, such as those of Convolvulus sepium, C. soldanella, Iris pseudacorus, Calla palustris, &c. It would thus appear that, in so far as buoyancy is concerned, Nature has for the most part ignored the small seed and has confined herself mainly to the dry indehiscent fruit. We have already seen that this is also true of the same great sorting-process in the tropical islands of the Pacific, and it doubtless applies all over the world.

We have now to learn the significance of this distinction amongst British plants between those with and those without buoyant seeds or seedvessels. When we regard the stations of these 80 plants of the buoyant group we find that about 70 per cent. of them are placed by the river, or the pond, or the sea, the fresh-water stations much predominating. But if we include the plants of the moist meadows adjoining the rivers, such as Ranunculus repens, Rhinanthus crista galli, some Cyperaceæ, &c., the buoyant fruits or seeds of which are regularly swept into the stream in the time of flood, we shall raise the proportion possessing a water-side station to 80 per cent. On the other hand, about two-thirds of the 240 plants of the non-buoyant group, which are enumerated in [Note 10], live away from the water-side; but the proportion of plants with a relatively dry station would be considerably higher than this figure for the whole flora, since my investigations were especially directed towards plants frequenting wet stations, and the number of them is excessive in the list.

Supposing, however, that our materials were restricted to the 260 plants tested by myself, we should obtain highly instructive results, since in a general sense the floating powers of their seeds or fruits were tested to the finish. We place them, let us say, in a bucket of water, and after six months we find that in not more than forty plants are the seeds or seedvessels still afloat. These forty plants, excluding two or three littoral plants, are nearly all plants of the borders and vicinity of rivers and ponds. (They are indicated in the list given in [Note 10] by the numbers vi. and xii., the last being those where the flotation experiment was prolonged to a year and over.)

It would thus seem—I am now quoting mainly from my paper in Science Gossip for May, 1895—that there are gathered at the margins of rivers and ponds, as well as at the sea-border, most of the British plants that could be assisted in the distribution of their seeds by the agency of water. This great sifting experiment has been the work of the ages, and we here get a glimpse at Nature in the act of selecting a station. But the curious character of the sorting process becomes yet more apparent when we discover that the buoyancy of the seeds or fruits of species of the same genus may become a matter of station.

We will first take the four British species of Stachys (arvensis, betonica, sylvatica, and palustris). Of these the fruits of S. palustris alone possess any buoyancy, being able to float for weeks. It is the only species that finds its characteristic home at the water-side; and as observed by Sernander its reproductive shoots occur in the Scandinavian fresh-water drift.

Galium illustrates the same principle. Whilst in my experiments the fruits of G. aparine and of another species growing in a dry station displayed little or no floating power, those of G. palustre, which alone grows at the water-side and in wet situations, have great buoyancy. As my observations show, they float unharmed through the winter in our ponds and rivers, and, according to Sernander, are often found in the Baltic sea-drift. (See [Note 12].)

The achenes of Potentilla afford another example. Those of P. tormentilla and of another species from dry situations have but little floating power. On the other hand, those of P. comarum float indefinitely. The last also came under my notice in the floating drift of ponds in February; and we learn from Sernander that they occur in the fresh-water and salt-water drift of Scandinavia.

As a further instance, I will take the two British species of Iris. The familiar river-side Iris pseudacorus has seeds that float unharmed in our ponds and rivers from the autumn to the spring, and often for a year or more. On the other hand, the seeds of Iris fœtidissima, which has its home in the shady wood, sink at once even after drying for months.

The nature of the sorting-process is especially well shown in some of the families, as for instance with the Labiatæ. Let the reader put on one side the four species with buoyant fruits, namely, Lycopus europæus, Mentha aquatica, Scutellaria galericulata, and Stachys palustris, and on the other side all the species with non-buoyant fruits, such as Salvia verbenaca, Thymus sp., Calamintha officinalis, Nepeta glechoma, N. cataria, Prunella vulgaris, Stachys arvensis, S. betonica, S. sylvatica, Galeopsis tetrahit, Ballota nigra, Lamium purpureum, L. album, Teucrium scorodonia, and Ajuga reptans, and he will at once perceive that he has separated the regular water-side plants from those growing in drier stations.

