PLANT SOCIETIES 



313 



damp air and soil. All of tliem transpire freely, and many 

 of them cannot live at all under the moisture conditions 

 which suit ordinary plants. 



Some aquatics have their leaves wholly submerged, 

 others, such as the duckweed and the pond-lilies (Fig. 218), 

 have them floating, and still others, like the sedges in the 

 same picture, have their leaves freely exposed to the air. 

 A few plants have both 

 water-leaves and air-leaves 

 (Fig. 219). Soine aquatic 

 plants are rooted in the mud, 

 while others have no roots 

 at all, or, like the duckweed, 

 have only water-roots. 



The leaves of land-plants 

 in very rainy, subtropical 

 climates are exposed to the 

 attacks of parasitic spore- 

 plants which flourish on 

 their surfaces. To ward off 

 the attacks of these it is 

 necessary to keep water from accumulating on the surfaces 

 of the leaves. This result is secured by a waxy deposit on 

 the epidermis and also by the slender prolongation to drain 

 off surplus water, shown in Fig. 221. That this slender 

 leaf tip is useful in the way suggested is proved by the fact 

 that when it is cut squarely off the leaf no longer sheds 

 water freely. 



385. Xerophytes. A xerophyte is a plant which is 

 capable of sustaining life with a very scanty supply of 

 water. Since the first plants which existed were aquatics 



FIG. 219. Submerged and Aerial Leaves 

 of a European Crowfoot (Ranunculus 

 Purshii). The leaf with thread-like 

 divisions is the submerged one. 



