HYDROPHYTES. 329 



algas, which have been swept to the quieter parts of the North 

 Atlantic by currents after having been detached by the waves. 

 Such plants may often live for a long time and may even 

 continue their development. 



Plants adapt themselves to currents, such as those in fresh- 

 water streams, by their slender form, which is characteristic 

 of plants in flowing waters, as seen in filamentous alga? and 

 the much divided leaves of higher plants. Currents of water 

 act as a stimulus upon certain plants, producing a direct 

 reaction in the mode of growth. 



448. The composition of the water affects chiefly the dis- 

 tribution of plants, in a manner similar to the presence of 

 salts in the soil. In the ocean waters the percentage of salts 

 is extremely variable in different regions ; in some places it 

 is as low as 0.5 per cent. , while in others it reaches 4 per cent. 

 In fresh waters the differences in kind and amount of dis- 

 solved salts are chiefly due to differences in the soils which 

 the streams drain. 