If he does the same with the Umbelliferæ he will find that when he is separating Hydrocotyle vulgaris, Cicuta virosa, Œnanthe crocata, and Angelica sylvestris from Æthusa cynapium, Pastinaca sativa, and Chærophyllum sylvestre, on account of their buoyant fruits, he is also distinguishing them on account of their stations. On the other hand, there are apparently weighty exceptions to this rule in the non-buoyancy of the fruits of the three British species of Apium (graveolens, nodiflorum, inundatum), which grow in streams and marshes. Or, again, if we look at the sea-coast representatives of the family, we find that whilst the fruits of the Samphire (Crithmum maritimum) float buoyantly for months, those of Eryngium maritimum seemingly set the law at defiance, and all sink in less than a week or ten days, even after months of drying. To regard these as exceptions, however, is to miss the essential point of the principle concerned. It is not thereby implied that all water-side plants, whether by the sea or by the river or by the pond, have buoyant fruits or seeds, but that nearly all plants with such fruits or seeds have been gathered at the water-side. It will be shown in the next chapter that several other influences go to determine the station of a plant on a beach or by a river. This is true of the Compositæ, which, if we except our two species of Bidens (cernua and tripartita), come under the play of other determining causes, as indicated by the little or no buoyancy displayed by the fruits of Aster tripolium, Senecio aquaticus, and Carduus palustris.

Within the limits of a genus we can, however, point to other examples of this principle. Take, for instance, Convolvulus arvensis, the common weed of our fields. Its seeds, whether fresh or dried for months, have no buoyancy. On the other hand, those of Convolvulus soldanella float unharmed in sea-water for half a year and more. Its seeds have come frequently under my notice among the stranded drift of the Devonshire beaches, and also on the coasts of Chile; whilst Sernander includes them amongst the drift of the beaches on the Norwegian coasts. It is remarkable that Convolvulus sepium, which accompanies C. soldanella over much of its great range, has seeds that are sometimes able to float unharmed for long periods, even for years (Notes [13], [41], [49]). Though not strictly a water-side plant, it grows commonly over other plants on the banks of the Thames; and when it fruits its seeds occur typically in the floating drift of that river. According to Gray, it is almost a river-side plant in the United States, where it is found “especially on the moist banks of streams.” Not all the seeds of C. sepium, however, are buoyant; and in its varying behaviour in this respect it resembles the inland species of Ipomœa, which are referred to in the previous chapter.

The British species of Euphorbia also seem to behave in accordance with the principle that when a genus has littoral and inland species, the first-named alone possesses buoyant fruits or seeds. Thus, whilst the sound fruits of E. helioscopia and of another species found commonly as a garden weed are non-buoyant, those of E. paralias, the familiar beach-plant, float for several weeks, and are to be noticed among the stranded drift of the coasts frequented by this plant. (See [Note 90] for later results.)

The structural characters connected with the buoyancy of the seeds or seedvessels of some of the British plants are dealt with in [Chapter XII.] Here it may be remarked that this capacity is often associated, as with the Pacific island plants, with a “buoyant” tissue, that is either absent or less developed in the case of the non-buoyant group.

Enough has now been said to show in a general fashion how Nature through the agency of buoyant seeds and fruits has affected the stations of plants of the British flora. Allowing this line of inquiry to develop itself as the work proceeds, we will here pause and close the chapter with a reference to some of the principal points that have been brought into prominence.

(a) The proportion of flowering plants of the British flora that possess buoyant seeds or seedvessels is very small, probably not more than 10 per cent.

(b) In so far as buoyancy is concerned, Nature has for the most part ignored the dehiscent fruit with small seeds, such as we see in the Cruciferæ and the Caryophyllaceæ, and has chiefly endowed with floating power the dry indehiscent fruit, such as we see in the Umbelliferæ and in the Labiatæ.

(c) In the great sorting-process that has been in operation through the ages, nearly all the plants with buoyant seeds or seedvessels have been located at the water-side, principally by ponds and rivers, but also on the sea-beach. On the other hand, the great majority of the plants with seeds or seedvessels that sink have found a home in drier stations.

(d) The character of the operation is well displayed in certain genera possessing species of the water-side and species of drier situations, and in the case of genera having both coast and inland species. In both instances the species by the water-side possesses buoyant seeds or fruits, whilst that of the station in a drier locality or removed from the coast has seeds or fruits that sink.

(e) Yet it is necessary to remember that the principle involved is not that all water-side plants have buoyant seeds or fruits, but merely that plants thus endowed gather at the water-side. There are many plants with non-buoyant seeds or fruits on our beaches and beside our ponds and rivers.

