42 LEAF FALL 



by the water and the crude foods being absorbed by nearly all 

 parts of the plant body, do not require elaborate bundles for their 

 conduction. We now understand why shade leaves are thin and 

 broad and soft whereas leaves of desert plants and those exposed 

 to severe drying winds of summer or winter are thick, compact, 

 firmer and often leathery and hairy. 



One of the most interesting adaptive features of leaves is seen 

 in the leaf fall of our deciduous trees and shrubs. In the tropics 

 the leaves remain on the perennial plants often for long periods, 

 but in temperate climates the severe winters necessitate the 

 annual dropping of the leaves except in a comparatively few 

 evergreens where the thick leathery leaves are able to endure 

 such conditions. In our deciduous plants, when the conditions 

 are no longer favorable for the performance of leaf work, the 

 cells at the base of the petiole begin to divide and a delicate 

 layer of cells, the separating layer, is formed across the petiole 

 (Fig. 28). Various conditions induce this growth for there is 

 more or less leaf fall at all seasons of the year. Doubtless lack 

 of nutriment has much to do with it. If a leafy branch of horse 

 chestnut is placed between moist paper in a few days the separa- 

 ting layer will have formed and the leaf will have dropped from 

 the branch. The innermost delicate cells of the separating layer 

 may become changed into cork cells just before or directly after 

 leaf fall while the outer cells of this layer may break down 

 through mucilaginous modification or they may become rounded 

 off so that water collects in the intercellular spaces. Thus the 

 leaf is attached to the stem chiefly by the veins since the delicate 

 separating layer offers little support. If now the water in the 

 intercellular spaces of the separating layer should freeze, the 

 expansion of the w'ater as it freezes would so rupture the remain- 

 ing tissue that we would have the familiar sight of the leaves 

 falling in a shower in the morning after a frost either of their 

 own weight or with the slightest breeze. It will also be noticed 

 that owing to the formation of the cork layer or the drying up 

 of the delicate cells of the separating layer, the scar formed by 

 the fall of the leaf is nicely healed and closed against any loss of 

 fluids or the entrance of any organism (Fig. 28, C). 



