Bruguiera rheedii (Blume)
This species is reduced in Hooker’s Flora of British India to Bruguiera gymnorhiza (Lam.), and thus viewed it has a very wide range in the Old World, corresponding very much to that of Rhizophora mucronata, namely, tropical East Africa, tropical East Asia to the Liukiu Islands, the Indian Archipelago, New Guinea, tropical Australia, and Western Polynesia, as in New Caledonia, Fiji, Tonga, and Samoa. There are four or five species of the genus, but all are confined to the Eastern Hemisphere, none occurring in America.
As with the species of Rhizophora, this plant is indebted for its present dispersal to the floating seedling, which, however, often falls from the tree whilst still attached to the fruit, but is generally freed in a day or two. The seedlings float for a long time in sea-water. I kept one of them afloat for 117 days, when it was quite sound and healthy. They appear to be better fitted than the species of Rhizophora for the “rough-and-tumble” of ocean transport, since the plumule is much less prominent, projecting only one line (2·5 mm.) or less, whilst with the two Fijian species of Rhizophora the plumule measures from seven to twelve lines (18 to 30 mm.). In the latter part of the year they are to be found in abundance in the floating drift of rivers, and there they readily develop the first leaves and roots. They are also frequent in the sea off the coasts, and they are stranded in large numbers on the beaches, where they readily strike into the sand when partially buried amongst the vegetable drift.
The empty flowers and the germinated fruits containing the cotyledons are very common in floating drift. They look much alike, but the flowers are much smaller and possess the long style, whilst the fruits contain the cotyledons at the bottom of the seed-cavity.
As with Rhizophora, there is a rather curious adjustment of the buoyancy of the seedling to the density of sea-water. About 75 per cent. of those afloat in the fresh-water of rivers assume the vertical position, the plumular end protruding between two and five lines (5 to 12 mm.) above the surface, while the remainder float horizontally or nearly so. In sea-water about 50 per cent. float either vertically or steeply inclined, and the other half float horizontally.
With regard to the times of flowering and fruiting, it may be remarked that the trees are mostly in flower during the hot months from November to February, and that the fruiting is in active operation in the latter half of March. The floating seedlings occur in abundance in the river-drift at the end of the year, a circumstance which corresponds with the fact that a period of six months passes between the fertilisation of the ovule and the fall of the seedling into the water.
Fertilisation, or, more correctly speaking, the discharge of the pollen, takes place after the opening of the flower, and not before, as in the case of the species of Rhizophora. The flower-bud is at first erect, but subsequently it begins to bend downwards, and ultimately it hangs more or less vertically. The provision to secure fertilisation under these circumstances is rather curious. Without some such contrivance as is below described, the pollen would merely fall out of the flower. Each petal has its sides rolled or folded inwards so as to completely inclose two stamens. In the bud the folded petals are white and flexible, but as the flower expands they redden and become dry and elastic, and are only prevented from flying open with a spring by the interlocking of the hairy tips of their lobes. Whilst the folded petals are becoming stiff and elastic during the opening of the flower, the inclosed stamens are at the same time preparing themselves for their function. The anthers are dehiscing and the filaments are acquiring elasticity. All is now ready, and a slight shake or a touch puts the mechanism into action. The petals unfold themselves with a spring, and the stamens thus suddenly exposed and released fly forward, and a little shower of pollen is thrown towards the centre of the flower. This process is accomplished in ordinary fine weather during the first twenty-four or thirty-six hours after the expansion of the flower. When the opening occurs in the early morning, half of the stamens will be found released in the evening and the rest on the following day. During the next day or two the petals and the stamens fall out of the flower. In wet weather, the petals never acquire elasticity, and in consequence do not unfold. In this case pollenisation is never effected, and the folded petals soon fall to the ground, carrying the stamens within them. Cross-fertilisation would be much more likely to occur with species of Bruguiera (if, as is probable, the same process of pollenisation is usually followed) than with species of Rhizophora, since the stamens are securely inclosed in the petals for some hours after the expansion of the flower.
