3 8 LEAF FALL 



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 cells of the layer may 



FIG. 28. Leaf fall : A, branch of horse-chestnut showing scar formed 

 by the fall of the leaf. The dots on the scar show the position of the 

 vascular bundles that are finally broken by the weight of the leaf. At the 

 left the base of the petiole is shown. B, diagram of a section through a 

 twig of hickory s, separating layer at base of petiole; v, vascular bundles. 

 C, enlarged view of the separating layer c, cork cells that heal the wound 

 caused by the fall of the leaf. The granular cells are the outer region of 

 the separating layer, and they are beginning to break down, as seen in the 

 upper part of the figure, at x, thus causing the fall of the leaf; st, cells of 

 the stem containing starch. 



break down 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 



