Observations of a Naturalist in the Pacific Between 1896 and 1899, Volume 2 / Plant-Dispersal

by H. B. Guppy

OCEANIA
John Bartholomew & Co., Edin^r.

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].