(f) We have now learned from the British flora that the “locating” of plants with buoyant fruits or seeds on the beaches of the tropical islands of the Pacific, and indeed of tropical regions generally, is but a part of a much wider principle by which plants thus endowed are placed at the water-side, whether by a river or a pond or by the sea.

(g) It is with this distinction between a fresh-water and a salt-water station that we shall be occupied in the next chapter; and it is of great interest, since it leads us to discover that the wider principle is in its turn part of a far larger scheme.

Note.—It must be clearly understood that by water-side plants the true aquatic plants, such as the Water-lilies, the Myriophylls, the Potamogetons, &c., are not implied. It will be seen from the list in [Note 10] that in most cases the seeds or fruits of aquatic plants have little or no floating power. This is true, for instance, of Ranunculus aquatilis, Nymphæa, Nuphar, Myriophyllum, Ceratophyllum, Callitriche, Naias, Zannichellia, Ruppia, and half the Potamogetons.

CHAPTER IV
THE LESSON OF THE BRITISH FLORA (continued)

The choice of station of the water-side plant possessing buoyant seeds or seedvessels.—Determined by its fitness or unfitness for living in physiologically dry stations.—In the internal organisation of a plant lies the first determining influence of station.—The grouping of the British strand-plants.—Whilst the Xerophyte with buoyant seed or fruit finds its station at the coast, the Hygrophyte similarly endowed makes its home at the river or pond side.—The grouping of the plants of the river and the pond.—Summary.

By following up the clue supplied by the floating seed, we have arrived at the conclusion with respect to the British flora that plants with buoyant seeds or fruits gather at the water-side. But we have yet to inquire why some of these plants are “located” at the sea-coast and others on the borders of ponds and rivers. Mere buoyancy aided by chance has not determined the choice. There are definite principles at work in the economy of plant-life that make the selection for each plant.

Rivers in all parts of the world carry to the sea in great abundance the seeds and fruits of the plants that are stationed at their borders; and such seed-drift is found in quantity washed up on the beaches in the vicinity of the estuary. One finds, for instance, on such beaches in the South of England the stranded fruits and seeds of Bidens cernua, Alnus glutinosa, Sparganium ramosum, Iris pseudacorus, &c., mingled with those of true beach plants like Cakile maritima, Convolvulus soldanella, Euphorbia paralias, &c. Yet we would be much surprised if either the Bidens or the Alder or the Sparganium were to establish itself on the sandy beach, even though they have had through the ages innumerable opportunities of doing so. We thus see that mere buoyancy of fruit or seed cannot determine a station on a sea-beach, and that some other factor makes the choice. The nature of this factor I will now endeavour to explain; but in so doing it will be necessary to employ a few technical terms, which it is not easy to dispense with altogether.

It may be doubted whether Professor Schimper could have conferred a greater benefit on the student of plant-distribution than in his clear delineation of the connection between the habit or organisation of a plant and its station. Nature has imposed an important structural distinction between plants that have been endowed with the means of checking excessive transpiration or water-loss in stations where there is risk of drought, as in deserts and in similar arid localities, and those that live in stations where such safeguards are not needed. Hence arises the distinction between Xerophytes on the one hand, and Hygrophytes on the other. This contrast is shown not only in minute structural features, but also, as my readers are aware, in the external characters, as in hairiness, succulency, a leathery cuticle, the occurrence of thorns, and in several other characters of the plants of the steppe and the desert. This important subject is dealt with by Professor Schimper in his recent work on Plant-Geography; but it was from his earlier work on the Indo-Malayan strand-flora that I learned this valuable lesson in plant-distribution.

It has been ascertained, however, that a safeguard against excessive water-loss by transpiration is not only needed by plants living in arid localities, but also by those placed at the coast. Both the shore plant and the plant of the steppe and the desert present the same xerophilous organisation, provision against excessive transpiration being also required by the beach plant to prevent the injury of the green cells from the accumulation of salt in the tissues. It would thus appear that plants of the Hygrophytes that possess buoyant seeds or fruits are gathered at the borders of ponds and rivers, whilst those of the Xerophytes that are similarly endowed find their station on the sea-shore. This important distinction penetrates very deeply into the conditions defining the stations of plants. The connection between the plant of the coast and the plant of the steppe or the desert is strikingly shown on those occasions when the beach plants extend inland over parched and arid plains, such as occurs for instance in North Africa, and in the larger islands of Fiji, as described in [Chapter V.]

