28 THE FACTORS [Part 



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

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

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

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

 of most flowers is arrested when the light is weak '. 



A flowing movement of the water also impedes the formation of flower 

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

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

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

 explained. 



4. WATER AND THE DISPERSAL OF SEEDS. 



The species of plants that inhabit waters and shores frequently have coi 

 trlvances in the construction of their fruits or seeds enabling them to flo 

 for a long time and thus facilitating their dispersal by water-currents. 1 

 highly adapted cases such fruits or seeds possess various floating orgar 

 rarely in the form of a floating bladder with a wate 

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

 frequently in that of floating tissue, formed by a thi 

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

 intervening, as in fruits of Cocos nucifera, Cerbe 

 Odollam, Barringtonia speciosa, Terminalia Catapj 

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

 Cycas circinalis. Yet many floating fruits and seec 

 Fig %% a Morinda among which are some that remain for a long tin 

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



floating; natural size. ,. . , ^- i i ■ i r t i_ ._• j 



b Morinda citrifolia: dispense With any particular kind ot adaptation and o\ 

 stone with a floating their low specific gravity to an air-containincj water-tig 



bladder; natural size. i & » 



<r The same magnified, space between the pericarp and the seed, or betwei 



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



case of many inland fruits and seeds which have no connexion wi, 



the water °. | 



I'ruits or seeds possessed of prolonged floating capacity are frequc 

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

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

 recorded types. 



The great importance of marine currents in regard to the dispen. 

 of seeds was first recognized in the case of tropical fruits and seeds '• 

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

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



' See Part I, Chap. III. 



' Schimper, IV. Numerous figures of floating fruits and seeds will be found in t ; 

 book; see particularly Plate VII. 



