. Outlines of plant life : with special reference to form and function . Botany. 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 transpi
. Outlines of plant life : with special reference to form and function . Botany. 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 Please note that these images are extracted from scanned page images that may have been digitally enhanced for readability - coloration and appearance of these illustrations may not perfectly resemble the original Barnes, Charles Reid, 1858-1910. New York : Henry Holt and Company
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