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.