. A textbook of botany for colleges and universities ... Botany. STEMS 6S9. of water. The absorptive organs of epiphytes, as seen in the lichen thallus, in the aerial roots of orchids, and in the leaves of mosses and bromeliis, have been treated elsewhere. Most orchids and bromelias have highly cutinized epidermal walls, which, with other protective features, reduce transpiration to such an extent that Ihe plants do not dry out for weeks. In many orchids the leaves or stems or both are greatly thickened and contain large quantities of water, the stems fre- quently showing bulbous enlargement.
. A textbook of botany for colleges and universities ... Botany. STEMS 6S9. of water. The absorptive organs of epiphytes, as seen in the lichen thallus, in the aerial roots of orchids, and in the leaves of mosses and bromeliis, have been treated elsewhere. Most orchids and bromelias have highly cutinized epidermal walls, which, with other protective features, reduce transpiration to such an extent that Ihe plants do not dry out for weeks. In many orchids the leaves or stems or both are greatly thickened and contain large quantities of water, the stems fre- quently showing bulbous enlargement. In most bromelias the leaves form a sort of cistern, which retains water for weeks after a rain; since these cistern epiphytes absorb water almost entirely through the leaves, the advantage of the habit is obvious. Epiphylls ; injury due to epiphytes. — Epiphytes occurring on leaves are known as epiphylls (fig. 970), and are especially characteristic of the moist tropics; lichens, mosses, and even vascular plants occur in this strange position. Epiphylls are very injurious to the leaves on which they grow, cutting off light and im- peding gas exchange. Epiphyllous lichens show all gradations between epiphytism and parasitism, some forms being strictly external to the leaf, while other forms destroy the cuticle and thus have a position directly on the outer wall of the pidermis; in still other cases (as in Strigula) the lichen tissues may penetrate the mesophyll. Stem epiphytes also often injure their supporting plants, checking gas exchanges through the bark or breaking the branches by their weight. Even in cold climates the beard lichens (such as U nea and Alectoria) may enfold the leafy twigs of conifers to such an extent as to cause their death; in some cases the hyphae of Usnea penetrate the living cells of the supporting plant. Subordinate categories of epiphytes. — Plants epiphytic for only a part of their existence (as Picus, p. S^S) ^rs known as hemi-epiphytes. In t
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Keywords: ., bookcentury1900, bookdecade1910, booksubjectbotany, bookyear1910