The causes of the buoyancy of fruits and seeds, as pointed out in [Chapter XII], are so various, that it appears at first sight impossible to connect them with the xerophilous or hygrophilous organisation of a plant, or, in other words, with any structural characters associated with particular stations; yet behind all lies the general principle that, given a plant of the buoyant group, if it is a Xerophyte it finds its way to the coast, and if a Hygrophyte it makes its home by ponds and rivers. In the case of a tropical littoral flora, such as we find in a Pacific island, the large proportion of plants with buoyant fruits or seeds gives so much prominence to the subject of their distribution by currents that the question of “station” is often masked. On the other hand, in the shore-flora of a temperate region like that of Great Britain, the plants with buoyant seeds or fruits are in the minority, and the question of “station” is the first to obtrude itself.

In establishing the principle that most of the plants with buoyant seeds or fruits have been gathered at the water-side, it was never implied that all the plants by the river or by the pond or at the coast are thus characterised. There is much to learn from the circumstance that whilst nearly all plants with buoyant seeds or fruits are placed at the water-side, not all water-side plants have buoyant seeds or fruits. In the first place, it is to be inferred in the light of what has been said above that the first determining principle in the selection of a station is concerned not with the buoyancy of the seeds or fruits, but with the xerophytic or hygrophytic organisation of a plant. In other words, it is the fitness or the unfitness of a plant for living in situations where the loss of water by transpiration requires to be checked that primarily determines the station at the coast. We thus see in the internal organisation of the plant the primary determining influence on station. Buoyancy of seed or fruit comes subsequently into play, the Xerophyte and the Hygrophyte, thus endowed, ultimately finding their way, the first to the beach, the second to the bank of the river or to the margin of the lake or pond.

In the next place, when we regard the composition of the British coast-flora, and examine the distribution of the plants in other situations than on the beach, we obtain some interesting results. There is first a group of plants, including such as Armeria vulgaris, Artemisia maritima, Cochlearia officinalis, Erodium maritimum, Matricaria inodora, Plantago coronopus, Polycarpon tetraphyllum, Raphanus maritimus, Spergularia rubra, Silene maritima (see [Note 15]), and others, all of which occur not only at the coast and on the adjacent hill-slopes, but also often far inland, and sometimes at considerable elevations in mountainous districts, as in Central Europe. It is on this occurrence of certain shore-plants in alpine regions that Prof. Schimper lays much stress in his memoir on the Indo-Malayan strand-flora (p. 28), and in his later work on Plant Geography (Engl. edit., p. 716), when pointing out that here temperature does not play a determining part, and that in both stations, whether on the sandy beach or on the mountain-top, the same xerophilous organisation is needed to obviate the risk of impeded water-supply. He quotes in this connection the observation of Battandier that many alpine species from the Atlas Mountains occur on the Algerian beaches, but not in intervening regions. Mr. Druce, in his discussion of the British species of Sea-Thrifts and Sea-Lavenders (Armeria, Statice), brought the subject of the occurrence of maritime plants on mountain summits again to the front; but he did not advance any general explanation, and seems to regard it as the result, as it doubtless is, of the recurrence of suitable stations (Jour. Linn. Soc. Bot., Dec. 1900).

Very few of these plants have any capacity for dispersal by currents, a subject dealt with in [Note 16]. Several of them have dehiscent, small-seeded fruits which, as pointed out in the previous chapter, hardly ever come into the buoyant category. I have experimented on the greater number of them, and in only one species, Matricaria inodora (var. maritima), do the results indicate a capacity for dispersal over wide tracts of sea.

If we look again at a list of British shore-plants, we find another group of plants frequenting salt marshes and muddy shores, and found also often far inland, as in the saline plains of Central Asia. Here we have such plants as Aster tripolium, Glaux maritima, Plantago maritima, Salicornia herbacea, Salsola kali, Samolus valerandi, Scirpus maritimus, Suæda fruticosa, S. maritima, Triglochin maritimum, T. palustre, &c. It becomes in this connection a subject of peculiar interest to the student of plant-distribution when he reads in Mr. Hemsley’s paper on the flora of Tibet (Jour. Linn. Soc. Bot., vol. 35) that amongst the British shore-plants above-named the two species of Triglochin and the same species of Glaux and Salsola occur in the salt marshes of the Tibetan uplands at elevations of 15,000 to 16,000 feet, Scirpus maritimus also being found in the swamps of the lower levels. We have the same thing, affecting much the same plants, illustrated in America. Thus we learn from Asa Gray that Salicornia herbacea, Scirpus maritimus, Triglochin maritimum, &c., which are common in salt marshes on the coast of the United States, occur also in the interior of the continent in the vicinity of salt-springs.

