LEAVES 



583 



the exact period varying widely, as the season is long or short. LeaJ 

 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 grounds and deciduous elsewhere ; 

 or the tree top may be deciduous and the basal limbs evergreen. Decidu- 

 ous trees (figs. 844, 845) scarcely need subdivision (except that some 

 tropical forms may have two or more periods of leaf shedding and re- 

 newal each year), but evergreens may be subdivided into (i) the 

 tender-leaved evergreens of the ramy tropics, such as tree ferns and 



FIG. 842. A dia- 

 grammatic vertical sec- 

 tion through the basal 

 region of a mature leaf 

 of the cottonwood 

 (Populus deltoides), 

 showing the initial 

 stages of leaf fall; the 

 absciss layer develops 

 along the plane, a ; the 

 cortical tissues (c~) sepa- 

 rate first, and the leaf 

 falls when the vascu- 

 lar region (v~) is rup- 

 tured 1 , somewhat mag- 

 nified. 