Nearly eight weeks pass between the date of fertilisation and the commencement of germination. This is somewhat similar to the period given for Rhizophora mangle, namely, nine weeks, and it obviously leaves little or no time for any stage of quiescence or dormant vitality in the case of the seed. The changes which the fruit undergoes in this interval are a considerable increase in girth and a thickening of the calycine walls, together with a contraction of the mouth of the tube. However, I found no method sufficiently accurate for recording the rate of increase of the fruit.
It is known that germination is in progress when the end of the hypocotyl begins to lift up the lining membrane at the bottom of the calycine tube (see Figs. 21 to 26). The floor of the tube begins to bulge up, but since this cannot be well seen at first, a better index is afforded in the elevation of the style which accompanies it. The top of the style preserves previous to this time a constant level with regard to the tips of the calycine teeth. But this does not indicate the actual beginning of germination. As shown in Fig. 21, the seed lies about two and a half lines (6 mm.) below the floor of the calycine tube, and the tip of the hypocotyl has to penetrate the intervening tissues before it can push up the lining membrane and raise the style. Judging from the subsequent rate of growth, seven or eight days at least, and perhaps as much as two weeks, are requisite for this purpose. It is not necessary to give further details here, and it may be at once stated that the average of numerous observations on the length of the interval between fertilisation and the elevation of the style was sixty-four days, the range being fifty-nine to sixty-nine. After deducting ten days for the time occupied for the radicle in reaching the floor of the calycine tube (see Figs. 22 and 23), we obtain, as already remarked, nearly eight weeks as the time elapsing between fertilisation and germination.
The radicle or hypocotyl, therefore, in the first stage of germination pierces the tissues above it and reaches the floor of the calycine tube. It does not, however, pierce the lining membrane of the tube but pushes it upward until it ruptures about 4 millimetres below the base of the style which is carried up with it. Thus a kind of cap is formed, as shown in Fig. 24, which does not fall off from the end of the hypocotyl until it has protruded rather more than an inch. The hypocotyl attains a length varying between 5 and 11 inches, the average being about 8 inches.
The whole period may be thus divided up:—
| (1) Period between fertilisation and germination | 71⁄2 weeks. |
| (2) Period between the beginning of germination and the protrusion of the point of the hypocotyl at the floor of the calycine tube | 11⁄2 weeks. |
| (3) Period occupied in the growth of the hypocotyl 8 inches outside the fruit and terminating in the fall of the seedling | 18 weeks. |
| —— | |
| Total | 27 weeks. |
The total period of twenty-seven weeks between fertilisation and the fall of the seedling is thus six weeks shorter than that estimated for Rhizophora mangle. On comparing the two tables it will be seen that the difference mainly lies in the length of the second period, namely, that between the commencement of germination and the protrusion of the hypocotyl from the fruit. With Rhizophora mangle the fruit grows considerably in length during this period of the germinating process. On the other hand with Bruguiera rheedii there is, during this period, practically no increase in the length of the fruit, and the radicle has only to penetrate the tissues, 21⁄2 lines in thickness, between the seed and the floor of the calycine tube.
In the mode of separation of the seedling there are very marked differences between this species of Bruguiera and the species of Rhizophora. With Bruguiera rheedii the four small cotyledons, which are united at the base, are, however, left behind at the bottom of the seed-cavity, when the seedling is detached. But there is no expulsion of the seedling, the connection being ultimately severed at the contracted base of the cotyledons inside the fruit. When the seedling is full-sized the nutritive supply begins to fail, and in consequence the pressure of the sides of the fruit on the inclosed plumular end of the seedling becomes slacker, the union with the cotyledons becomes weaker, and the connection of the fruit with its peduncle at the basal joint becomes slighter. Usually the fruit falls before the seedling is ready to drop out, and the connection is severed after a few days’ flotation in the water; but sometimes the union between the seedling and fruit is weaker than that between the fruit and its peduncle, and in that case the seedling falls and leaves the fruit containing the cotyledons on the tree. The whole process of separation is much simpler than with species of Rhizophora. Here it is mainly a matter of the failure of the nutritive supply, whilst with Rhizophora it is almost a process of parturition.
