i55« A TEXTBOOK OF THEORETICAL BOTANY 



persed by rivers, the property of eventually sinking is almost as important 

 as that of floating, if the fruits are not to be carried out to sea. Flotation 

 for a few hours may be quite sufficient. Fleshy fruits such as berries are 

 often floatable but their seeds are more generally dispersed by birds. 



Devices which increase the power of flotation are not numerous and 

 are on the whole simple. Bracts which persist as coverings for the fruit, 

 even after it has been shed, as in the Grasses, are often useful in this respect, 

 especially if they are unwettable and enclose air around the fruit, which 

 increases buoyancy. Many of the common waterside grasses, Glyceria, 

 Orysa, Pholaris, etc., are dispersed in this way. Especially remarkable is 

 the peculiar grass Coix, which has unisexual spikelets, the female, which 

 produces only one seed, being enclosed in a large, white, bony bract which 

 makes an excellent float. The perigynial sac in Carex is also a valuable 

 float for the small, light fruits. The pericarp is, however, the usual flotation 

 agent and in many fruits it contains air-spaces or has tissues formed of 

 dry, dilated cells which give at least temporary buoyancy, until they 

 decay or water penetrates the air-spaces and the fruit sinks. Akenes of the 

 aquatic Ranunculus, Sagittaria, Potamogeton and Alisma all float by such 

 means. 



Some of the widest dispersal by flotation has taken place in the sea 

 and in this connection the Coconut deserves the first mention. According 

 to Ridley's view Cocos nucifera is not an Old World tree but may have 

 originated on the west coast of tropical America. Apart from its carriage 

 by man, its present wide dispersal, especially in the Pacific and Indian 

 Ocean areas, seems to have been chiefly due to sea flotation. The tree 

 commonly grows on or close to beaches and there is no doubt that the 

 fruits do roll down the beach to within reach of the tide and get carried 

 away. It is only in its complete state, with the leathery epicarp and fibrous 

 mesocarp intact, that it is capable of surviving for long in sea water. The 

 stripped " nut " itself rots in the sea in a few weeks. Similar hard coverings 

 are also formed on the fruits of two other tropical sea-coast trees, Barring- 

 tonia and Nipa, which are dispersed by ocean currents. Only sea-coast 

 plants like these would readily establish themselves in places where they 

 were drifted up. This explains why the Coco-de-mer, Lodoicea, although 

 it was for long known only by its floating fruits, has not established itself 

 beyond the Seychelles. It is an inland plant and its huge nuts only reach 

 the sea by being carried down by flooded rivers. 



Coconuts sometimes reach European coasts in the Gulf Stream drift 

 and there is an amusing story that a sacred relic which was for centuries 

 venerated at Skalholt in Iceland as the skull of St. Thorlac, eventually 

 proved to be no more than a far-wandering coconut. 



Several species of Mangrove trees are generally dispersed by sea, 

 especially Cerbera adoUam (Fig. 1420) and Heritiera littoralis (Fig. 1421), 

 whose fruits may be found plentifully on beaches throughout tropical Asia. 

 The former has a pulpy outer coat which quickly decays, but the fibrous 

 inner covering permits the fruit to float for months. The latter has a light 



