CH. XII CAUSES OF BUOYANCY OF SEEDS AND FRUITS 105 



but because, as Professor Schimper remarks, the same structure 

 often occurs with inland fruits and seeds possessing little or no 

 floating power. In many of these cases, as he points out, the 

 question of adaptation to dispersal by ocean currents cannot, there- 

 fore, be raised. The third group may be named the adaptation 

 group, because it is on these examples of buoyant seeds and fruits 

 that this investigator chiefly based his contention that in the main 

 the structures concerned with buoyancy repre";ent adaptations to 

 dispersal by currents effected through the agency of Natural Selec- 

 tion. It is accordingly to this group that Professor Schimper 

 especially directed his attention, and the result of his observations 

 made in the home of the plants and of his investigations in the 

 laboratory has been the elucidation of many difficult points in the 

 structure of their fruits and seeds. To the two " mechanical " 

 groups he did not pay the same attention ; and as their examina- 

 tion came more within the limiits of my own capacity as an inquirer 

 I have worked them out with some detail, the subdivisions of the 

 first group being my own as well as much of the material. 



Synopsis of the btioyaiit fniits and seeds of littoral plants of the 

 tropical Pacific classified according to the cause of buoyancy. (The 

 authorities are indicated by the initial letter, S = Schimper, 

 G = Guppy. Details are given under some of the species in 

 latter part of volume.) 



First Group. — The floating power is derived from un- 

 occupied space in the cavity of the seed or fruit, no 

 part of the seed or fruit as a rule possessing independent 

 floating power. 



Sub-group I., where the seed is concerned. 



Section I. The seeds have little or no albumen, and 

 neither the tests nor the seed-contents have any 

 buoyancy. The cotyledons are generally large, 

 foliaceous, and crumpled or folded, or otherwise 

 arranged, so that the seed-cavity is incompletely 

 filled. 



S. G. Hibiscus tiliaceus. 



G. Hibiscus diversifolius. 

 S. G. Thespesia populnea. 

 S. Suriana maritima. 

 G. Kleinhovia hospita, variable. 

 S. G. Colubrina asiatica. 