Facts of this sort are well known, and I merely refer to them here in order to emphasise the importance of this little group of British littoral plants, those of the salt marsh. Their very wide distribution is connected with the frequent recurrence of suitable conditions, not only in space, but what seems of greater import, also in time. One can scarcely doubt when the Saltwort (Salsola kali) is seen on the Devonshire coast, on a beach in Chile, and in the elevated regions of Central Asia that here a very ancient type of plant finds its still more ancient conditions of existence. In the capacity which most of the plants of the salt marsh possess of germinating in sea-water, this group of littoral plants is sharply distinguished, as far as my observations show, from the other groups of British shore-plants. For instance, in my experiments the seeds of Aster tripolium, Salicornia herbacea, and Triglochin maritimum germinated freely in sea-water, whilst those of Spergularia rubra, Cakile maritima, Convolvulus soldanella and others failed to do so (see [Note 19]). It will also be noticed with respect to this group of littoral plants that, except in the case of Scirpus maritimus, the seeds or fruits have little or no floating power, the exception offered by Salsola kali being not very striking. This feature is brought out in the Table given in [Note 10]; but some of the details of my observations are given in [Note 17].

There yet remains a third group of the British shore-plants, namely, that comprising the plants that rarely stray far from the beach and often possess seeds or seedvessels that will float for months. Here we have such species as Arenaria (Honckeneya) peploides, Beta maritima, Cakile maritima, Crambe maritima, Crithmum maritimum, Convolvulus soldanella, Eryngium maritimum, Euphorbia paralias, Glaucium luteum, Lathyrus maritimus, Polygonum maritimum, &c. The seeds or seedvessels of quite half of these species will float for months unharmed in sea-water, but in a few, as with Cakile maritima and Eryngium maritimum, they float for only a week or two, whilst in others again like Glaucium luteum they have no buoyancy. (Some details of the buoyancy experiments on these plants are given in [Note 18]; and the long list in [Note 10] may be first consulted.)

It is not necessary to enter here into more detail with respect to British shore-plants. Enough has been said to disclose cleavage-lines in what might have appeared as a homogeneous plant-formation. We can thus discern the elements of at least three groups amongst the plants of our beaches, each group bearing the impress of an independent history:—

(a) The plants of the beach and of the inland plain or of the distant mountain peak, excluding those of the salt marshes. Armeria vulgaris, Silene maritima, and Spergularia rubra may be taken as examples. The currents here as a rule take little or no part in their dispersal.

(b) The “saline” group, including the plants of the saline plains and the salt marshes of the interior of continents. Of these Glaux maritima, Salsola kali, and Triglochin maritimum are examples. The capacity of germinating in sea-water is a distinguishing character of most of the plants; and but few of them possess seeds or seedvessels that are markedly buoyant.

(c) The true beach plants that rarely stray far from the beach, of which Arenaria peploides, Cakile maritima, and Convolvulus soldanella are examples. Many of them have buoyant seeds or fruits capable of dispersion over wide areas through the agency of the currents.

The reader will be able to extend this subject for himself if he is so inclined, but we have gone far enough together to learn that the plants with buoyant seeds or fruits are in the minority on our beaches, scarcely a third of the total being fitted for dispersal by the currents over broad tracts of sea. The British strand-flora thus differs strikingly from the littoral flora of a Pacific island, or indeed of any ordinary tropical coast, and in this respect it is to be regarded as typical of the temperate regions. It has been remarked before that on a beach in the tropics we would expect to find that quite three-fourths of the plants are provided with buoyant fruits or seeds distributed far and wide over the tropical seas by the currents.

