36 THE GROWTH OF THE TREES. 



the heavier fluid will attract to itself the lighter one until 

 they both become of the same degree of density. In the cells 

 of the young roots there is living organic matter, mucilage and 

 protoplasm, and the fluid is naturally denser than the liquid 

 they attract from the soil. The flow is, therefore, necessarily 

 into them. The leaves, however, throw off into the air as 

 vapours a vast amount of the water they contain ; especially is 

 this demand made in dry weather. In fact, they exhale more 

 freely than any other part of the tree. The organic matter 

 which then remains in them is, as will be readily seen, more 

 dense than that of the stalks which have not given out their 

 moisture so freely. The leaves, therefore, call on the stalks for 

 an upward flow of the contents of their cells. In the same way 

 the stalks call on the stems, and so on is the demand made 

 until the watery fluid of the root-ends is reached and drawn up- 

 ward to the leaves, or buds or any growing part of the tree. 



After the sap has been assimilated by these parts growth 

 begins, and in their own mysterious way they shape themselves. 

 Later the sap flows downward through the cambium layer, and 

 is again sent to parts where the tree needs it most. 



The assimilation of the crude sap is done in the green part 

 of the tree, and only is it accomplished when the brightest day- 

 light or the rays of the sun are shining upon them. New tissue 

 is then building, while useless matter is ejected. The tran- 

 spiratory organs of the leaves, innumerable minute openings 

 called stomata, are on their under surfaces. They open and 

 close. Then, too, the carbonic acid gas and water that the 

 tree has absorbed from the earth and air are digested and 

 given out abundantly as oxygen gas. This is finally the grand 

 purpose of the vegetable world ; to convert inorganic matter 

 into that which is organic, or to produce the food that is nec- 

 essary for all animal life. 



