114 PLANT FOODS AND PLANT GROWTH 



Transfer of Water. — The process by which water is trans- 

 ferred from one part of the plant to another part is very imper- 

 fectly understood, but we do know that it is transferred from 

 place to place and t-uat it is carried upward in great quantities 

 against the tremendous force of gravity. We know that the 

 force necessary to lift this column of water to the top of very 

 tall trees must be enormous. We know also that this activity 

 is much greater at certain seasons of the year than at others. 

 We have all observed the bleeding of trees and vines when 

 pruned too late in the spring, and know that considerable injury 

 may result from this practice. 



Transpiration. — We have said that a considerable amount 

 of water is given oif through the leaves by transpiration. This is 

 also true of the new shoots and such other green parts as the 

 plant may possess. The transpiration of water should not be 

 confused with evaporation. The latter process takes place from 

 the free surface of a body of water and is controlled by the 

 physical conditions of the air, but the former is controlled by 

 the structural character of the plant and the living protoplasm 

 of the plant cells. The water in the plant must pass through 

 the cell-walls into the intercellular spaces of the mesophyll to 

 which we have previously referred (Chapter IV), and then 

 passes out through the stomata by a process of diffusion. 



Of course, when the air is dry the transpiration of water will 

 be much greater than when moist. The number of stomata 

 varies greatly in different species of plants, those growing in 

 dry places usually having fewer than those growing in wet places. 

 The amount of water which plants take from the soil and give 

 out through the leaves during a growing season is enormous, 

 but extremely variable when we compare plants living under 

 different conditions. It is estimated that some of our common 

 farm crops take up and give off an average of 300 pounds of 

 water in the production of one pound of dry material, although 

 in some plants it is three times this amount. 



