THE STEM AND THE LEAF 83 



their actively transpiring leaves were to remain at work while 

 the ground was almost or quite frozen, the tree would suffer 

 a fatal loss of water. 



It has been found that the larch (which sheds its leaves) is 

 more resistant to such conditions than are most of the ever- 

 green conifers, although the latter have leaves of a highly xero- 

 phytic type. In a rainless summer some shrubs retain or shed 

 their leaves in proportion to the amount of soil moisture with 

 which they are supplied. The Euphorbia splendens (fig. 67) is 

 a commonly cultivated plant which well illustrates this capac- 

 ity to adjust the amount of leaf surface to a varying moisture 

 supply. In regions like Southern California and the coast of 

 the Mediterranean, where the long, hot summers are nearly 

 rainless, some trees and many shrubs are summer deciduous, 

 losing almost or quite all of their leaves at the beginning of 

 summer. Twigs in this leafless summer condition have been 

 found to lose in a given time only about one thirty-sixth as 

 much water as they do when in full leaf. 



The shedding of the leaf is a somewhat complicated process ; 

 a waterproof layer of tissue is formed at the base of the leaf- 

 stalk, and this cuts the leaf off from communication with the 

 stem. Before this layer is formed, the plant food in the leaf 

 has often been conveyed into other parts of the plant, so that 

 when the leaf falls, it takes with it little of value. 



PROBLEMS 



1. Show that the roots, stem, and leaves of ordinary trees cooperate 

 to do the work of food-making. 



2. Why is it that trees which are left standing after their forest neigh- 

 bors have been cut away are more liable to injury from windstorms than 

 are those which have always grown in the open ? 



3. Discuss the various ways of climbing, and show which are gener- 

 ally the most effective in securing a good light supply under great 

 difficulties. 



4. From what you have seen, does it seem probable that the different 

 kinds of plants in a piece of woodland all require about the same inten- 

 sity of light ? Discuss this matter, giving examples. 



