THE TREATMENT OF THE SHOOT 163 



finger-nail upon the external tissues that a layer of soft tissue 

 has already been formed over the exposed wood. 



This layer has been formed from the youngest cells of the 

 wood, which must have remained undamaged while the thin 

 cambium cells were ruptured, have thickened their outer walls 

 immediately after the operation, and have thus protected 

 themselves against the drying effects of the atmosphere. Some- 

 times this protective layer is strengthened from the beginning 

 by the collapse of the outermost layer of cells, while the cells 

 immediately beneath begin to multiply and form the new 

 tissue. We see the exposed medullary ray cells beginning to 

 divide ; the cells which were previously destined to be wood 

 fibres divide again and form short cells, and even in the young 

 vessels thylloses are formed and divide up to form new cells ; 

 in fact, all the cells constituting the new wood develop into a 

 homogeneous callus-like layer of cells, which at first looks like 

 a layer of cortex cells. But as these cells multiply a differen- 

 tiation begins to take place. The innermost layer alone remains 

 meristematic and forms a connection with the normal cambium 

 of the undamaged stem above and below the wound. A new 

 cambium layer is therefore formed over the entire exposed area, 

 and is continuous with the normal cambium. It forms the 

 new elements somewhat shorter at the commencement in 

 exactly the same way as the normal cambium, and thus a 

 new layer of bast begins to cover in the wood of the exposed 

 area. 



This account of the process of healing explains two rules 

 which have to be observed. As the material for the formation 

 of the new layers of cells has to come from the reserve 

 material stored up in the wood, the most favourable time for 

 the operation is that period of cambial activity at which a large 

 amount of material is again becoming stored up in the wood, 

 and that is the period immediately preceding" the second period of 

 growth. In the spring the cambium, it is true, is very active, 

 and the bark can be easily removed, but the reserve substances 

 are already transported away, and are being used up in the 

 unfolding of the buds, and there is therefore too little material 

 for the healing of an extensive wound. 



The second rule, to which also sufficient attention has not 



