XEROPHYTB SOCIETIES. 



203 



surface of the globular, cylindrical, or flattened stems (see 

 §36). 



lis. Hairy coverings. — A covering of liairs is an effective 

 sun screen, and it is very common to find jjlants of xerophyte 

 regions character- 

 istically hairy (see 

 §35). The hairs 

 are dead struc- 

 tures, and within 

 them there is air. 

 Thi.s causes them 

 to reflect the light, 

 and hence to ap- 

 pear white or 

 nearly so. This 

 reflection of light 

 by the hairs dimin- 

 ishes the amount 

 which reaches the 

 working region of 

 the plant (see Fig. 

 109). 



149. Body habit. 

 — Besides the va- 

 rious devices for 

 diminishing ex- 

 posure or leaf sur- 

 face, and hence 

 loss of water, 

 enumerated above, 

 the whole habit of 

 the plant may em- 

 phasize the same purpose. In dry regions it is to be observed 

 that dwarf growths prevail, so that the plant as a whole 

 does not present such an exposure to the dry air as in 

 regions of greater moisture (see Fig. ITo). Also the pros- 



FlG. 170. Two plants of a common scouring rush {Equi- 

 seiv7n\ showing the effect of environment ; the long, 

 unhranched one having grown in normal mesophyte 

 conditions ; the short, bushy branching, more slender 

 form having grown on the dunes (xerophyte condi- 

 tions). — After CowLES. 



