8 THE LIFE HISTORY OF A TREE 



are more easily regulated and, as will be seen later, by 

 manipulating the crown the woodman can cause the bole to 

 grow into the shape he desires. 



In a forest the crowns of the trees forrn a cover over the 

 ground, and this cover is called the leaf-canopy. 



The food of a tree. The youngest roots are covered with 

 fine root hairs which take in water, and minerals (including 

 compounds containing nitrogen) dissolved in it, from the soil. 

 The water passes up the stem into the leaves, and these take 

 in from the air carbon dioxide gas, which dissolves in the 

 water contained in the leaves of the plant. The leaves, and 

 often other parts of the tree, contain a green coloured matter 

 called chlorophyll, which has the remarkable power of being 

 able to cause the elements provided by water and carbon 

 dioxide to become united into complex carbon compounds 

 which form a large part of the actual food of the tree. The 

 principal substances thus formed are known as carbohydrates, 

 and of them sugar is the most important. There are several 

 different kinds of sugar, but sugar of some sort is the chief 

 body to arise in consequence of the activity of chlorophyll. 

 Sunlight, a suitable temperature, and the presence of a trace 

 of iron salt are all essential for the process ; indeed, chlorophyll 

 itself is not formed unless at least these three conditions are 

 complied with. It does not, however, need so bright a light for 

 its own production as is demanded for the synthesis (or for- 

 mation) of the sugars. The latter process, being dependent 

 on light, is generally spoken of as Photosynthesis, and it is 

 one of the most important of the activities of green plants. 



When sugar is produced in a leaf faster than it can be 

 removed to other parts of the plant, of course its concentration 

 in the cell sap rises. If this were allowed to go on, its further 

 production would soon be arrested. But this stoppage is 

 prevented by the fact that when the concentration reaches 

 a certain point, new activities of the protoplasm of the living 

 cell are awakened, and the formation of insoluble starch at 



