. A textbook of botany for colleges and universities ... Botany. LEAVES 583. the exact period varying widely, as the season is long or short. Leaf fall, especially in deciduous trees and shrubs, is brought about by the development of a special layer of separation or absciss layer at the base of the petiole, representing the final phase of leaf activity. The cells of this layer differ from the adjoining cells in their greater turgescence and in possessing denser cytoplasm and more abundant starch, and in the relative thinness and slight lignification of their walls. Soon the walls become mucila
. A textbook of botany for colleges and universities ... Botany. LEAVES 583. the exact period varying widely, as the season is long or short. Leaf fall, especially in deciduous trees and shrubs, is brought about by the development of a special layer of separation or absciss layer at the base of the petiole, representing the final phase of leaf activity. The cells of this layer differ from the adjoining cells in their greater turgescence and in possessing denser cytoplasm and more abundant starch, and in the relative thinness and slight lignification of their walls. Soon the walls become mucilaginous and the cells then disintegrate along the plane of separation (fig. 842) ; the rupture of the conductive tract by wind or otherwise completes the process, the leaf falling to the ground. At leaf fall, and sometimes before, the wound is healed by the development of a protective cork layer; thereafter, the place of leaf attachment is marked by a leaf scar, whose shape and structure vary with the spe- cies (figs. 1057-1059). In some compound leaves (as in the hop tree and the Virginia creeper) absciss layers may develop first at the base of the leaflets, leaving the stems with a number of bare petioles. Sometimes the absciss layer is imperfectly if at all developed, so that the dead leaves remain on the tree, as in the beech and in various oaks. In most herbs there is no definite absciss layer, the leaves remaining attached until after death. Deciduous and evergreen trees. —Deciduous trees, as commonly understood, shed all their leaves at once, at the beginning of an unfavorable season, while evergreens shed their leaves from time to time, or, if all at once, only after the new leaves have developed (fig. 843). While the distinction between evergreen and deciduous trees is well marked in cold tem- perate climates, such is not the case in the tropics, where the same species or even the same individual may be evergreen one year and de- ciduous the next, or evergreen in low
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Keywords: ., bookcentury1900, bookdecade1910, booksubjectbotany, bookyear1910
