WATER REQUIREMENTS FOR PLANT GROWTH 187 



groups, each of which can be subdivided into smaller groups. 

 They are called societies and are as follows : 



Xerophytes. — This society includes those plants which re- 

 quire but very little water. The anatoinical structures for retain- 

 ing water, such as fleshy organs for storage of water, reduction 

 of leaf surface, abundance of trichomes and sunken stomata, are 

 very prominent in these plants. The most conspicuous subordi- 

 nate division is the desert and dry plain group, which includes 

 the cacti, sagebrush, yuccas and similar plants. Other important 

 groups are the xeropJiytic thickets of the southwestern United 

 States, which are frequently spoken of as the chaparrals; the 

 xerophytic forests, composed of pines, spruces, firs and related 

 plants, and the less prominent rock societies, consisting mostly 

 of lichens and mosses. Many thousands of acres of most excel- 

 lent agricultural land are now occupied by the xerophytic plants. 

 In some parts of the world it is possible to reclaim this land by 

 means of irrigation. Many thousands of acres within the bound- 

 aries of the United States have already been reclaimed. 



Mesophytes. — This society includes those plants which re- 

 quire a medium amount of water and includes the great mass of 

 land plants. The most prominent subdivisions are the meadow 

 and prairie societies, which include numerous grasses and other 

 herbaceous plants which are very generally associated with 

 them ; the deciduoiLs forests, which include our common trees, 

 such as the oats, hickories, walnuts, elms, maples, beeches, 

 chestnuts, cherry, catalpa, sycamore and poplars ; the mesopJiytic 

 thickets, which include our common shrubby plants, such as 

 hazel, alders, willows, spice bush, viburntuns and most berry 

 bushes ; and the rain forests of tropical countries which include 

 the dense jungle growths. The great majority of our agricultural 

 crop plants belong to the mesophytes. 



