. An introduction to vegetable physiology. Plant physiology. 20 VEGETABLE PHYSIOLOGY complexity in the different groups. In the smallest forms, which are only herbaceous in habit, we find the protective mechanism taking the shape of a thickening and cuticulari- sation of the outer walls of the cells of the outermost layer (fig. 22). The protection secured is twofold; evaporation of water is prevented, and so an economy of the supply ia secured, while the dangers incident to cold or heat are minimised. In plants of sturdier habit per, cork layer. ^ protection affor(Jed by this outermost layer o
. An introduction to vegetable physiology. Plant physiology. 20 VEGETABLE PHYSIOLOGY complexity in the different groups. In the smallest forms, which are only herbaceous in habit, we find the protective mechanism taking the shape of a thickening and cuticulari- sation of the outer walls of the cells of the outermost layer (fig. 22). The protection secured is twofold; evaporation of water is prevented, and so an economy of the supply ia secured, while the dangers incident to cold or heat are minimised. In plants of sturdier habit per, cork layer. ^ protection affor(Jed by this outermost layer or epidermis is replaced after a while by a more complicated tegumentary sheath. Certain cells become specialised and form layers of cork (fig. 23) which ,pe ..-pe Fig. 23.—Outee Pobtion or Cortex of Young Twig op .-pe -pe Pig. 24.—Section of Baku of Quercus sessiliflora. (After Kny.) pe, cork layers arising at different depths in the cortex. arise successively at gradually increasing distances from the exterior, and in the case of trees finally lead to the con- struction of a bark (fig. 24). The corky formations are. 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 Green, J. Reynolds (Joseph Reynolds), 1848-1914. Philadelphia, P. Blakiston
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