THE GROWTH OF THE PLANT 95 



Another important function of the leaves is that 

 of throwing off surplus moisture by transpiration. 

 As this moisture which was derived from the soil 

 through the agency of the root hairs is given off in 

 the form of vapor, all of its dissolved mineral salts 

 are left behind in the leaves, where they are com- 

 bined with the carbohydrates produced by photosyn- 

 thesis to form the complex food substances required 

 for the nourishment of the plant cells. The throw- 

 ing off of water during transpiration causes the 

 leaves to also act as pumping engines and induces 

 the constant upward movement of the crude sap. 

 In moving trees or plants when in leaf this pumping 

 action of the leaves must not be forgotten and the 

 leaf surface must be greatly reduced by pruning. 

 Otherwise the plant will wither and die from want 

 of water before new root hairs can be formed with 

 which to absorb it. Deciduous trees should always 

 be moved when out of leaf. Even then the top 

 should be heavily pruned to avoid too great a dis- 

 play of leaf surface in the early spring before the 

 new root system has had time to develop. 



In some special cases, as with the cabbage, leaves 

 may become so altered as to serve as receptacles for 

 storage of reserve food materials. 



Roots, stem, and leaves are all organs that are 

 concerned in the growth of the individual plant, but 

 the chief office of the flowers is the reproduction of 

 new individuals in order to secure the perpetuation 

 of the species. Flowers present many variations in 

 form, structure, size, and color. In most cases, how- 

 ever, there are four sets of organs. The outer, called 



