XEROPilYTES AND HALOPHYTES. 



239 



tion. — Certain leaves are adapted to a permanent profile 

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

 stead of the surfaces. (See ^ 243. ) Others assume a profile 

 position when the illumination 

 becomes too intense. These 

 positions, by placing the leaf 

 surface oblique to the direction 

 of the light rays, reduce the 

 amount of evaporation very con- 

 siderably. 



338. 4. Coverings, consisting 

 of living or dead scale-leaves, 

 stipules, leaf-bases or entire 

 leaves, reduce transpiration by 

 obstructing the free exchange of 

 air, or by holding water and so 

 keeping moist the surfaces they 

 cover. 



339. 5. Structural modifica- 

 tions. — These may occur either 

 in the epidermis or some inter- 

 nal tissues. (a) The epidermis 

 may greatly reduce evaporation 

 by the formation of hairs in such 

 profusion as to form a cover for 

 the surface (figs. 200-202). 

 Hairs intended to protect from 

 evaporation are usually dead and 

 filled with air. Reflecting light 

 from many points, they look white, and the surface seems hoary, 

 or woolly, or silky. Hairs in the form of scales which overlap 

 reduce the rate of evaporation by covering the stomata (fig. 

 203). Further adaptations of the epidermis are to be found in 

 the water- proofing of part or all of the outer wall of the epider- 



FiG. 198. — Shoot of larch, with ripe 

 cone ; showing needle-shaped leaves 

 on dwarf branches; scale leaves on 

 main axis. Natural size. — Afttr 

 Kemer. 



