The Water of Plants and its Movements. 61 



expansion of the tissues of the stem, as well as of the air 

 and water within it. This creates so much pressure in 

 some trees and shrubs that water flows freely from 

 wounds in the wood, bearing with it, of course, the ma- 

 terials it Jiolds in solution. This happens when we tap 

 a sugar maple tree in spring. Alternate rise and fall of 

 temperature increase the flow of sap, because with each 

 contraction, new supplies of water or air are drawn into 

 the stem, and thus the pressure is maintained. Sap 

 ceases to flow on the opening of the buds, because trans- 

 piration from the foliage (75) quickly relieves the ab- 

 normal pressure. 



The popular idea, that the flow of sap in spring is due 

 to a rapid rise of water through the stem at that season, 

 is erroneous. The sap is really rising through the stem 

 much faster in midsummer than in early spring. 



80. The Current of Prepared Food. The food of the 

 protoplasm in the different parts of the plant is prepared 

 almost wholly in the leaves (121). We know, however, 

 Hi at growth occurs in the stem and roots as well as in 

 the leaves. It is clear, therefore, that when the stem 

 and roots are growing, a movement of food matter must 

 occur from the leaves into these organs. This movement 

 may be demonstrated by a simple experiment. If a 

 notch deep enough to pass through the bark and a little 

 into the wood, is cut into the stem of any of our common 

 woody plants during spring or summer, a callus or cush- 

 ion of new cells (73) will soon form on the upper side 

 of the notch, but not on the lower, showing that the ma- 

 terial from which new cells are formed is passing down- 

 ward. Close examination will show that this callus 