Haberlandt, in the memoir before quoted, describes quite a different mode of detachment in the case of Bruguiera eriopetala. Here the seedling falls normally whilst still attached to the fruit, and the separation is subsequently effected by the expansion of the mouth of the calyx-tube due to the swelling of the “endosperm-neck” from the entrance of water.
Summary
(1) There are four typical mangroves of the Rhizophoraceæ in Fiji, Bruguiera rheedii, Rhizophora mucronata (the Asiatic species), Rhizophora mangle (the American species), and the Selala, a seedless form intermediate between the two species of Rhizophora just named, but nearest to the Asiatic species.
(2) It is shown that the sterility of the Selala is connected with the impotent character of the pollen; and since the ovules appear capable of fertilisation this is held to indicate that cross fertilisation has not been in operation in producing the barren form.
(3) Good reasons are given for the belief that the Asiatic species of Rhizophora is the parent of the Selala, not as the result of a cross between the Asiatic and American species, but as connected with dimorphism, the Asiatic species producing two kinds of offspring, one of them with impotent pollen.
(4) In support of this view it is pointed out that there are two forms of Rhizophora mangle in Ecuador, one of which comes near the Fijian Selala, though producing seed. There could thus be no question of crossing, since but one species occurs there.
(5) The Selala reproduces itself in a vegetative fashion when growing, as it often does, in an inclined position. The parent trunk dies and the primary branches supported by the aërial roots, remain alive and in their turn give rise to secondary branches similarly supported.
(6) Although, as a rule, only one of the four ovules of Rhizophora becomes a seed, occasionally a fruit contains more than one seed. With R. mangle in Fiji about one per cent. of the germinating fruits displayed more than one hypocotyl.
(7) As a result of a protracted series of observations in Fiji, it was established that in the case of a seedling of average length of Rhizophora mangle a period of thirty-three weeks elapsed between the date of fertilisation of the ovule and the detachment of the seedling from the tree. In the instance of R. mucronata it was placed at forty-two weeks. A period of thirty-eight weeks, or nine to ten months, is regarded as typical for the genus.
(8) It is established that normally there is no rest-period for the seed in the case of Rhizophora, the seed at once beginning to germinate on reaching maturity. In those exceptional instances, however, where there is more than one seed, it is shown that in some cases the seeds do not begin to germinate together, and that a rest-period of at least some weeks can be at times postulated for one of the seeds.
(9) An analogy exists between the process of expulsion ending in the detachment of the seedling of Rhizophora from the fruit and the process of parturition.
(10) Experiments show that Rhizophora seedlings can float unharmed in sea-water for a period of at least three or four months. Though nine-tenths or more float in sea-water, as much as a fourth or a half sink in fresh-water. As a rule they float vertically in fresh-water and horizontally in sea-water, the horizontal position safe-guarding the plumule against the risk of being withered up by the sun in a calm sea.
(11) It is shown that in the case of Bruguiera rheedii the seedlings when detached from the tree can float unharmed in sea-water for months. In their specific weight they display a similar fine adjustment to the density of sea-water, as is above described in the case of Rhizophora.
(12) With this species of Bruguiera, fertilisation takes place not in the unopened flower, as in Rhizophora, but after the flower’s expansion; and a very singular mechanism is here described which secures the completion of the process.
(13) A period of twenty-seven weeks elapses between the fecundation of the ovule and the detachment of the seedling from the tree in the case of Bruguiera rheedii; and it is shown that there is normally little or no room for any rest-period, and that, as with Rhizophora, the seed on reaching maturity begins to germinate.
(14) Though the seedlings of Rhizophora and Bruguiera could be transported in safety a few hundred miles across the sea, it is held that they could never cross the Pacific and reproduce the plant. That the American species of Rhizophora has reached the Western Pacific from the New World is not accepted. Rather is its present distribution regarded as representing its original wide range over much of the tropical zone.
CHAPTER XXXI
A CHAPTER ON VIVIPARY
The significance of vivipary.—The scale of germinative capacity.—A lost habit with many inland plants.—The views of Goebel.—The shrinking in the course of ages of tropical swamp areas.—The variation in the structures concerned with vivipary.—Abnormal vivipary.—Summary.
