462 



ECOLOGICAL GROUPS 



The leaves of land plants in very rainy sub-tropical climates 

 are exposed to the attacks of parasitic fungi. To ward off the 



^ _.____. attacks of these and to 



allow free transpiration, 

 it is necessary to keep 

 water from accumulating 

 on the surfaces of the 

 leaves. This result is 

 secured by a waxy de- 

 posit on the epidermis, 

 and also by the slender 



FIG. 355. The duckweed, a floating 

 aquatic plant 



prolongation to drain off surplus water, 

 shown in Fig. 356. 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. 



438. Xerophytes. A xeropliyte is a plant 

 which is capable of sustaining life with a 

 very scanty supply of water. Since the first 

 plants which existed were aquatics, we 

 must consider that xerophytes ae highly 

 specialized and modified forms adapted to 

 extremely trying conditions of life. A typi- 

 cal xeropliyte is one which can live in a 

 very dry soil in a nearly rainless region. 

 The ylicca and the cactuses (Figs. 50, 357) 

 are good examples of such plants. Less extremely xerophytic 

 are plants like the date palm (Fig. 53), which nourishes in the 



1 Ficus religiosa. 



Fid. 356. Leaf of an 

 East Indian fig tree, 1 

 with a slender, taper- 

 ing point to drain off 

 water 



After Schimper 



