In arid areas moisture is retained in the upper soil hori- 

 zon, and the vegetation is confined to those species which are 

 adapted to extreme economy of water. In areas of greater precipi- 

 tation, deeper penetration results in plant roots drawing upon a 

 greater volume of soil moisture. In low places a concentration of 

 moisture takes place and ground-water areas support those plants 

 which use more water than dry-land plants. Finally the water- 

 loving plants, living with their roots in water, are large con- 

 sumers of water. 



DROUGHT -RESISTANT PLANTS 



Certain plants are qualified to inhabit desert areas where 

 temperatures are high and precipitation is low. These plants 

 have guarded themselves against excessive transpiration in order 

 that they may conserve the limited supply of moisture available. .. 

 This economy is accomplished, in part, by reducing the area of 

 the transpiring surface through size of the leaf or in limiting 

 the amount of foliage. In some instances the plant lives through 

 the dormant season by storage of moisture in succulent tissues. 

 In others, modifications, as hairs on the leaves, waxy surfaces, 

 or closing or concealing of stomata are employed to prevent ex- 

 cessive transpiration. 



Investigations of desert growth by Cannon (9), have shown 

 three different systems of roots developed by perennials in their 

 struggle for survival under arid conditions. First, the spreading 

 type of lateral roots which are common to many cacti; second, the 

 long tap root which not only helps to anchor the plant against 

 winds, but draws moisture from depths below the surface; and third, 

 a generalized type which combines the other two, enabling the 

 plant to take advantage of all the moisture in the soil to a depth 

 of several feet. The creosote bush ( Covillea glutinosa ) is an ex- 

 ample of the generalized systems which probably accounts for its 

 rather wide distribution under varying conditions of moisture. 



Desert growth includes many plants having varying water 