It was remarked in [Chapter IX] that the study of the germination of the floating seed carried us to the borderland of vivipary; and we may now observe that our study of the mangroves, Rhizophora and Bruguiera, in the previous chapter, has brought us into contact with vivipary in its most complete development in the tropical swamps of our age. There is a great gap between the two extremes, represented by the occasional germination of a seed in a capsule or in a berry on the plant, and by the elaborate process of vivipary exemplified by Rhizophora; but most of the intermediate stages can be illustrated by known examples of vivipary. There is, however, no pretension to deal with this subject here in anything but a cursory fashion; but it will, I venture to think, add completeness to a work in which germination on and off the plant has been such a frequent theme if I endeavour to connect together some of the various sets of facts known to us concerning germination from the standpoint of vivipary.
The principal argument here followed has been already outlined in [Chapter IX], where I have remarked that it is possible to construct a scale of the germinative capacities of plants, presenting a continuous series beginning with the mangroves, where germination takes place on the tree, and ending with those numerous inland plants where seeds are liberated in an immature condition. It is suggested that vivipary was the rule under the uniform climatic conditions of early geological periods, and that with the differentiation of climates that has marked the emergence of the continents the viviparous habit has been lost over much of the globe, the mangrove-swamps alone illustrating the climatic conditions once prevailing. The rest-period of the seed is regarded as an adaptation to climatic differentiation and to seasonal variation; and even the seed-stage may be broadly regarded as the price paid for adaptation on the part of the evolutionary or determining power that lies behind plant-development. When discussing the germination of Cæsalpinia in [Chapter XVII], I have shown that the contraction and induration of the seed-tests appear merely as an adaptation to climatic differentiation and to seasonal variation, and that it would be quite possible by exposing the maturing seed to very warm and moist conditions to induce germination without any rest-period, as actually occurs with Rhizophora. One would then dispense altogether with the final processes of the contraction and induration of the seed-coats, as illustrated in the Leguminosæ; and the rest-stage would appear as an adaptation to secular differentiation of climate in the later epochs of the world’s history.
The significance of occasional vivipary was long ago pointed out by Goebel in his Pflanzenbiologische Schilderungen (teil I., 117-134, Marburg, 1889), when he observed that vivipary, as displayed in the mangroves, and particularly in the Rhizophoreæ, represented the fullest expression of a habit that is only occasionally exhibited by other plants under exceptionally moist conditions. His view was that the seeds of plants living in wet places are suited in a varying degree for rapid germination, and that vivipary presents itself as the most complete development of this capacity. If I regard the views of Goebel and of Kerner aright, vivipary as normally developed in the mangrove is to be traced in a descending scale to small beginnings, the principal determining condition lying in the great difference that exists amongst plants in the readiness of the seed to germinate. In the ascending scale we would have first the detachment of the immature seed, where the embryo is often in a rudimentary state, the ripening of the seed taking place in the soil. Then would come those plants where the seeds on being detached are quite mature and are ready to germinate as soon as they fall to the ground. Then would follow the stage represented by those plants where the seeds merely begin to germinate on the plant, such as occurs more or less normally with some mangroves like Laguncularia, and abnormally with a number of plants living in drier stations. After this come those mangroves, where, as in Avicennia, germination is completed on the tree or shrub, but the seedling at once liberates itself from the parent. Last of all there is the stage of the typical mangroves, Rhizophora and Bruguiera, where the seedling remains for months growing on the tree and hangs from the branches.
Vivipary, as above stated, presents itself as a matter of small beginnings. My own view, however, is that it is a matter of small “endings”; and that if we were to commence the scale not with the immature seed lying on the soil, but with the seedling suspended from the branches of a Rhizophora tree, we should record the various epochs in the history of vivipary throughout the plant-world. From this standpoint the occasional cases of incomplete vivipary displayed outside the mangrove-swamp represent a lost habit belonging to a primeval period when the climatic conditions were uniform over most of the earth, an age almost of eternal gloom, when the air was ever saturated with aqueous vapour, and when the sun’s rays were screened off by a dense cloud-covering that enveloped the globe, an age of which the existing mangrove swamps alone afford an imperfect indication. Yet even now we can say with Schimper that “dense and frequently repeated cloudiness apparently represents the most essential climatic condition for the occurrence of mangrove in the tropics” (Plant Geography, p. 409).
