Professor Schimper (p. 164) remarks that the buoyant seeds of Cæsalpinia bonducella all rattle when shaken, and that it is to the incomplete filling of the seed-cavity, thus indicated by the loose kernels, that the buoyancy of the seed is due. The rattling of the kernel was, however, quite exceptional in the seeds handled by me, even in the case of originally buoyant seeds kept for five years. Seeds with loose kernels were, in fact, more frequent with non-buoyant seeds than with those that floated. Thus in Fiji I found that whilst with the buoyant seeds 17 to 20 per cent. had loose kernels, with non-buoyant seeds the proportion was as much as 60 per cent.
The normal cause of buoyancy is, therefore, a large intercotyledonary cavity with the cotyledons lying in close contact with the seed-shell; but the two kinds of cavity may sometimes be combined. Out of a number of buoyant seeds of Cæsalpinia bonducella examined by me, 80 per cent. owed their buoyancy solely to a large central cavity (4 to 5 mm. across). In 6 per cent. it was due solely to the shrinking of the kernel away from the seed-shell; whilst in 14 per cent. it was to be attributed partly to a reduced central cavity (2 to 3 mm. wide), and partly to a space outside the kernel. The only difference noted in the structure of thebuoyant seeds of C. bonduc was that the two kinds of cavities were more often combined.
The reason of the absence of floating power was clearly indicated in the non-buoyant Hawaiian seeds, where there was no central cavity, or it was represented by a narrow slit. The solitary buoyant seed found in the beach drift had a typical large central cavity. With the non-buoyant seeds of the inland species of the mountains of Vanua Levu it was ascertained that two-thirds had loose kernels with the cotyledons closely appressed. In the others there was a lateral cavity outside the kernel, the central cavity being only represented by a slit, a hair’s width in breadth. In the non-buoyant seeds of C. bonduc, the central cavity was only 2 to 3 mm. wide, and the lateral cavities were small.
Respecting the influence of “station” in producing the differences in buoyancy, it cannot be said to be connected with the maturation of the seeds of inland plants under more humid conditions than those which prevail at the coast. In Fiji some of the littoral plants with buoyant seeds grow on the mangrove-trees in the shade and humidity of the swamps; whilst in Hawaii the inland plants of Cæsalpinia bonducella with their non-buoyant seeds thrive in exposed arid situations in districts of little rainfall, such as on scantily vegetated lava-flows. With non-buoyant seeds, where there is little or no cavity, the cotyledons are always thicker and moister than in the case of the seeds that float. Though associated with differences in station, as implied in the terms “coast” and “inland,” the cause of the difference in buoyancy is not connected with different degrees of humidity, but with some other cause or causes acting on the spot which, while they favour the drying of the kernel in coast plants before the seed-coats finally set, impede it in the inland plants. That the seed does not subsequently acquire floating power, even after years of drying, was shown in several of my experiments.
The light, unopened prickly pods of both species float buoyantly, even when the inclosed seeds have no floating power. In an experiment on Cæsalpinia bonduc in Fiji the pods remained afloat after a month in sea-water. With those of C. bonducella in Hawaii I found that they floated for several weeks, and in one case a pod was afloat after three months. The pods dehisce on the plant; but they sometimes do not open sufficiently to allow the seeds to fall out. The pods, however, have to be torn off from the plant, and are not likely to occur in the drift. Indeed, they never came under my notice in any locality in the drift, and as an effective aidto dispersal they must be disregarded. The buoyancy of the seeds and their well established distribution by currents render unnecessary an appeal to the floating pod.
The following is a summary of the foregoing remarks on Cæsalpinia bonducella and C. bonduc.
(1) The two species in Fiji are not always sharply distinguished, since intermediate forms occur, and here probably lies the explanation of the confusion that has sometimes occurred in diagnosing the species.
(2) Both are typical littoral plants, distributed over most of the tropical zone, and occurring in company in most of the Pacific archipelagoes; but they at times extend far inland.
(3) Though it is not unlikely that sea-birds may have aided in their dispersal, the oceanic currents have been the great agencies in their dispersal, as is indicated by the frequent transport of seeds in the Gulf Stream drift across the Atlantic, and by their occurrence in beach drift in various parts of the world.
(4) Having regard to the present arrangement of the currents and the distribution of the two species, reasons are given for the belief that their original birthplace was in the interior of the American continent.
(5) Notwithstanding the stony hardness of the seeds, when a notch is made in the outer skin a seed rapidly takes up water, and in a few days it becomes a soft and much swollen germinating mass. The author is inclined to think that this was the original condition of the seed, and that the rest-stage is an adaptation to secular differentiation of climate in later epochs.
(6) Unlike the seeds of other Leguminous littoral plants, those of Cæsalpinia are not likely to germinate abortively when floating in warm tropical seas, a risk that restricts the distribution of several littoral species.
(7) As tested by experiment, the seeds of both species are often able to float unharmed for years; but on the other hand seeds not infrequently have no floating power.
(8) Observation, however, shows that buoyancy goes with station, and that the general rule here applies that the seeds of coast plants float and those of inland plants sink.
(9) The nature of the influence of “station” on the seed-buoyancy is obscure; but it is evidently not connected with the usual differences between coast and inland localities, such as those concerned with exposure or shade, dryness of soil, relative humidity, and similar contrasts.
(10) The buoyancy of the seed is developed during the final shrinking process associated with its maturation, a large cavity between the cotyledons being usually produced.
Note.—Since most of the principal conclusions of this work are involved in my especial study of the littoral species of Afzelia, Cæsalpinia, and Entada, the reader is advised, if he wishes to form an opinion of the author’s method of investigation, to read this chapter carefully through. With most other shore-plants, though in not a few cases studied with the same detail, the exigencies of space have often limited me to the employment of the general results in the appropriate chapters without entering into details. Should he desire to test any view of his own relating to plant-dispersal, he could not do better than begin with the materials here provided.