XEROPHYTES AND HALOPHYTES. 319 



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 

 B ^fl^ may be secured among the leaves 



by reducing them either in area 

 f or in number or both, or by much 

 ranching, with little green tissue 



A 

 Fig. 357. — Transverse sections of a gra>-s leaf (Lasiagrostis). .1, open; A', 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 wi th bris t je-sh npr-' 1 "'■ "—d ie-shaped leav es (tigs. 10 1, 

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

 those with scale-like leaves I fig. 100 ) show thev arious phases 



of such adaptations^ KxtrenuT reduction oTsurface is secured 



by suppression of leaves. In this case any further adaptation 

 depends upon the stems, which must also provide fornutritive 

 work. The.se may take the form of leaves 1 see € 112); 

 or the branches may be thick, rigid, and fleshy 1 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 <a<tuses (fig. 

 1 10). 



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

 tion. — Certain lea\es are adapted to a permanent profile 

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



