72 THE PRINCIPLES OF AGRICULTURE. 



this season, there can be no evaporation to relieve the 

 pressure. The sugar of the sap is chemically formed in 

 the tree, out of the starchy substances stored during the 

 previous autumn. 9 



Nutrition. The nutritive substances which are de- 

 signed to serve as food for plants, and are dissolved in 

 the moisture of the soil, include a great variety. 



Each of these is needed in nearly all parts of the plant. 

 The most of them, however, must undergo chemical 

 changes before they are ready to be assimilated, or to 

 enter into the structure of the plant. Some of them 

 must first rise to the leaves to receive certain changes 

 by contact with the atmosphere, and must then pass 

 downward again to all points where they are required. 



The sap of plants, like the blood of animals, furnishes 

 a medium through which the elements of food may find 

 their way to the points where they are needed, but the 

 flow of sap is riot at all similar to the circulation of the 

 blood. The blood, in its circulation, actually carries sub- 

 stances to all parts of the body, and deposits them at the 

 points of destination, but there is no such complete sys- 

 tem of conveyance in the sap. It is true that the ele- 

 ments of plant food are aided, in passing from the soil 

 into the plant, by the upward movement of the sap ; but 

 when once in the plant they must act independently of 

 this movement, passing sidewise, downward, and in every 

 other direction. 



This is to be explained by the principles of diffusion 

 and osmose. 



As salt, when dissolved in water, will extend itself to 

 all parts. of the water, making its way through interven- 

 ing membranes, so these nutritive substances find their 

 svay to every part of the plant. 



