. Plant anatomy from the standpoint of the development and functions of the tissues, and handbook of micro-technic. ad lost33 per cent, of its original wreight and the latter i per Aloe leaf, according to Heberlandt, had in twenty-fourhours lost times more water where the epidermis was re-moved than where it was left on. EPIDERMIS AS WATERPROOF COVERING 65 As might be expected, the amount of cutinization, as well asthickness of the outer wall, depends upon the severity of thedemands made by the environment. In submerged water plantsthe outer wall is usually thin and little if at a
. Plant anatomy from the standpoint of the development and functions of the tissues, and handbook of micro-technic. ad lost33 per cent, of its original wreight and the latter i per Aloe leaf, according to Heberlandt, had in twenty-fourhours lost times more water where the epidermis was re-moved than where it was left on. EPIDERMIS AS WATERPROOF COVERING 65 As might be expected, the amount of cutinization, as well asthickness of the outer wall, depends upon the severity of thedemands made by the environment. In submerged water plantsthe outer wall is usually thin and little if at all cutinized, whileparts rising above the surface of the water show greater thicknessof wall and more cutinization. In land plants the waterproofingcharacters become more pronounced and reach their fullestdevelopment in desert regions, or in alpine and arctic regions,and bogs and salt marshes, where the water, although presentin abundance, is difficult of absorption on account of its lowtemperature or the inimical nature of the substances dissolvedin it. Plants of the same kind grown in dry and in moist atmos-. FIG. —A, portion of cross section of leaf of Avicennia growing in salty soil; outerwall of epidermis very thick. B, cross section through skin of apple. C, cross sectionthrough upper half of petal of Japan quince. D, upper, and E, lower epidermis of leaf ofHibiscus moscheutos. F, epidermis of leaf of Lactuca scariola in the sun; and G, in theshade. spheres are apt to differ decidedly in their epidermal defenses;and even the different parts of the same plant or the same mem-bers of a plant are apt to differ in this respect according to thedemands made upon them. Thus, the upper epidermis of aleaf has, as a rule, a thicker and more highly cutinized outer wallthan the lower epidermis (Fig. 32), and the epidermis of the petalsof a flower, which are to endure for so short a time, is insignifi-5 66 PROTECTION FROM INJURIES AND LOSS OF WATER cant in its defensive characte
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