228 BUDS AND STIPULES 



species in bygone ages, and under different circum- 

 stances. 



The Fall of the Leaf 



Having traced up the leaf from its earliest rudi- 

 ment, it remains, in conclusion, to say a few words about 

 its final stages. 



The fall of the leaf (41) is by no means a simple 

 process or a mere case of death. In the first place, the 

 chlorophyll-substance is too valuable to be lost and 

 thrown away. Sachs ! has satisfied himself that it passes 

 out of the leaf, down the petiole, and thus migrates into 

 the persistent parts of the plant, and is stored up for 

 future use. With the general disappearance of the 

 cell- contents the protoplasm and cell nuclei are dis- 

 solved, the chlorophyll corpuscles lose 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 examina- 

 tion to follow the materials (and especially the starch), 

 and also the most valuable chemical materials (espe- 

 cially the potash and phosphoric acid), down the petioles, 

 into the twigs, where 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. 340). 



1 Flora, 1863, p. 300; Phys. of Plants, tr. by H. M. Ward, p. 318. 



