. A textbook of botany for colleges and universities ... Botany. LEAVES 615 Probably the most xerophytic of all plants are the crustose lichens (as £«(e«ia), which are either epiphytic or epilithic { growing on rocks), appearing embedded within the substratum. Most crustose lichens ab- sorb water chiefly through the upper surface, though some species have upper surfaces which are not readily wetted or which are covered with an impermeable crust. On dry rocks, at least, such plants absorb water chiefly during or immediately following precipitation, soon drying out again, and enter- ing a pe
. A textbook of botany for colleges and universities ... Botany. LEAVES 615 Probably the most xerophytic of all plants are the crustose lichens (as £«(e«ia), which are either epiphytic or epilithic { growing on rocks), appearing embedded within the substratum. Most crustose lichens ab- sorb water chiefly through the upper surface, though some species have upper surfaces which are not readily wetted or which are covered with an impermeable crust. On dry rocks, at least, such plants absorb water chiefly during or immediately following precipitation, soon drying out again, and enter- ing a period of inactivity; the active period may be longer in the case of bark lichens, owing to the greater retentiveness of the substra- tum. Foliose lichens (for ex- ample, Parmelia; fig. iiii) are more leaflike and are at- tached to rock or tree surfaces by evident rhizoids; perhaps also they are less xerophytic, though capable of withstand- ing prolonged desiccation. Foliose lichens commonly ab- sorb through both surfaces, but mainly through the lower surface, except in a few instances (as in species of Parmelia with black under surfaces). The gelatinous lichens (as Collema) absorb great quantities of water and remain for some time as mucilaginous masses. Possibly the rhizoids supplement the chlorophyll-bearing organs in absorption, but to what extent if any is unknown. Some mosses (as Andreaea and Grimmia) are epilithic xerophytes and with- stand prolonged desiccation without harm, reviving rapidly during rain as a result of leaf absorption. Bromeliaceae. — The most remarkable instance among seed plants of absorption by aerial leaves is in the Bromeliaceae, a family of tropical epiphytes, repre- sented in the southern United States by Tillandsia, especially T. usneoides, the. Figs. 904, —Absorptive scale hairs of Tillandsia: 904, a scale hair, as seen in surface view; 905, a scale hair, as seen in cross section; the outer cells (c) are dead, and absorb water readily
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Keywords: ., bookcentury1900, bookdecade1910, booksubjectbotany, bookyear1910