But, to return to the subject immediately under consideration, if my view is correct we ought to find indications of the lost habit in the anomalous structure of the seeds of some inland plants; and, indeed, it is shown in [Note 50] that this view can be taken of the singular structure of the seeds of the Myrtaceous genera, Barringtonia and Careya, and of the genera of some other orders, and can be extended by implication to several other plants possessing similar seed-structures.
With regard to the subject generally, it may be remarked that although normal vivipary is mainly restricted to the plants of a mangrove swamp, by no means all mangrove plants are typically viviparous. This habit in its most complex form is exhibited as a rule by plants with firm, somewhat fleshy, usually one-seeded, indehiscent fruits, such as we find with Rhizophora and Bruguiera; but plants with follicular fruits, such as occur with Ægiceras, may also display it in a fashion nearly as complex. Generally speaking, however, plants with hard, dry fruits, such as are owned by Excæcaria, Heritiera, and Lumnitzera, are non-viviparous, though to all appearances quite at home in a mangrove-swamp. Others again, like Carapa, Laguncularia, and Nipa, whilst displaying vivipary in a varying degree, in some cases as a general rule, in others only occasionally, exhibit no special structures connected with it. This point is well brought out by Schimper in his work on the Indo-Malayan strand-flora (p. 43), and no further mention need be made of it here.
The structures connected with vivipary vary greatly in their degree of specialisation. At the one end of the scale we have highly complex structures, such as are described in the preceding chapter. At the other end we have those cases of occasional germination on the parent plant where there is seemingly no special structure of any sort. That the complex arrangements concerned with the vivipary of Rhizophora, Bruguiera, Ægiceras, and Avicennia are adaptations is argued by Haberlandt and Schimper, both of whom devoted much attention to the study of these plants. This is seemingly indicated by the circumstance that complex structures concerned with vivipary are found in plants so divergent in their characters (the four genera above-named representing three orders, Rhizophoreæ, Myrsinaceæ, and Verbenaceæ) that they only possess their stations in common. It does not, however, follow that all mangroves that exhibit a complex form of vivipary are of the same antiquity. I should be inclined to regard those of the Rhizophoreæ as the more primitive types, whilst it is possible that plants of other orders, though ancient denizens of a mangrove-swamp, may be more recent intruders into the mangrove-formation after the differentiation of a dry-land flora.
Of particular interest in this connection are the cases of abnormal vivipary, or of “precocious germination,” that have been recorded from time to time respecting a number of plants not denizens of a mangrove swamp, none of which would appear, according to Schimper’s views, to present anything of the nature of an adaptation. Goebel mentions a number of instances, such as that of wheat-grains germinating on the stalk in a wet summer, and that of Dryobalanops camphora, the Borneo camphor-tree, when during a prolonged wet season in Java the seed germinates in the fruit on the parent tree. Amongst other examples he cites the Cacti, Epilobium, Agrostemma, and Juncus, the last case coming also under my observation in a wet season in England. One may here notice the instance of Dracæna, of which Mr. Hemsley, in April, 1902, exhibited at a meeting of the Linnean Society of London a specimen showing the seeds germinating in the berries on the plant.
Several cases of this kind came under my notice in Fiji. Pulpy fruits rather favour the precocious germination of seeds. Thus I sometimes found the seeds germinating in the Mandarin orange and in the Papaw fruit (Papaya) shortly after they had been gathered. But more interesting examples were displayed in those instances where the seed was found germinating on the plant. When the Convolvulaceæ grew in wet situations, as on the borders of a mangrove swamp, the seeds were sometimes observed germinating in the capsule. This came under my notice with Ipomœa glaberrima (Boj.) and with I. peltata, more particularly in wet weather. With some other plants, like Hibiscus diversifolius, that grow in wet places, this at times occurs. A species of Croton, employed as a support for the Vanilla plants in a plantation near Suva, displayed seeds germinating on the plant. I was informed that the seeds of the common cultivated Luffa (L. cylindrica) growing in a garden on Vanua Levu sometimes germinated in the fruit still attached to the parent. It is possible that the seeds of the parasitical genus, Myrmecodia, may occasionally germinate on the plant, since I found them germinating inside some of the small berries that had been lying forgotten within a newspaper for a fortnight.
