SUMMARY 



229 



These cells eventually adhere less and less closely 

 together, so that at last the slightest touch or the 

 gentlest wind brings the leaf to the ground. That this 

 is not a mere case of death of the leaf, but, on the 

 contrary, is a vital process, we may easily convince 

 ourselves by breaking a branch during summer. In 



FIG. 340. LONGITUDINAL SECTION THROUGH THE JUNCTION OF STEM 

 AND LEAF OF THE HORSE CHESTNUT IN AUTUMN. 



a, parenchyma forming the outer cortex of the twig ; b, external cork layer of the 

 same ; c and e, parenchymatous tissue at the base of the leaf-stalk ; d, separating 

 layer, the continuity of which is already broken in the upper part. 



that case it is killed, the new layer of cells is not 

 formed, and the leaves remain firmly attached to the 

 twig. 



It is also of interest to notice how plants utilise the 

 fact of the temporary nature of their leaves. Leaves 

 share with the bark (also transient) the functions of the 

 excretory organs of an animal. In their cells are de- 



