Chapter XLII. 



Xerophytic Plants. 



The plants classified under the general name of xerophytes 

 are, in their selection of habitats, just the opposite of aquatic 

 plants. They grow typically under conditions of slight mois- 

 ture and some of them are able to maintain themselves in the 

 most arid regions of the world. Thus, a distinctive desert 

 vegetation, such as may be found in the Sahara, or in the deserts 

 of Arizona, has come into existence. A number of plants, 

 although not inhabitants of the desert, find it difficult to obtain 

 sufficient moisture and assume the xerophytic type of structure. 

 Good examples of these are the orchids that live in tree tops 

 and dangle their roots in the damp air of tropical forests. Since 

 this is a slow way of accumulating moisture, such plants are 

 often cactus-like in form. Not being- exposed to the attack 

 of grazing animals, as are the cacti, they do not become armed 

 with thorns and spines, but they have often the same massive 

 bodies that are found in true desert plants. A great variety 

 of secondary conditions serve to modify and regulate the ap- 

 pearance of xerophytic structural characters. Thus, some xero- 

 phytic plants living in sand differ decidedly from others living 

 upon rocks. Those developed in soil rich in nitrogen differ 

 from those that have but a scant supply of this element avail- 

 able for their roots. The saline substances present or absent 

 in the soil may modify the plants growing upon it. For this 

 reason a difference arises between the vegetation of limestone 

 and granitic regions and some plants are known to be indica- 

 tive of limestone soils just as others are of the presence of quartz. 



Desert plants. A variety of devices are employed by xero- 

 phytic plants to regulate the evaporation or transpiration of 

 moisture. Sometimes during extremely dry seasons desert 

 plants abandon their leaf structures entirely, thus responding 

 to the arid conditions quite as plants of temperate regions re- 

 spond to the approach of winter. Trees in deserts often drop 



