40 Plant Life and Evolution 



tion, and for this purpose there have been developed 

 in the higher plants special organs, roots, for the 

 absorption of water. There have also arisen ex- 

 tremely efficient tissues for its distribution through- 

 out the plant. 



Provision has also to be made for checking ex- 

 cessive loss of water through evaporation. Where 

 plants are completely submerged, whether they 

 are simple seaweeds or more highly developed 

 plants like the pond-weeds, eel grass, etc., there is 

 free communication between the water within 

 the cells and that outside the plant. The spe- 

 cific gravity of the plant is not greatly different 

 from that of the surrounding medium which buoys 

 it up without the need of its developing special 

 supporting tissues, and the seaweed or other sub- 

 mersed plant collapses completely when removed 

 from the water which has served to support it. 



Modification Due to Aquatic Life. The differ- 

 ence in habit of the same species grown in water 

 and out of it is strikingly shown in a good many 

 amphibious plants. Thus the common yellow pond 

 lily (Nuphar), grown in deep water, has a leaf 

 stalk long and flexible, and the leaves lie flat upon 

 the surface of the water. The same plant growing 

 in shallow water, or, as sometimes occurs, simply on 

 the exposed mud, has short and stout leaf stalks 

 which bear the narrower and firmer leaves com- 

 pletely erect. Another common aquatic plant, the 

 arrow-head (Sagittaria), often has two sorts of 



