DISPERSAL BY WATER. 847 



water, during transportation by aerial currents, may be floated along, and finally 

 deposited by the stream. Numbers of fruits and seeds of the most various kinds of 

 plants are invariably found to have been deposited on the banks of sand by the 

 sides of mountain-torrents, and on the margins of rivers and rivulets after the water 

 has subsided from a state of flood. Many of them, it is true, have no chance of 

 developing, but perish, either because the conditions are unfavourable, or because 

 they have lost their capacity for germination in the transit; others do, however, 

 germinate, and some even thrive luxuriantly. But such seeds can only be said to 

 have been accidentally dispersed by running water, and must not be considered as 

 instances of adaptation to that method of dispersal. 



The same statement applies generally to the chance deposition of the fruits or 

 seeds of land-plants in the sea. They may be carried away to a great distance by 

 ocean-currents, may float about for months, and finally be stranded on some remote 

 coast. Experiments have frequently been made with a view to ascertain which 

 fruits and seeds retain their power of germination notwithstanding prolonged 

 immersion in salt water. As a result of these experiments it has been established 

 that the seeds of Asparagus officinalis, Hibiscus speciosus, and several other plants 

 do not lose their capacity for germination after immersion in sea- water for a period 

 exceeding a year in duration, a fact which is in itself of great interest. But such 

 results are without significance in relation to the dispersion of fruits and seeds, 

 unless it be also ascertained that the fruits and seeds in question keep afloat upon 

 the surface of the water. For most fruits and seeds sink at once, and sooner or 

 later undergo decomposition at the bottom of the sea. The number of fruits or 

 seeds capable of keeping afloat on the surface for any length of time is extremely 

 small. Of the fruits which are found floating on the sea we may mention first the 

 hard-coated fruits of the group of Palms named Lepidocarynse. They have a 

 smooth, scaly, completely closed envelope which is impermeable to water, and looks 

 very like a coat-of-mail, and, owing to the fact that this envelope is not in im- 

 mediate contact with the fruit, but is separated from it by a layer of air, the fruits 

 are able to float on the surface of the water. The large fruit of the Cocoa-nut 

 Palm also is rendered buoyant by a substantial layer of fibres, which incloses a 

 quantity of air, and is itself coated by a layer with fatty contents which prevents 

 the infiltration of water. If fruits of this kind fall into the sea and are cast up by 

 the waves, the seedlings inclosed in them may develop and become denizens of the 

 shores to which they have drifted, provided the conditions, in respect of climate and 

 soil, are such as to permit it. As a matter of fact, fruits cast up by the sea on to 

 remote islands in the Tropics have been known to develop without any human 

 interference. 



The phenomena connected with the dispersion of fruits and seeds in still water 

 are altogether peculiar. Currents arising from the slope of the ground do not occur 

 in such water, whilst currents set in motion by the varying temperatures of different 

 layers of water, for the most part, ascend and descend merely, and can occasion very 

 little horizontal displacement of fruits and seeds. The wind is the only agency in 



