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, or meso- 
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 suchas a cactus 
This plant is rather leafy when well oie houbeleck: som 
supplied with water but nearly leaf- Tepresentative xero- 
less when forced to live with a phyte, and a young 
very scanty water supply 
: bean plant or mus- 
tard plant for a typical mesophyte, if left unwatered, 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 
Fic. 67, Euphorbia splendens 
