54 PRINCIPLES OF PLANT CULTURE 



one cuts into such a tree trunk no sap appears. Under 

 certain conditions in early spring, however, the sap " flows " 

 freely and this creates so much pressure in some trees and 

 shrubs that water drips from wounds in the wood, bearing 

 with it, of course, the materials it holds in solution. 

 This happens when we tap a sugar maple tree in spring. 

 Alternate rise and fall of temperature above and below 

 32 F. are necessary to start the flow of sap in the maple, 

 hence this occurs only for a few days in early spring. 

 With the birch and some other trees and the grape vine, 

 the strongest sap-flow or bleeding comes a little later in 

 the spring. This spring sap pressure or flow seems to be 

 for the purpose of forcing the water into the stems of the 

 plants in sufficient quantity to supply the unusual need 

 which comes with the opening buds and rapidly expand- 

 ing leaves, because with each contraction new supplies 

 of water or air are drawn into the stem, and thus the pres- 

 sure is maintained. Sap ceases to flow on the opening 

 of the buds, because transpiration from the foliage (74) 

 quickly relieves the abnormal 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. 



79. The current of prepared food (elaborated sap).- 

 The food of the protoplasm in the different parts of 

 the plant is prepared almost wholly in the leaves (120). 

 We know, however, that 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 



