THE PHYSIOLOGY OF PKUNING. 



331 



be not too deep ; it will be found that the leaves above the cut will not 

 flag for lack of water. 



What, then, is the function of the outer series of tubes ? To consider 

 this we must now leave the roots and devote our attention to the leaves. 

 Leaves have been called the lungs of plants, but this simile hardly 

 describes their functions correctly. Leaves are the organs in which the 

 crude sap from the roots is transformed by the agency of light into food 

 products from which roots, stems,. leaves, in fact all parts of the plant, are 

 built up. The three great necessities for this process are light, air, and 

 water. The vital importance of light is easily appreciated when we 

 study the arrangement of leaves upon a stem. It will be seen that the 

 scheme of disposition is such as to ensure for each the maximum amount 

 of light. The importance of light lies in the fact that it is the only source 

 from which the chemical energy required for the processes of construction 

 of food material can be obtained. Before, however, we consider this in 

 more detail we will briefly see what part air takes in the living plants. 



It is commonly thought that leaves and wood are solid masses of 

 tissue, but this is of course far from being so. Every leaf has its ven- 

 tilating system, for all living parts of plants breathe as animals do. Air 

 passes through the spaces between the leaf cells, and its entrance and exit 

 are carefully regulated by the stomata of the leaves, which open and close 

 according to requirements. The bark requires also its supply of fresh 

 air, and the small corky dots, "lenticels," on the branches of all fruit 

 trees are so many inlets for fresh air. 



The process of combining the raw materials taken in by the root and 

 the carbon dioxide from the air to make the building material, " elaborated 

 sap," is called assimilation, and its exact nature is still under discussion, 

 but for our purpose "what happens" will suffice without considering 

 " how it happens." 



The place in which the work of assimilation is done is the protoplasm 

 of the cell : so far can we trace this remarkable process which still baffles 

 chemical research. Given a sufficient supply of water, certain inorganic 

 salts, air and light, the protoplast produces a substance nearly akin to 

 sugar. This process goes on for a certain time until the solution of sugar 

 in the cell sap gets stronger and stronger, and as the sugar increases the 

 energy of the protoplasm decreases until it comes to a standstill. The 

 problem has now to be solved how to get rid of this sugar so that the 

 formative process may continue. It is solved in a simple way. The 

 sugar is changed into starch, and in this form it has no deterrent effect 

 upon the protoplast which is free to continue its work, which it does as 

 long as the three conditions, light, air, and water are present. But when 

 the light goes at the end of the day the process stops and the leaf has 

 now a large supply of starch stored within its tissues. How is it to get rid 

 of this supply so that it may be free upon the return of daylight to con- 

 tinue its work ? Owing to the size of the starch grains and to the fact 

 that, as we have seen, the tubes along which the sap is conveyed are not 

 continuous, it is evident that the starch cannot be passed down the leaf 

 stalk into the roots and stems in its existing state. This difficulty is 

 solved by changing it back once more into a sugar solution, in which form 

 it transfuses through the plant with great ease. This, then, is the 



