28 



Propagation. 



of the light. It now becomes essentially changed in character, 

 enters into new combinations, and is charged with the materials for 

 the newly forming wood ; it descends, not through the sap-wood, 

 but through the inner or living bark, and deposits a new layer be- 

 tween the bark and the wood. This new layer being soft and fresh, 

 interposed between them, causes that separation known as the peel- 

 ing of the bark. 



The sap is capable of flowing sidewise, through lateral openings 

 in the 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. 1 7. This lateral motion explains 

 the reason why a graft set in the longitudinal cleft of a stock, re- 

 ceives the sap from the split surfaces of the cleft, and succeeds as 

 well as when cross sections of both are brought into contact. 



Fig. 17. Fig. 1 8. Fig. 19. Fig. 20. i-ig. 21. 



The down-ward flow of sap, causing swellings, callus, and root* 



I. CUTTINGS. 



When a ligature is bound closely round a branch, the obstruction 

 which it imposes to the descending juices, causes an enlargement 

 or swollen ring above the ligature, as in Fig. 18. The same result is 

 produced if a small ring of bark is cut out, as in Fig. 19. If a shoot 

 is taken from the tree before the leaves expand, and plunged into 

 moist earth till it commences growth, the descending current exud- 



