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.