THE TREATMENT OK THE SHOOT 149 



invariably set in in the branch, and subsequently in the stem. 

 Even artificial covering of the wound is not always sufficient 

 to prevent this, as the wood which is laid bare will produce 

 large cracks when it contracts in seasoning, and so offer a 

 point of attack for Fungi. 



^ 31. In what way can the natural process of healing be 

 accelerated ? 



First of all, we must see how the natural process of healing- 

 is effected. The age of the injured organ is of the foremost 

 importance, as the process of healing takes place more rapidly 

 the younger the injured tissues are. If a root, for instance, 

 is deprived only of its meristematic apex, the entire wounded 

 surface will begin to form new cells, and the root-tip will 

 be entirely renovated. If, however, the axis is cut below the 

 apex, where the tissues have already become differentiated and 

 the woody cylinder has made its appearance, we notice not only 

 a retarding of the process of healing, but we observe also that 

 all the tissues are not able to take part in the closing of 

 the wound. The outermost, that is, the oldest regions of the 

 cortex, the woody cylinder, and the pith too, are not able to 

 enter again into cell division. 



In a branch of one year's growth the formation of cells which 

 will gradually cover in the wound is restricted to the cambium 

 layer. It is not, it is true, only the cambium layer of the 

 anatomists, i.e., the single layer of meristematic cells, but also 

 the cells adjoining it on either side, which I'opresent the 

 youngest cells of the bast and of the wood ; still it is only a 

 comparatively nai-row ring of cells just outside the wood which 

 possesses the power of forming the healing tissues. In some 

 plants (the lime), under favourable circumstances the pith too 

 of the first year's twigs can take part with its outermost 

 layers in the formation of new cells, but usually this faculty is 

 not of much consequence. The tissue formed by the cambium 

 and spreading over the cut surface consists at first of delicate 

 meristematic cells (callus) ; later on, the cells become differenti- 

 ated in such a way that those which are placed externally form 



