The Second Group.
Here are included those seeds and stone-fruits that possess buoyant kernels. Professor Schimper points out that since this is a feature both with inland as well as coast plants such a character cannot be viewed as an adaptation to dispersal by currents. The plants concerned belong mostly to the Leguminosæ, and we find here some of the most widely spread of strand species, such as Canavalia obtusifolia and Sophora tomentosa, as well as some of the giant climbers of the coast forests belonging to the genera Dioclea and Strongylodon. The kernels when divested of their coverings float buoyantly, but they soon absorb water and sink usually in a day or two, a circumstance indicating that it is to the impervious coverings that they indirectly owe their capacity to keep the seed or fruit afloat. It is noteworthy that seeds of Strongylodon lucidum from Fiji display beneath the raphe a trace of an internal layer of loose cellular tissue which, however, has no appreciable effect on the buoyancy; whilst with seeds of Dioclea (violacea?) from the same locality there is a thick layer of loose tissue which aids the floating power of the kernel but is not of itself sufficiently aeriferous to buoy up the seed.
This leads one to refer to two other plants belonging to this group, Calophyllum inophyllum (Guttiferæ) and Ximenia americana (Olacineæ), where, though the floating power is mainly due to the buoyant kernel, it is also aided by a layer of air-bearing tissue inside the hard shell of the “stone” of the drupe. Professor Schimper places these fruits in the third or adaptive group on account of the layer of buoyant tissue, but it would be more correct to class them according to the predominant cause of their buoyancy. It can be shown that with a non-buoyant kernel the “stone” no longer floats. This double cause of the floating power renders an explanation very difficult, since it would seem indefensible to give conflicting interpretations of their nature. With Ximenia americana there is another great difficulty. Its drupes are known to be dispersed by fruit-pigeons (Introd. Chall. Bot. p. 46); and judging from the rare occurrence of the “stones” in the drift there is good reason to believe that bird agency in the Western Pacific is predominant in the dispersal of the plant. It is by such test cases as this that we must put to the proof the reality or non-reality of the influence of adaptation on seed-buoyancy.