THE STRUCTTJEE OF TIMBEE. 273 



shoot, and that it chiefly finds expression in the vertical elonga- 

 tion of the cells that are produced by division in the region of 

 the shoot close to the apex. The ultimate lignification of such 

 elongated cells induces firmness and rigidity, but does not occur 

 until distension has practically ceased. 



Coming next to diameter- growth, or growth in thickness of a 

 tree, we find that it occurs all over the outside of the wood of 

 the stem and branches. Passing outwards from the wood to the 

 bark of a tree, we come to a point where it is difficult to say 

 whether the tissues are wood or what is popularly called bark. 

 It is in this thin layer of cells known as the cambium that all 

 the wood and all the bark are formed. Here we have energetic 

 cell-division going on, the cells produced on the inside ultimately 

 thickening and lignifying to form wood, those on the outside re- 

 maining more pliable and forming the so-called bark. The exact 

 position of this cambium layer may be best determined by taking 

 a branch or stem during the period of growth and peeling off the 

 bark. The separation takes place along the zone of cambium, 

 which is practically destroyed by the operation. 



As in the case of growth in height, so in the case of growth 

 in thickness, no activity is manifest except during the gonial 

 weather of spring, summer, and autumn. Taking a typical tree 

 such as the Scotch pine, and speaking perfectly generally, we 

 have the following points to note in connection with growth in 

 thickness. In spring, whenever the climatic conditions become 

 favourable, the cambium becomes active and starts to manufac- 

 ture fresh wood cells. This process goes on during summer, to 

 cease only when the autumnal weather becomes unfavourable 

 for its continuance. During the period when the cambium is 

 active it manufactures a sheet of wood which is disposed all over 

 the tree, and which, when vieAved on a cross section, is known as 

 the annual wood ring. Now, although this ring of wood is so 

 far alike throughout that it contains the same kind of cells in 

 all its parts, it presents some notable differences in appearance 

 according as the portion is produced in an early or a late period 

 of the season of growth. 



The portion . which is formed first is known as the spring- 



