460 



ECOLOGICAL GROUPS 



436. Difficulties in ecological grouping. It seems at first sight 

 a simple matter to group plants in regard to their need of 

 water. There can be no difficult} 7 in recognizing as hydrophytes 

 all plants like the bladderworts, water cresses, certain mosses, 

 and most alg which live only in water. Cactuses, aloes, and 

 similar plants are recognized at sight as xerophytes. But the 

 chief difficulty is in dividing mesophytes from the other two 

 assemblages, into which they shade by indefinite gradations. In 



a single mesophytic thicket, for example, 

 one may find such hydrophytes as the 

 pepper bush (Clethra) and such moderate 



I \ ; 





FIG. 352. Aquatic plants : pond lilies with floating leaves, 

 and sedges with aerial leaves 



xerophytes as the catbriers (Smilax). In order to know whether 

 the plants of a region have plenty of water or not, we must know 



not only how many inches of yearly rainfall there are, but also 



/ </ </ i 



what the soil is like, what is the temperature of the soil and air ? 

 whether or not there are dry winds, and whether there are fogs or 



t/ 



heavy dews. A lichen on a bare rock may be living almost under 

 desert conditions, while a pitcher plant in a bog near by has its 

 roots in standing water (or in ice) nearly all the year round. 



