XII CAUSES OF BUOYANCY OF SEEDS AND FRUITS 117 



buoyancy is derived from the buoyant nucleus or kernel ; and the 

 third where it arises from air-bearing tissues in the coats of the seed 



or fruit. 



(2) The first and second groups, in which the question of 

 adaptation to distribution by currents through the agency of 

 Natural Selection is not raised, since the same structural characters 

 are found in seeds and fruits of inland plants not dispersed by the 

 currents, are termed the mechanical or non-adaptive groups. The 

 third is distinguished as the adaptive group, because it is here that 

 Schimper finds evidence in favour of the Selection Theory. 



(3) The first group is best represented by the Convolvulaceous 

 and the Leguminous types. In the former, which is well illustrated 

 by Ipomea pes caprse, the seed-cavity is imperfectly filled by the 

 crumpled embryo, the result of the shrinking process during the 

 final setting of the seed. In the latter, which is exemplified by 

 Entada scandens and Csesalpinia bonducella, the seed displays a 

 large central cavity produced by the arching outward of the 

 cotyledons during the shrinking process accompanying the last 

 stage of the maturation of the seed. As an instance of fruits 

 belonging to the group, those of Heritiera littoralis may be cited. 

 An uncommon type is presented in the " stones " of the drupes of 

 Premna taitensis, and in the pyrenes of Morinda citrifolia, where 

 the buoyancy arises from empty seed-cavities resulting from the 

 failure of some of the seeds. 



(4) The second group with buoyant kernels includes mostly 

 widespread Leguminous species, such as Canavalia obtusifolia and 

 Sophora tomentosa. 



(5) The third or " adaptive " group comprises many of the 

 characteristic littoral trees and shrubs of the Pacific islands, such 

 as Barringtonia speciosa, Guettarda speciosa, Terminalia katappa, 

 Tournefortia argentea, &c., that contain in their fruit-coverings a 

 buoyant cork-like material often bound together by fibres, but 

 which proves on examination to resemble cork only in appearance. 

 In another type, illustrated by the fruits of Cycas circinalis and the 

 seeds of Anona paludosa, the buoyant tissue forms a layer inside 

 the shell of the seed or "stone." 



(6) Some fruits like those of Ximenia americana and Calo- 

 phyllum inophyllum illustrate both the so-called mechanical and 

 adaptive principles in their structure ; whilst with the first-named 

 species they are as well adapted for dispersal by frugivorous birds 

 and are known to be a favourite food of fruit-pigeons. The same 

 difficulty arises with the fruits of some other characteristic littoral 



