XEROPHYTES AND HALOPHYTES. 



323 



adaptations to reduce evaporation. In the leaves of some 

 xerophytes the guard cells of the stomata are motile only 

 when young, becoming thick-walled and fixed when the leaf 

 is mature. The stoma itself sometimes becomes closed, also. 



-~c* 



Fig. 366. 



Fig. 367- 



Fig. 366. — Portion of a transverse section through a node of sugar-cane, showing rods 

 of wax secreted by the epidermis. Magnified 142 diam. — After De Bary. 



Fig. 367. — Transverse section of a portion of the margin of a leaf of Aloe socotrina. 

 c, thick cuticle ; below c, cutinized layers of wall of epidermis, ep ; /, parenchyma 

 cells with chloroplasts ; cr, a crystal cell with needle crystals of oxalate of lime ; sp, 

 guard cells of stoma, sunk below surface ; a, intercellular space under stoma. Magni- 

 fied about 175 diam. — After Tschirch. 



(5) The internal tissues of the leaves may be more compact. 

 This reduces transpiration by restricting the area of the air 

 passages. Such dense structure is secured by multiplying 

 the number of the palisade layers and by the more regular 

 form of the spongy parenchyma (fig. 359 and ^[ 167). 



B. Adaptations for taking up water. 



439. Absorption. — 1. Some plants are adapted to im- 

 mediate absorption of moisture in the air or of liquid water 

 falling on their aerial parts. Such are, usually, the 

 algae, lichens, and mosses which grow in exposed situations. 

 2. Certain of the higher plants are furnished with hairs 

 adapted to the prompt absorption of rain or dew, e.g., Spanish 



