Ii6 TEXTBOOK OF BOTANY 



a layer of cells lying between the wood and the bast. The 

 cells of this layer divide so as to form two layers, three layers, 

 and so on. Then some of the cells between the vascular 

 bundles begin to divide in the same way. Thus a continuous 

 zone of dividing cells, called the cambium (represented by a 



FIG. 67. A cross section through a portion of a pine stem in which the 

 different tissues have developed. The narrow dark zone is the cambium 

 layer, separating the wood (on the inner side of the cambium) from the 

 bast (on the outer side). At the center, inside the wood, is the pith. 



narrow dark ring in Figure 67), is formed which cuts through 

 the middle of each vascular bundle ; outside this zone lie the 

 bast, cortex, and epidermis, and inside it are the wood and 

 pith. The formation of new layers of cells pushes the wood 

 and bast farther apart, and so makes the stem thicker. 



Most of the new cells that are formed on the inner side of 

 the cambium zone develop into wood cells, and most of those 

 formed on the outer side of the cambium become b'ast cells. 



