PROPAGATION. 23 



During the growing season of a fruit-tree, the sap enters at 

 the fibrous roots, passes up through the alburnum or sap-wood, 

 ascends to the extremities of the branches, and is distributed 

 through the leaves. Emerging thus from the dark and minute 

 vessels of the wood, it is spread out and exposed to the action 

 of the light. It now becomes essentially changed in charac- 

 ter, enters into new combinations, and is charged with the 

 materials for the newly forming wood; it is distributed, not 



FIG. 18. FIG. 19. FIG. 20. FIG. 21. FIG. M. 



The Downward Flow of Sap, Causing Swellings, Callus, and Roots. 



through the sap-wood, but through the inner or living bark, 

 and building up the new plant tissues. This new layer being 

 soft and fresh, interposed between them, causes that separa- 

 tion known as the peeling of the bark. 



The sap is capable of moving sidewise, laterally through 

 the various vessels or microscopic tubes. Hence some trees 

 may be cut at one point more than half through on one side, 

 and at another point more than half through on the other side, 

 without intercepting the upward flow of sap, as in Fig. 18. 

 The lateral motion explains the reason why a graft set in the 

 longitudinal cleft of a stock receives the sap from the split 

 surfaces of the cleft, and succeeds as well as when cross sec- 

 tions of both are brought into contact. 



