ORGANIC RESPONSE 275 



supported by the medium. Some float freely on the 

 surface, such as the familiar " duck- weed " or Lemna. 

 Others are entirely submerged, though often rooted 

 to the bottom. Such plants have a great develop- 

 ment of leaflike or chlorophyll-bearing tissue, 

 which makes up for the deficient supply of sunlight 

 beneath the surface of the water. Some of the 



FIG. 98. Hydrophytic vegetation. 



marine algae of this class grow to enormous dimen- 

 sions and develop floats which buoy them up. 

 At the other extreme from the hydrophytes we have 

 the xerophytes, comprising vegetation characteristic 

 of deserts, where water is extremely scanty. Only 

 those forms that are able to avail themselves of the 

 meager supply of water in such a situation, have been 



