TEMPERATURE, LIGHT, AND MOISTURE 125 



Water and plant forms. Although the character of the soil 

 determines in great measure the kind of plants that will thrive 

 in it, and temperature may have much to do in determining 

 their distribution, water is of still greater importance, since it 

 affects both the form and structure of vegetation. The plants 

 of the world - may be separated into three ecological groups, 

 known as xerophytes, hydrophytes, and mesophytes, accord- 

 ing to the amount of water to which they are adjusted. The 

 xerophytes, or drought plants, can 

 thrive with a very limited sup- 

 ply of moisture. They inhabit 

 deserts and other dry locali- 

 ties and are characterized by 

 many adaptations for conserv- 

 ing moisture, such as condensed 

 stems, reduced leaf surface, 

 thick epidermis, an extensive 

 root system, and various tis- 

 sues for water storage. Many 

 xerophytes are leafless and the 

 stems perform the work of pho- 

 tosynthesis, others have leaves 

 for a part of the year but drop 

 them when seasons of drought occur. In the species which 

 retain their leaves, the latter are usually covered with hairs, 

 scales, or bloom. Xerophytes often have their stems under- 

 ground in the form of rootstocks, bulbs, and corms, and the 

 production of thorns and spines by woody species is common. 

 The cactus, yucca, and houseleek are xerophytes. All xero- 

 phytes do not live in deserts, however. A rocky ledge or a 

 sand dune, though located in a region of abundant rainfall, 

 may hold so little moisture that the plants that grow upon 

 them are exposed to desert conditions and may have all the 

 characteristics of drought plants. The lichens which grow upon 



FIG. 89. Yucca glauca, a xero- 

 phyte of the Western plains 



