248 OUTLINES OF PLANT LIFE. 



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 algse and 

 the much-divided leaves of higher plants. Currents of water 

 act as a stimulus upon certain plants, producing a direct re- 

 action in the mode of growth. 



350. 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 o. 5 per cent. , while in others it reaches 4 per 

 cent. In fresh waters the differences in kind and amount of 

 dissolved salts are chiefly due to differences in the soils which 

 the streams drain. 



351. Summary. — Water plants may grow in the mud or 

 slime at the bottom ; submersed, and either free or attached; 

 or floating and either free or attached. The light, temperature, 

 movements of the water and the composition of the water are 

 the principal factors to which water plants must adapt them- 

 selves. 



