152 GENERAL BOTANY 



lower slopes and the borders of streams. It is thus often pos- 

 sible to see on a single mountain slope all gradations between 

 mesophytes and xerophytes, and to gain a more definite idea 

 of the factors which control the great mesophytic and xero- 

 phytic plant associations of the earth's surface. 



Hydrophytes. Hydrophytes are plants which are wholly or 

 partially submerged in ponds, lakes, and streams and are thus 



FIG. 74. Hydrophytic vegetation 



Pond lilies, with mesophytic vegetation in the background. From Bergen and 

 Caldwell's " Practical Botany " 



subjected to conditions very different, as regards water supply, 

 from those on land. Like the typical xerophytes, they are sub- 

 jected to extreme conditions, which profoundly affect their form 

 and internal organization. In a typical submerged hydrophyte, 

 like the Elodea or the pond weed (Potamogetori), the stem and 

 leaves are of a very delicate nature, since the plant is protected 

 by its surrounding water medium. 



Partially submerged water plants, including water lilies, 

 pickerel weed, and some grasses and sedges, partake more nearly 



