200 SPRING. 



these in the wood, and at other times it is as difficult 

 to trace them in the bark. In the pine they are easily 

 traced in both, and in the lime they are hardly dis- 

 tinguishable in the wood, while in the bark they can 

 be separated with the greatest ease. 



It is the process, from the soft sap which is found 

 between the wood and the bark at the first vegetating 

 of the tree in the spring, to the separation of these two 

 parts of the substance just before the tree resumes its 

 repose, with the elongation of the twigs by pith and one 

 layer of cambium, that also separates into wood and 

 bark, which is the whole annual increase of the indivi- 

 dual tree ; and when the tree is all in equally vigorous 

 health, the action is probably simultaneous in the 

 whole of it. When a part is partially diseased, as when 

 the young shoots of a laurel have been nipped by the 

 winter's frost, it is longer in coming into action ; and 

 if the injury has been severe enough, the part does not 

 come into action at all, but shrivels and separates. 

 This often happens in blotches upon the boles and 

 branches of fruit trees when they are in a soil that does 

 not suit them, or have been injured by insects. The 

 vigour of the plant depends upon the unbroken state 

 of the surfaces at which this annual action and pro- 

 duction of cambium takes place. If a complete divi- 

 sion be made down to the wood on a branch, the 

 growth of the branch is lessened ; and if the same 

 operation be made on the bole, no more new layers, 

 either of bark or of wood, are formed on the portion 

 below the division. It is not, however, that the 

 power of making wood resides only in the upper parts 

 of the tree ; for trees, in the cambium of which germs 



