62 Principles of Plant Culture. 



sap, 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 

 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 cushion of new cells (72) 

 will soon form on the upper side of the notch, but not 

 on the lower, showing that the material from which new 

 cells are formed is passing downward. Close examina- 

 tion will show that this callus forms just outside the 

 union of the bark and wood. In all plants having more 

 than one cotyledon (45), this current is through the 

 sieve tubes in the inner layers of the bark. The pre- 

 pared food matter is dissolved in the water that satu- 

 rates the cell-walls, and passes from the leaves to other 

 parts of the plant by diffusion (63). 



