XEROPHY'JES AND HALOPHYTES. 



319 



during the dry [jeriod and form them again upon the return 

 of the grow'ing 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 j and mosses. 



435. 2. The constant reduc- 

 tion of exposed surface. — This 

 .may be secured among the lea\'es 

 /by reducing them either in area 

 or in number or both, or by much 

 branching, with little green tissue. 



B^^^^ 



Fig. 357. — Transverse sections of a grass leaf {La^iitgreslis). A, open: A', rolled, 

 when dry. The white plates are the rihs of mechanical tisstie 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 wlien the leaf is 

 rolled, evaporation through them is hindered. Magnified i6 diam,— After Kenier. 



Plants with bristle-sliapcd o)" needle-shaijed leaves (figs. loi, 

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

 those with scale-like lea\es (fig, 109) show the various phases 

 of such adaptations. Extreme reduction of surface is secured 

 by suppression of leaves. In this ca^c any further ada]italion 

 depends upon the stems, which must also [irovide for nutritive 

 work. These ma)- take the form of lca\es (see* 112); 

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

 or they may be thread-like or needle-shaped, as in the as[)ara- 

 gus (fig. 105) ; or the stems themseh'es may reduce their area 

 by becoming fleshy and cylindrical, prismatic, or spheroidal, 

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

 1 10). 



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

 tion. — Certain leaves are adapted to a permanent j^irofile 

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



