IV.] 



THE FALL OF THE LEAF 



73 



existence. As a rule, cuttings cannot be struck from the 

 young wood in the spring, because it then possesses no 

 reserve material to draw upon. The leaves of deciduous 

 trees afford another example of the migration of food 

 materials ; when the year's growth is over and the tree 

 is preparing to shed its leaves for the winter, a layer of 

 comparatively large cork cells forms between the stem 

 of the leaf and the branch, and it is by the rupture of 

 this dividing layer that the leaf becomes detached from 

 the tree. But long before the leaf is detached, as 

 it begins to get old, the valuable food materials it 

 contains — the carbohydrates, the nitrogen, phosphoric 

 acid, and potash — are withdrawn from the leaf and stored 

 away in the stem, their place being taken by comparatively 

 abundant and valueless materials like lime and silica. 

 In consequence, dead leaves, from a manurial point of 

 view, are much poorer than living leaves, as the following 

 analyses (Table IX.) of the young, mature, and dead 



Table IX. — Materials contained in 500 Leaves of the Plane 

 Tree, gathered at different dates. 



leaves of the plane will show. The economy of this 

 process is obvious ; such substances as the starch formed 

 by the leaf, the nitrogen, phosphoric acid, and potash 

 taken from the ground, are required by the active cells in 

 order to carry out the vital processes of the plant ; they 



