XII CAUSES OF BUOYANCY OF SEEDS AND FRUITS 113 



Geography, p. 28. A very suggestive instance of this nature is 

 described under Brackenridgea in Note 46 and in Chapter XIII. 



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 Leguminosse, 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 (Guttiferae) and Ximenia ameri- 

 cana (Olacinese), 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 inde- 

 fensible 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. 

 VOL. II I 



