40 LEAF FALL 



the currents of the water. Aquatic plants have Httle need of 



xonduction^^-^^he palisade arrangement of tissues isalso favor- 

 able to photosynthesis. In intense light the movements of the 

 protoplasm usually distribute the chloroplasts along the sides 

 of the palisade cells so that they are edgewise to the light, but 

 in feeble light they are distributed throughout the cell with their 

 broad surfaces to the light. Just why this disposition of the 

 plastids is made is not known but that it is of vital importance 

 to the formation and distribution of substances in the cell is not 

 to be questioned. 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 delicate cells of the separating layer may be- 

 come changed into cork cells or the outer cslls of the layer may 

 break down or they may become rounded ofT 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 



