NATURE OF PLANTS 85 



of the cambium cylinder. This growth results in the formation 

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

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

 enlargement of the stem (Fig. 55). This growth is repeated 

 each spring in such plants as continue to enlarge from year to 

 year, as in the shrubs and trees. The great majority of all the 

 cells that are to be formed in a year's growth are cut off from the 

 cambium cylinder early in the spring, the first of these cells, and 

 in fact the majority of them become xylem cells while a few of the 

 later formed cells are added to the phloem. Consequently the 

 xylem increases faster than the phloem and forms the bulk of the 



cam 



Fig. 54. Cross-section of a stem of castor bean in which the formation of 

 the cambium cylinder, cam, as a ring of regular cells, has been completed; 

 p, pith; V, vascular bundles; st, stereome; c, cortex. Compare Fig. 40. — 

 H. O. Hanson. 



tissues of the stem. The amount of the xylem added to the stem 

 each year is generally indicated by the bands or annual rings (Fig. 

 56). This is due to the fact that the first cells formed in the 

 spring have thinner walls and often contain a great many ducts, 

 whereas the later formed cells are for the most part small and pro- 



