THE CALLUS 



83 



cells from the inner bark; but a wound which 

 exposes the wood is healed by growth from the 

 cambium. 



The cambium, then, is the active, living tissue 

 of the plant cylinder. The wood cells soon be- 

 come lifeless, and have no power to grow or to 

 multiply. It is apparent, therefore, that when a 



75. Cross-section of callus on an apple tree. 



limb an inch or more in diameter is cut off, the 

 exposed hard wood can never heal, as a w T ound 

 heals in flesh. The pressure on the cambium 

 being relieved, however, excessive growth arises 

 from it and from the inner bark, and a mass of 

 tissue, known as a callus, rolls out over the wound 

 and covers it. Fig. 74 is an excellent picture of 

 this callus ring. The ring will eventually cover 

 the wound; and if a longitudinal section of the 

 healed wound w^ere then made, we should find the 

 condition shown in Fig. 75, the end of the old 

 stub remaining as sharp -cut as it was when left 



