XEROTHYTES AND HALOPHYTES. 



3'9 



during the dry period and form them again upon the return 

 of the growing season. The fall of leaves in our woody vege- 

 tation is a similar adaptation to the cold season. The rolling 

 or curling of leaves is a common mode of avoiding evapora- 

 tion. It is common in grasses (fig. 357) and mosses. 



435. 2. The constant reduc- 

 tion of exposed surface. --This 

 may be secured among the leaves 

 by reducing them either in area 

 or in number or both, or by much 

 branching, with little green tissue. 



FIG. 357. Transverse sections of a grass leaf {Lasiagrostis). A, open; B, rolled, 

 when dry. The white plates are the ribs of mechanical tissue above and below a 

 stele, one in each ridge ; the shaded areas are green tissue. The stomata are located 

 low on the sides of the narrow grooves between the ridges, so that when the leaf is 

 rolled, evaporation through them is hindered. Magnified 16 diam. After Kerner. 



Plants with bristle-shaped or needle-shaped leaves (figs. 101, 

 358), those with permanently rolled leaves (fig. 359), or 

 those with scale-like leaves (fig. 109) show the various phases 

 of such adaptations. Extreme reduction of surface is secured 

 by suppression of leaves. In this case any further adaptation 

 depends upon the stems, which must also provide for nutritive 

 work. These may take the form of leaves (see ^[ 112); 

 or the branches may be thick, rigid, and fleshy (fig. 360) ; 

 or they may be thread-like or needle-shaped, as in the aspara- 

 gus (fig. 105) ; or the stems themselves may reduce their area 

 by becoming fleshy and cylindrical, prismatic, or spheroidal, 

 as in the various forms of Cereus and melon cactuses (fig. 

 no). 



436. 3. Movements of parts to reduce the illumina- 

 tion. Certain leaves are adapted to a permanent profile 

 position, that is, with the edges turned toward the sky,, 



