NATURE OF PLANTS 



41 



of the separating layer should freeze, the expansion of the water 

 as it freezes would so rupture the remaining 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 for- 

 mation of the cork layer or the drying up of the delicate cells of 



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. 



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). 