Perhaps the most curious case of abnormal vivipary observed by me in Fiji was that concerned with the Coco-nut palm. Though not known to many residents in the island, this habit was described to me by Mr. Matthew Simpson, a planter on Vanua Levu, who told me that he had noticed nuts germinating on the tree in unusually dry seasons. Coco-nut palms displaying the nuts germinating on the tree came under my observation near Bale-bale, Savu-Savu Bay. In these cases the mature fruit, instead of falling, remains attached and dries on the stalk. In one case the seedling was about eighteen inches high. This seems to be what takes place normally according to Blume with Nipa fruticans, the swamp palm of Indo-Malaya. Goebel quotes this author to the effect that the fruits are not separated from the head before germination is so far advanced that sea-water can no longer injure the seedling. The fruits, we are told, may remain for years attached in a state of incomplete germination.
Summary
The scale of germinative capacity, that begins with the seedling hanging from the branches of a mangrove like Rhizophora and ends with the detached immature seeds of many inland plants that only germinate after lying for some time in the soil, is regarded as supplying a record of the various epochs in the history of vivipary throughout the plant-world. In the occasional cases of incomplete vivipary occurring among inland plants and in the singular structure presented by the seeds of certain genera of the Myrtaceæ and other orders we perceive indications of a lost viviparous habit belonging to a primeval period when vivipary was the exception and not the rule, an age when the same climatic conditions prevailed over much of the globe. At such a period the sun’s rays were screened off by a dense cloud-covering that enveloped the earth, and the atmosphere was ever charged with moisture. With the differentiation of climate that has marked the emergence of the continents during the secular drying of the earth, the viviparous habit has been alone retained within the confines of the mangrove-swamp, where the conditions once almost universal now survive; and as an adaptation to the differentiation of climate and to the resulting seasonal variation the rest-period of the seed has been developed.
CHAPTER XXXII
THE WEST COAST OF SOUTH AMERICA
The littoral floras of the West Coast of South America.—The Convolvulus soldanella zone of Southern Chile.—The plantless or desert zone of Northern Chile.—The Sesuvium zone of Peru.—The Mangrove zone of Ecuador and Colombia.—The two varieties of Rhizophora mangle, the “mangle chico” and the “mangle grande.”—The floating vegetable drift of the Guayaquil River.—The Humboldt current and the climate of the West Coast of South America.—The advance northward of the arid climatic conditions of the Peruvian sea-border.—The retreat of the mangroves.—Evidence of ancient coral reefs on the coast of Peru.—The shore plants and stranded seed-drift of the Panama Isthmus.—Summary.
My acquaintance with the strand-flora of the west coast of South America began at Corral, the port of Valdivia, in Southern Chile in lat. 40° S., and terminated at the mouth of the Guayaquil River, in Ecuador, about 2° south of the equator. During the period December 23, 1903, to March 17, 1904, I examined the coast plants at sixteen localities in this region, which covers 38 degrees of latitude and thus measures about 2,300 miles. Travelling in a steamer to Callao that was trading on the coast I had opportunities of staying for periods ranging from half a day to a couple of days at a considerable number of places; and a week spent at Valparaiso gave me a good opportunity of examining the beaches north and south of it. At Lima I spent some weeks, and from that centre examined the shore-plants at Callao, Ancon, and Chancay to the northward. North of this I had not the same opportunities, until we passed the Peruvian and Ecuadorian boundary; but from a visit to the shore at Paita, from the general look of the country in places as we coasted along, and from information derived from other sources, I was able to obtain a fair general idea of the prevailing character of the beach plants. After my previous experience to the southward, one could fairly gauge the character of the beach-flora from the appearance of the land behind. In the Gulf of Guayaquil and in the vicinity of the city of that name I spent about three weeks in the investigation of the coast flora.
