7 6 



THE CAMBIUM ZONE 



from the cambium. While the vascular bundles are very small 

 it will be noticed that the parenchyma cells separating the vascu- 

 lar bundles begin to divide so as to form a line of cells connecting 

 the cambium of each bundle (Fig. 52). In some cases these 

 divisions are at first somewhat irregular but soon the growth 

 results in the formation of cells with parallel walls and in this 

 way a ring of regular cells, the cambium zone, is formed which 

 continues to divide as already noted in the cambium of the 

 bundles (Fig. 53). Consequently new cells are now added not 

 alone to the vascular bundles but also along the entire extent 



cam 



FIG. 54. Cross-section of stem of castor bean three weeks older than 

 one shown in Fig. 53. Note the changes that have occurred in the stem 

 and especially the numerous cells that have been added to the xylem. 

 ck, cork ; ph, phloem ; cam, cambium zone. H. O. Hanson. 



of the cambium zone. This growth results in the formation 

 of a layer of xylem on the inside of the cambium zone and a 

 layer of phloem on the outside, and so brings about the principal 

 enlargement of the stem (Fig. 54). This growth is repeated 



