INTRODUCTION 



33 



their normal outlines, the starch disappears, and the 

 colouring matter changes, leaving in many cases a large 

 number of small bright yellow granules, to which the 

 yellow tints of autumn are due. In many cases the 

 cell sap becomes bright red. Sachs has been able by 

 chemical examination to follow the materials (and 

 especially the starch), and also the most valuable 

 chemical materials (especially the potash and phos- 

 phoric acid), down the petioles, into the twigs, where 



Pia. 17. — LoiigituJiual sectiou through the junction of stem and leaf of Horse 

 Chestnut in autumn, a, parenchyma formuig the outer cortex of the twig ; b, 

 external cork layer of the same ; c and e, parenchymatous tissue at the base of 

 the leaf-stalli ; d, separating layer, the continuity of which is already broken in 

 the upper pait. 



they are preserved, evidently to nourish the growing 

 buds of the following spring. While these changes are 

 taking place a new layer of cells is formed across the 

 base of the petiole (Fig. 17, d). These cells gradually 

 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 that case it is killed, the new 

 layer of cells is not formed, and the leaves remain 

 firmly attached to the twig.^ 



' Avebiiry (Lubbock), Buds and Stipules. 



D 



