238 OUTLINES OF PLANT LIFE. 



Perennial plants with perennial shoots may drop their leaves 

 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. 197) and mosses. 



336. 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 



l^SSSSS 



A 

 Fig. 197. — Transverse sections of a grass leaf {Lasiagrosiis). 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 i6 diam. — After Kerner. 



green tissue. Plants with bristle-shaped or needle-shaped 

 leaves (figs. 63, 198), those with permanently rolled leaves 

 (permanent form similar to temporary rolling shown in 

 fig. 197), or those with scale-like leaves (fig. 71) 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 ^ 96) ; or the branches may be thick, rigid, 

 and fleshy (fig. 199) ; or they may be thread-like or needle- 

 shaped, as in the asparagus (fig. 67) ; or the stems them- 

 selves may reduce their area by becoming fleshy and cylin- 

 drical, prismatic, or spheroidal, as in the various forms of 

 Cereus and melon cactuses. 

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



