AN OUTLINE OF PHYTOBIOLOGY. 19 



V. Locomotio7i by Wafer Currents. 



To utilize the movement of water currents for 

 locomotion, it is needful that the parts to be scattered 

 shall float, and be able to resist decay for a considerable 

 time, but finally sink to the bottom. The floating is 

 usually efifected by the development of air-holding tissue. 

 Water currents are of three kinds : 1. Those caused by 

 falling rain, both as it falls and as it runs along the 

 ground ; 2. Those caused by wind on non-flowing water ; 

 3. Those of flowing water. Of these the first and second 

 are not of much importance, and there is probably more of 

 the incidental than of adaptation in the locomotion they 

 efi'ect ; the third is more efiicient. 



The various types are as follows : 



1. Seeds or fruits of wind-scattered land-plants are very 

 light, and when accidently they fall in water, they float, and 

 may be carried far. But they rarely reach conditions favorable 

 to germination and hence this incidental method is of little 

 value. It may, however, explain the reported fact that some 

 heavy-nut trees which are migrating northward, are found upon 

 branches of the St. John which flow from the south and not on 

 those flowing from the north. 



2. Floods may tear out root-stalks, fruits and other parts 

 of plants and carry them to situations in which they can grow 

 when the floods subside, but here, also, we can have no adapta- 

 tion — only incidental locomotion. 



3. Entire plants or their vegetative parts may be carried by 

 currents. This occurs with many algpe. The plant may let go 

 its hold on the bottom, float down stream and later take root 

 again, as in Nasturtiiom lacustre ; or it may float normally 

 throughout its life as in Lemna and Salvinia, the water hyacinth 

 of Brazil, which makes floats of its swollen leaf-stalks, and others. 

 Detachable buds are common in water-plants, which are either 

 lateral and protected by bracts as in Potamogetons, or condensed 