We pass on now to briefly discuss from the same standpoint the British plants that find their homes on the borders of rivers and ponds. It is here that the hygrophytes with buoyant seeds or fruits gather together, just as the xerophytes with similar seeds or fruits collect on the beaches. We have seen before that only a portion of the beach plants belong to the buoyant group, and the same applies to the plants at the edges of rivers and ponds. The plant-formation is no more homogeneous there than it is in the case of the strand-flora. Let us see if we can discern some lines of division there also, or in other words let us endeavour to connect the absence or presence of floating power in the fruits and seeds with some variations in the placing of the plants. We still pursue the clue to the study of the complicated problems connected with plant-stations by taking the floating seed as our guide.

We will carry ourselves in thought to the Thames-side between Teddington and Twickenham at the end of August, 1892. The river is at the high-water level, and we see flourishing at the margins, sometimes a little above the water and sometimes a little within its reach, Ranunculus repens, R. sceleratus, Spiræa ulmaria, Lycopus europæus, Scutellaria galericulata, different species of Rumex, Alnus glutinosa, Iris pseudacorus, Sparganium ramosum, and different species of Carex, with several other plants, all contributing their seeds or fruits to the drift that floats in the river from the autumn to the spring.

But besides these plants there are a number more or less submerged in the stream, including Nasturtium amphibium, N. sylvestre, Stellaria aquatica, Myosotis palustris, and Veronica beccabunga; and as the water falls other plants still more submerged come into view on the exposed flats, such as Nasturtium officinale, Apium nodiflorum, and Polygonum hydropiper. None of these plants are represented by their seeds or fruits in the floating river-drift. Several of them possess dry dehiscent fruits with small seeds, such as Nature ignores in the matter of buoyancy, and the small fruits of Myosotis, Apium, and Polygonum have little or no floating power.

We have thus here a clear dividing line between the plants with buoyant seeds or fruits that were more or less exposed above the high-water level, and those that were more or less submerged at that state of the tide. That which occurs in the Lower Thames twice in the day within the reach of the tide represents what happens in the higher part of the river during the seasonal floods, but in the last case the effects cannot be so readily distinguished. We thus perceive that the buoyant seed or fruit is as a rule only characteristic of the plants of the river-side that grow more or less exposed above the water, whilst those plants liable to periodic submergence have seeds or fruits that sink.

In this connection it is of especial interest to observe that as a general rule the truly aquatic plants of English rivers contribute little or nothing to the floating seed-drift. I pointed this out several years ago, in my paper on the Thames, as an agent in plant-dispersal, and it has been already noticed in this work (page [30]). We look in vain amongst the floating winter drift of our rivers for the seeds or fruits of Ranunculus aquatilis, Nuphar luteum, Nymphæa alba, and of the species of Myriophyllum, Limnanthemum, Callitriche, Ceratophyllum, Zannichellia, and of several of the Potamogetons, all of which give character in summer to the aquatic vegetation of the river. In their place we find only the seeds and fruits of the plants growing on the banks.

There is, however, another small group of river plants, which in their structure and habits and in the behaviour of their floating fruits come between the true aquatics and the plants of the river-banks. They belong mostly to the Alisma family, and Alisma plantago and Sagittaria sagittifolia may here be specially mentioned. Their fruits display great variation in their floating power; and on this point M. Kolpin-Ravn, writing to me in 1895, made the following interesting suggestion, that since these plants approach true aquatics in structure they may be also regarded as approaching them in the inconstancy of the buoyant capacity of their fruits, those of aquatics having typically little or no floating power.

Seed-buoyancy, however, does not play quite such an important part in the plant-economy of a river as the examination of the floating drift would lead one to expect. Only a portion of the bank-plants have buoyant seeds or fruits, whilst amongst the true aquatics, the semi-aquatics, and the plants periodically submerged, the rule of non-buoyancy prevails. And, indeed, when we look at all the possible stations for the plants of the British flora, we discover that seed-buoyancy can rarely be connected with station. It is, however, in those few stations that plants with buoyant seeds have mainly gathered. There it is, probably, that the remnants of a past floral age find a refuge, since it would seem likely that the tendency has been in the course of geological time for the development of dry stations for plants at the expense of the wet stations.

The following is a summary of some of the points discussed in this chapter:—

(1) In the case of the strand-flora of a Pacific island, and indeed in that of an ordinary tropical region, the large proportion of plants with buoyant seeds or fruits tends to mask all other issues, and we are seemingly only concerned with dispersal by currents.