If it were not for the interposition of the great rainless deserts of Northern Chile and for the scantily vegetated, scantily watered and semi-sterile condition of almost the whole coast of Peru, the botanist would be presented with a splendid opportunity of studying the distribution of shore-plants along a meridian stretching through some fifty degrees of latitude from Patagonia to Ecuador. As it is, drought and sterility in one form and another reign over about half of this great stretch of continental coast. This is reflected in the beach-flora; and though the observer will often have his interest attracted by the wonderful climatic anomalies arising from the presence on the coast of the cold Humboldt current, to which the sea-border of North Chile owes its desolation and the coast of Peru its semi-sterility, yet for a long time he will feel as if Nature had hardly dealt fairly with him.
THE WEST COAST
OF
SOUTH AMERICA
John Bartholomew & Co., Edinr.
Along the sea-border corresponding to the deserts of North Chile there would seem to be practically no plants growing on the beaches, except here and there where some stray plant from the saline districts inland intrudes on the coast. Along the whole sea-border of Peru from Arica north to Tumbez on the borders of Ecuador, the coast-districts, though more or less rainless, receive the benefit of the drizzly garuas and sea-fogs, and the sterility of the land immediately backing the beaches is much less pronounced than with the sea-border corresponding to the deserts of Northern Chile. This difference shows itself in a peculiar type of littoral vegetation, a strand-flora that is very scanty but one where on the beaches Sesuvium prevails. North of Tumbez the mangrove-formation predominates along the sea-borders of Ecuador and Colombia to Panama, excepting on a stretch of sterile coast extending north from the Gulf of Guayaquil to the equator.
Though in one sense the botanical observer will be disappointed with the littoral floras of the west coast of South America, in another sense when he remarks the manner in which the coast-vegetation reflects the abrupt changes in the prevailing climatic conditions he will be fascinated by the interesting problems presented to him. We are accustomed to connect a tropical coast with mangroves, coral-reefs, and beaches of calcareous sand supporting a luxuriant littoral flora. Climatic conditions banish all these from the tropical west coast of South America until within four degrees of the equator, and then with startling suddenness the dominion of the mangrove begins, the neighbouring hills commence to be clothed with tropical jungle, and the climate is completely changed. Mr. John Ball, who sailed along this coast about twenty years ago, referring to this remarkable phenomenon on the borders of Peru and Ecuador, remarks that no such abrupt and complete change both in climate and vegetation is known elsewhere in the world, and he adds that few parts of the American coast better deserve careful examination (Naturalist in South America). This subject has since been discussed at length by Dr. Wolff in his “Geografia y Geologica del Ecuador,” and by Baron von Eggers in a paper to be subsequently quoted, two very competent observers, but the latter considers that the subject still requires a systematic investigation, and suggests that an observing station should be established on this coast by the combined meteorological societies of Europe. A sojourn of more than a week in the swamps at Puerto Bolivar, a few miles from Tumbez, enables me to appreciate the nature of the problem, and to throw a little light on the line of investigation required.
But to return to the general subject of the littoral floras of the west coast of South America, I may say that beginning with the island of Chiloe in lat. 42° S., this coast may be divided into four zones.
(1) The Convolvulus soldanella zone of Southern Chile, which extends as far north as Coquimbo about 30° S. lat.
(2) The Plantless or Desert zone stretching north to the vicinity of Arica in lat. 18° 30ʹ, and corresponding to the coast of Northern Chile.
(3) The Sesuvium zone, extending north from Arica to the 4th parallel of south latitude in the vicinity of Tumbez, a sea-border of semi-sterility that comprises the entire coast of Peru.
(4) The Mangrove zone, stretching from Tumbez, on the frontiers of Ecuador, to the equator and on to Central America, but interrupted at first by a strip of sterility on the coast extending from the Gulf of Guayaquil to the borders of Colombia, or, strictly speaking, to the equator.