THE CARE OF STREET-TREES 



119 



the cambium unite and form a complete layer over the sur- 

 face of the wound. This layer is a continuation of the 

 cambium of the growing stem, and during the next season 

 a layer of growth will be added over the wound continuous 

 with the annual ring added to the tree. The wood produced 

 over wounds differs in structure from normal wood and is 

 called callus wood. Eventually, however, the successive 

 layers become more like natural wood. 



The callus overgrowing the end of a severed branch 

 never coalesces with the old wood. 

 It simply seals up the remaining stub 

 of the branch, which becomes like 

 so much dead material buried in the 

 wood of the tree. Fig. 18 and Plate 

 27, Fig. 3, show the transverse and 

 the longitudinal sections of healed 

 wounds caused by the removal of 

 branches. It will be seen that the 

 stubs remained exactly in the same 

 condition as when the limbs were 

 cut off, and that the layers of tissue 

 of the subsequent growth of the trees 

 have overcapped them. 



The importance of the proper healing of wounds cannot 

 be overestimated. As has been pointed out before, limbs of 

 trees originate in many cases from the very centre of the 

 tree-trunks. After a limb is removed the remaining stub, 

 which becomes lifeless, is like a cylindrical block of wood 

 driven into the tree with the end exposed to the weather. 

 If nature did not provide for the healing of the wound, or 

 rather its overgrowing with new tissue, the stub would form 

 a soil for fungus spores and the entrance of insects, and 



Fig. 18. Transverse sec- 

 tion through callus on a 

 horse-chestnut, showing 

 stub A overcapped by 

 tissue. 



