. A textbook of botany for colleges and universities ... Botany. LEAVES 559 level, the stomata of conifers agree with those of many xerophytes, and are, perhaps, to be regarded as xerophytic stomata. The stoma-bear- ing under surface of xerophytic leaves is often hairy, contrasting w^ith the smooth upper surface (as in Antennaria or in Populus alba, fig. 820), while in other cases hairs may develop only in pits (as in Neriunt, fig. 807) or in furrows. Similarly, wax deposits often are observed on stoma- bearing surfaces, and waxy or resinous excretions may even clog up the stomata. Occasionall
. A textbook of botany for colleges and universities ... Botany. LEAVES 559 level, the stomata of conifers agree with those of many xerophytes, and are, perhaps, to be regarded as xerophytic stomata. The stoma-bear- ing under surface of xerophytic leaves is often hairy, contrasting w^ith the smooth upper surface (as in Antennaria or in Populus alba, fig. 820), while in other cases hairs may develop only in pits (as in Neriunt, fig. 807) or in furrows. Similarly, wax deposits often are observed on stoma- bearing surfaces, and waxy or resinous excretions may even clog up the stomata. Occasionally intrusive growths, known as tyloses, due to the bulging of adjoining cells into the stomatal air cavity, develop to such an extent as almost to block up the air passages (fig. 803); especially is this the case in xerophytic leaves. The stomata of vernal herbs (as Medeola) are likely to be less protected than are those of trees or of estival herbs (as Achillea), the latter being more like those of xerophytes. In most hydrophytes and in some mesophytes (as in ferns) the inner cutin ridges of the guard cells are lacking; in a few cases, even the median ridges are absent. Rarely, ex-. FiG. 803.—A cross section through a stoma from the upper leaf surface of Pilea elegahs, showing the closure of the stomatal passageway by the protrusion of a mesophyll cell (w) into the stomatal cavity (c); such outgrowths are known as tyloses; note the great thickening of the wall (w) just beneath the stoma; s, guard cells; p, chlorenchyma cells with chloroplasts (d) and nuclei (n); highly magnified. — From Habeelandt. cept in some hydrophytes, are the stomata lifted above the epidermal level. The stomata of cotyledons are relatively uniform in structure, thus seeming to correspond with the uniform conditions under which cotyledons are developed. The influence of external factors upon the structure of stomata. — A slight decrease in the size of the guard cells and a slight increase in the cut
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Keywords: ., bookcentury1900, bookdecade1910, booksubjectbotany, bookyear1910