(2) But in the British strand-flora where plants with buoyant seeds and fruits are in a minority, constituting less than a third of the total, it is seen that the issue is primarily an affair of station, an inference that may be applied generally to temperate regions.

(3) All British shore-plants may be regarded as owning certain characters in common which may be collectively designated the xerophilous habit, and we may extend this view to other temperate strand-floras.

(4) But this xerophilous habit is also characteristic of inland plants in certain localities, as of those of the steppe, the desert, the rocky mountain-top, and of other exposed situations, in all of which checks to the loss of water by transpiration are required. Whilst the risks of drought are thus guarded against in the case of plants stationed in arid localities, the risk of injury to the plant from the accumulation of salt in the tissues is obviated in the instance of the plants of the coast.

(5) On the other side we have the hygrophilous habit characteristic of plants living under conditions where checks to transpiration are relatively little needed. All the plants of the margins of rivers and ponds belong here, and indeed all plants living under moist conditions.

(6) This distinction between the xerophilous and hygrophilous habits penetrates deeply into all questions connected with stations, and lies behind all matters relating to the buoyancy of seeds or fruits. It is the fitness or unfitness of a plant for living in dry situations that primarily determines the station. If a xerophilous plant has a buoyant seed or seedvessel it finds its way ultimately to the coast; if it is hygrophilous and its seeds or fruits can float, then it is finally established on the side of a pond or river.

(7) The composite character of the British strand-flora is to be explained on the above principles. We have in the first place the plants confined to the sandy beach, many of which possessing buoyant seeds or fruits are dispersed by the currents. Next come the plants of the sandy beach which are found also far inland in open plains and on mountain-tops; and afterwards come the plants of the salt-marsh and mud-flats of the coast, which appear again in the saline plains and swamps in the interior of the continents.

(8) The plant-formation of the river’s border displays also lines of division, and is by no means homogeneous; and indeed other factors besides those connected with seed-buoyancy have here been in operation.

(9) In only a few of the possible stations of British plants can a direct connection be traced with seed-buoyancy. Yet it is at these few stations, such as at the coast and by the pond or river, that the plants with buoyant seeds and fruits have mainly gathered.

(10) The plants now frequenting wet stations may often be regarded as the remains of an age when moist conditions for plant-life prevailed.

CHAPTER V
THE FIJIAN STRAND-FLORA

The inland extension of the beach plants.—The grouping of the coast plants.—Their modes of dispersal.—The zone of change.—Summary.

Having learned from the British flora the real significance of the buoyant seed or fruit in a littoral flora, we will now return to the Pacific and proceed to deal with the composition and general character of the strand-plants.

Speaking of the Malayan strand-plants, Professor Schimper remarks (pp. 11, 12) that both in outward appearance and in anatomical structure they are xerophilous in character, whether in the case of those of the mangrove-swamp or in those of the beach. Since the tropical shore-flora of the Pacific islands is essentially Malayan, the identity usually extending to the species, the same conclusion may be applied to its character. The xerophilous habit may show itself externally in a variety of ways, as in hairiness, leaf-structure, a leathery cuticle, succulency, &c.

From this xerophilous habit of the Pacific strand-flora we should expect to find that many of the plants stray far from the coast, wherever the suitable conditions for their type of organisation occur, whether in the inland plain or on the mountain-top. This is indeed the case; but in dealing with this subject it will be necessary to discuss in some general detail the littoral floras of the Fijian, Hawaiian, and Tahitian groups in succession.

The Fijian Strand-flora
THE INLAND EXTENSION OF THE BEACH PLANTS

Viewed from the old standpoint of “station,” where one would distinguish sharply between the coast and the inland plants, the Fijian strand-flora exhibits a number of inconsistencies, all at first sight extremely puzzling. When, however, we regard their xerophilous character and reflect that this habit, and not mere fitness for growing at the coast, is the primary determining factor of their station, much that is strange appears normal and plain.

Let me refer in this connection to the impression that the distribution of the Fijian shore-plants made on Mr. Horne, the director of the Botanic Gardens of Mauritius, who spent a year in the botanical investigation of the group about a quarter of a century ago. In his account of the group (pp. 59, 60) he says that several of “what are known as sea-shore plants” are found far in the interior of the larger islands; and amongst others he names such characteristic beach plants as Cerbera Odollam, Hibiscus tiliaceus, Ipomœa pes capræ, and Pandanus odoratissimus. On the other hand, he remarks that several species of inland plants occur at the coast, and that several plants growing on the mountain-tops are found near the sea. This apparent confusion of station he seems to attribute to the circumstance that the mountains of Fiji are not high enough for the development of an alpine flora. But such a view could not be held now, since the effect of an alpine flora would be the introduction of further elements of confusion in the occasional occurrence of some of the alpine plants on the sea-coast, as we find in Hawaii.

