38 



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 ma\ be- 

 come changed into cork cells <>r the miter cells of the layer may 



4^^' : - 



Fig. 28 Leaf tall: ./. branch of horse-chestnut showing scar funned 

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

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

 [eft 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 

 ( . enlarged \i<\\ of the separating layer — <\ cork cells that heal the wound 

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

 eparating layer, and they are beginning to break down, as Been in the 

 upper part of ili<- figure, at x, thus causing the fall of the leaf; st, c* 

 -in containing starch. 



break down or they may become rounded off so that watei 

 collects in the intercellular spaces. Thus the leaf is attached 

 to the stem chief!) by the veins since the delicate separating layVi 



