lyo General Botany 



Xerophytes. The xerophytes are the characteristic plants of 

 deserts and dry plains, but they are by no means confined to these 

 regions. They occupy sand dunes and sand plains along the 

 Atlantic coast and on the shores of the Great Lakes. They may 

 even be found locally on rock cliffs and on dry, exposed hilltops. 

 In fact, they may occur in any situation in which a reduced water 

 supply in the soil is accompanied by atmospheric conditions that 

 promote rapid transpiration, or in which the plants are periodi- 

 cally, or continuously, subjected to drought. The stems of plants 

 that thrive in these habitats are reduced both in size and in the 

 amount of branching. The leaf area is reduced, or temporary, 

 or leaves may be entirely wanting. 



The cactus type. The cactuses represent the extreme type of 

 drought-resistant, succulent plants. Leaves are wanting except 

 during the early stages of growth, and then they occur only as 

 small scales at the nodes. The stems are columnar, often ridged 

 and fluted, and always thick and fleshy. The photosynthetic 

 work in cactuses is done by the chlorenchyma of the cortex. As 

 the green surface is small compared with the green surface in 

 mesophytic plants, food manufacture is slower and growth is 

 correspondingly less. Yet some of the cactuses of Mexico attain 

 heights of 50 feet (Fig. 97). The cactus form points clearly to 

 one of the most characteristic features of desert plants ; namely, 

 water storage. A single large plant may contain from i to 25 

 tons of water. As the plant loses moisture so slowly, it may 

 continue to live for several years without an additional supply 

 of water. At every node there are usually clusters of spines 

 and spicules. A heavy cuticle and one or several layers of epider- 

 mal cells prevent rapid transpiration. 



The shrub type. The mesquite, greasewood, and sagebrush 

 represent a second type of extreme xerophyte with much- 

 branched, hard woody stems. These plants are characteristic of 

 semi-deserts and of those parts of deserts in which soil moisture 

 is more constant. As a rule these plants are deeper rooted than 