Yet this apparent mingling of the littoral and inland floras in Fiji becomes intelligible when we perceive that the seeming confusion of station is mainly restricted to the xerophilous plants of the arid inland plains and of the bare mountain-tops. The rank humid forests that cover so much of the interior of the islands, and the luxuriant vegetation of the mountain-gorges, are not here concerned. Such a mingling occurs it is true under certain conditions; but in the general physiognomy of the flora the distinction between the shore and inland plants holds good. The same shore plants that are distributed far and wide over the Pacific here present themselves; and although some of them extend far inland, where the scantily-vegetated plains descend to the coast, this does not deprive them of the right of being still regarded as littoral plants.

Still, when we look at a fairly complete list of the shore-plants of Fiji, numbering in all about eighty, we perceive that about two-thirds of them also occur inland, either in Fiji or in some other tropical region; and if we reflect that many of the residue are plants of the mangroves that would not be found inland except under estuarine conditions, it becomes evident that with this reservation there are very few littoral plants in Fiji that do not at times leave the coast.

Cæsalpinia Bonducella may be taken as a type of those shore-plants that stray far away from the coast, even into the interior of continents, since in India it reaches the Himalayas. Although Terminalia Katappa and Calophyllum Inophyllum often owe their existence inland in different parts of the tropics to man’s agency, this cannot be said of most others, as Cassytha filiformis, Casuarina equisetifolia, Cycas circinalis, Ipomœa pes capræ, Pandanus odoratissimus, Premna tahitensis, Tacca pinnatifida, Tephrosia piscatoria, Vitex trifolia, &c., when they occupy the extensive inland plains that slope to the coasts on the lee sides of the large islands of Fiji. Plants, like Hibiscus tiliaceus, are found in a Pacific island almost as frequently away from the beach as on the beach itself; and this is true of most other regions of the tropics where it occurs.

Other plants that appear to be altogether confined to the sandy beach in Fiji, break away on rare occasions from their usual station and appear on the bare rocky summits of hills near the coast, even though the hill-slopes are densely wooded. On such bare hilltops in Vanua Levu, varying from 500 to 1,100 feet in elevation, one is surprised at times to find shore creepers and climbers like Canavalia obtusifolia and Derris uliginosa associated with other beach-plants more frequently found inland, such as Tephrosia piscatoria and Vitex trifolia, and in the company of climbing species of Morinda and of small trees of Fagræa Berteriana. When the “talasinga” (sun-burnt) districts, as the Fijians term the plains on the north sides of the islands, extend a long distance from the coast into the heart of the island, they carry with them their peculiar vegetation and the intruding beach-plants up to considerable elevations above the sea. We then find familiar beach-plants like Cerbera Odollam and Ipomœa pes capræ growing far inland at heights of 1,000 feet and over above the sea. (See Notes [20] and [21].)

One is never quite sure of the behaviour of shore-plants in Fiji when the “talasinga” plains lie behind the beach, since even Scævola Kœnigii, usually a steadfast beach-plant, occurs at times some miles inland. (See Notes [20] and [55].) There are, however, a few that never came under my notice inland, such as Pemphis acidula, Triumfetta procumbens, and Tournefortia argentea. The extension of sea-coast plants for any distance inland depends a good deal on the occurrence of scantily-vegetated plains, or of scrub-covered, rolling country at the back of the beaches; and doubtless that which I have described in the case of Fiji is to be found in other tropical coast-regions. Professor Schimper informed me by letter that he had noticed a similar inland extension of the shore-plants in the Seychelles.... I have only here touched on this subject. In Notes [20] and [21] the reader will find further details of the inland extension of the beach-plants, and in [Note 22] is given a general account of the “talasinga” plains, in which the wandering beach-plants mingle with the peculiar vegetation of the plains themselves. Covered with reeds and bracken, and dotted over with clumps of Casuarinas and Acacias, with the Cycad and Pandanus distributed irregularly over their surfaces, such level districts possess, as remarked by Seemann, a South Australian look.