28 THE FACTORS [Part I 



plants, also, that form their sexual organs under water are fertile in shallow 

 water, but sterile in deep water where their vegetative growth is luxuriant ; 

 we see this in Potamogeton rufescens, several of the Podostemaceae, Isoetes, 

 and other plants. This may be due to the action of light, as the production 

 of most flowers is arrested when the light is weak l , 



A flowing movement of the water also impedes the formation of flowers, 

 for instance in Potamogeton pectinatus. From Klebs' researches on various 

 Algae, it appears impossible that the cause of this should be the weakening 

 of the light by bubbles of air. The phenomenon has not yet been 

 explained. 



4. WATER AND THE DISPERSAL OF SEEDS. 



The species of plants that inhabit waters and shores frequently have con- 

 trivances in the construction of their fruits or seeds enabling them to float 

 for a long time and thus facilitating their dispersal by water-currents. In 

 highly adapted cases such fruits or seeds possess various floating organs, 

 rarely in the form of a floating bladder with a water- 

 tight wall, as in Morinda citrifolia (Fig. 33), more 

 frequently in that of floating tissue, formed by a thick 

 husk, the cells of which contain air, often with air-spaces 

 intervening, as in fruits of Cocos nucifera, Cerbera 

 Odollam, Barringtonia speciosa, Terminalia Catappa 

 (Fig. 34), Calophyllum Inophyllum (Fig. 35), seeds of 

 Cycas circinalis. Yet many floating fruits and seeds, 

 Fig m a Morinda among which are some that remain for a long time 

 umbellata : stone, not on the water, for instance Heritiera littoralis, altogether 



floating ; natural size. ,. . , . , . . , r , . . 



b MorTnda citrifolia: dispense with any particular kind 01 adaptation and owe 

 stone with a floating their low specific gravity to an air-containing water-tigfht 



bladder ; natural size. r , . , , . . 



c The same magnified, space between the pericarp and the seed, or between 

 the seed-coat and the kernel of the seed, as in the 

 case of many inland fruits and seeds which have no connexion with 

 the water 2 . 



Fruits or seeds possessed of prolonged floating capacity are frequent 

 in the littoral flora, particularly of tropical coasts, where they are often 

 of considerable size and have great diversity of form, within the few 

 recorded types. 



The great importance of marine currents in regard to the dispersal 

 of seeds was first recognized in the case of tropical fruits and seeds by 

 Linnaeus, who found some of those belonging to the tropical American 

 flora on the Norway coast, whither they had evidently been brought by the 



1 See Part I, Chap. III. 



2 Schimper, IV. Numerous figures of floating fruits and seeds will be found in this 

 book; see particularly Plate VII. 



