. A textbook of botany for colleges and universities ... Botany. STEMS 687 vascular development. In dry cultures the vessels are more numerous, larger, longer, and have thicker walls than in moist cultures; some of the smaller veins present in the dry cultures are absent in the moist cultures, remaining as undifferentiated parenchyma. Furthermore, ,jn the drier cultures lignification begins earlier and is much more pro- nounced, and the differential thickening of the walls is more conspicuous; also the endodermis, which often retains its cellulose walls in water or in moist air, has thicker an
. A textbook of botany for colleges and universities ... Botany. STEMS 687 vascular development. In dry cultures the vessels are more numerous, larger, longer, and have thicker walls than in moist cultures; some of the smaller veins present in the dry cultures are absent in the moist cultures, remaining as undifferentiated parenchyma. Furthermore, ,jn the drier cultures lignification begins earlier and is much more pro- nounced, and the differential thickening of the walls is more conspicuous; also the endodermis, which often retains its cellulose walls in water or in moist air, has thicker and more completely suberized walls. Finally, the hadrome elements die much sooner in dry than in moist cultures. Where growth and transpiration are pronounced from the outset (as in bulbous plants), the' first new vessels often are larger than where growth is slow (as in many seedlings). Observation tends to con- firm experiment regarding the influence of water upon vas- cular development. In sub- mersed hydrophytes, such as Elodea and Ceratophyllum, the vascular elements occupy a much smaller space and are much less differentiated than. Fig. 1017. —A cross section through the vascu- lar bundle of a stem of the waterweed (^Elodea canadensis); note that the vascular tract {v) is not obviously differentiated into leptome and hadrome, and that the vascular cells have thinner walls than the cortical cells (c); j, starch grains; a, intercellular air chamber within the vascular tract; a\ similar chambers-in the cortex; highly magnified. in land plants of similar size, the leaf bundles often being so small as easily to escape detection (figs. 1017, 1018). Although the duckweeds are regarded as vascular plants, their conductive tissues are much less developed than are those of mosses like Polytrkhiim, entire organs sometimes having no vascular tissue, as in the roots of Lemna. In hydrophytes the leptome generally is reduced less than the hadrome, though in rare cases there may be but
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