so8 TIMBER AND SOME OF ITS DISEASES. [CHAP. 



the wood. Supposing it has required three years to 

 heal over, it will be found that the new annual rings of 

 wood are a little thicker just below the slit ; this is 

 simply because the slit had relieved the pressure on 

 the cambium. The converse has also been proved to 

 be true i.e. by increasing the pressure on the cambium 

 by means of iron bands, the annual rings below the 

 bands are thinner and denser than elsewhere. 



But we have also seen that the cambium is not the 

 only living tissue below the bark : the cortical paren- 

 chyma (pa), and the cells (c) of the inner cortex 

 (technically the phloem) are all living and capable of 

 growth and division, as was described above. The 

 release from pressure affects them also ; in fact, the 

 " callus," or cushion of tissue which starts from the 

 lips of the wound and closes it over, simply consists 

 of the rapidly growing and dividing cells of this cortex, 

 i.e. the release from pressure enables them to more 

 than catch up the enlarging layer of cortex around 

 the wound. 



An elegant and simple instance of this accelerated 

 growth of the cortex and cambium when released from 

 the pressure of other tissues is exhibited in the healing 

 over of the cut ends of a branch, a subject to be dealt 

 with in the next chapter ; and the whole practice of 

 propagation by slips or cuttings, the renewal of the 



