THE CARE OF STREET-TREES 117 



the base of the stub, as shown in Plate 26, Fig. 2. The 

 stub of wood being no longer living tissue and exposed to 

 the weather, will dry, check, lose its bark, and the successive 

 annual layers deposited on the trunk will form a collar at the 

 base of the stub, as shown in Plate 26, Fig. 3. 



In Plate 26, Fig. 4, which is a section of the specimen in 

 the former figure, is shown the result of such a method of 

 pruning. The decay caused by the stub has been carried to 

 the heart of the tree. As time goes on the stub further rots, 

 and breaks off at the collar, forming an ugly knot-hole, Plate 

 26, Fig. 5. Knot-holes resulting from improper pruning are 

 very numerous. They form an admirable spot for the ger- 

 mination of fungus spores and the entrance of borers, and 

 are frequently the first cause of the serious injury and final 

 death of trees. 



The Right Way. Returning to Plate 26, Fig. 1, suppose 

 the limb had been cut off close to the trunk, along the line 

 CD, as far as possible parallel with the axis of the tree. 

 Separating the wood from the bark is the cambium layer. 

 All the tissue arising from the inner side of the cambium 

 layer goes to form the wood, while the outside produces the 

 bark. When a limb is cut off, as in Plate 26, Fig. 1, along 

 the line CD, the living cells of the cambium bordering on 

 the wound put forth an abnormal growth of tissue, called a 

 callus. It first arises from the exposed cambium, like a thin 

 ring, as shown in Plate 26, Fig. 8. With the growth of the 

 tree it rolls over the scar and finally overcaps it. 



How the Wound Heals. While the callus tissue is in the 

 process of overgrowing the wounded surface, it forms a 

 protective bark and a new cambium under it, which is con- 

 tinuous with the cambium of the growing stem. When the 

 margins of the overgrowing callus tissue meet, the edges of 



