82 



INTRODUCTION TO BOTANY 



Frequently each stoma is at the bottom of a depression, or 

 pit, in the epidermis, and is thus somewhat protected from 

 drying currents of air. 



80. Xerophytes. Plants like the cactus (fig. 66), the desert 

 Pelargonium (fig. 20), the crowberry (fig. 64), and a multitude 

 of others (many of them not marked by any such peculiarities 

 of form and structure as these are), which can resist con- 

 ditions of extreme drought, are called xerophytes. The only way 



in which one can get 

 a good idea of the 

 difference between 

 xerophytes and ordi- 

 nary plants, orjziSfl. 

 phytes. in their power 

 to endure a combi- 

 nation of high tem- 

 perature and scanty 

 water supply is to 

 compare their behav- 

 ior under conditions 

 of drought. A potted 

 plant such as a cactus 

 or a houseleek for a 

 representative xero- 

 phyte, and a young 

 bean plant or mus- 

 tard plant for a typical mesophyte, if left un watered, will afford 

 material for a highly profitable comparison of types. 



Are any xerophytes of economic value ? any mesophytes ? 

 any water or marsh plants? Give as many examples as 

 possible. 



81. Advantage of shedding leaves. When the soil tempera- 

 ture is nearly at the freezing point, most plants are unable to 

 absorb much water by their roots. It is probably owing mainly 

 to this fact that our ordinary winter deciduous trees have their 

 habit of shedding their leaves at the approach of winter. If 



FIG. 67. Euphorbia splendens 



This plant is rather leafy when well 

 supplied with water but nearly leaf- 

 less when forced to live with a 

 very scanty water supply 



