THE TREATMENT OF LEAVES 201 



first production of foliage there remains still a considerable 

 amount of starch stored up, chiefly in the base of the stem. 

 The formation of a second crop of leaves will use up the greater 

 part of this reserve material. By so doing the stem will be 

 greatly exhausted. There will also be no material at hand, for 

 a time at least, for the growth of the cambium, and conse- 

 quently only a very weak annual ring of wood will be de- 

 veloped. 



It is probably less harmful to the tree to lose its foliage by 

 a late frost. The leaves killed by the frost remain attached to 

 the tree, and it is therefore probable that a certain amount of 

 the mineral substance, at least, which has been expended on 

 their production will be reabsorbed by the branches. 



Such a reabsorption of substance from the leaves by the 

 stem has been proved in the case of foliage killed by excessive 

 heat and drought. This takes place when young trees are 

 growing in a very thin layer of soil covering rocks or scries. 

 The dried-up leaves will remain adhering to the branches 

 throughout the winter. 



The nitrogenous constituents and the phosphoric acid, it is 

 true, remain in the dead leaves, but the starch, and with it the 

 potassium, is in part reabsorbed by the tree. 



i^ 39. In what cases can the leaf be used for propagation ? 



As has been mentioned in dealing with the shoot, some 

 plants have a considerable tendency to produce adventitious 

 roots and adventitious buds. This faculty is in many cases 

 shared by the leaves too, and causes them to be used as cuttings. 

 At present we know of no general rule as to what kinds of 

 leaves can be used for cuttings, and we can only base our 

 remarks upon experience. It does not indeed follow that a 

 leaf which has been observed to produce roots will be suitable 

 for propagating ; for we know many hard-leaved plants the 

 leaves of which readily produce adventitious roots and remain 

 fresh for months, but which will not form adventitious buds. 



Begonias are perhaps most often propagated by leaf-cuttings, 

 and in their case it is chiefly the so-called leafy Begonias, like 



