NATURE OF PLANTS 91 



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 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, usually by the last of April. 

 The majority of these cells become xylem cells while comparatively 

 only a few of the cells are added to the phloem. Consequently the 



ck 



cam 



FIG. 55. Cross-section of stem of castor bean three weeks older than one 

 shown in Fig. 54. 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 cylinder. H. O. Hanson. 



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

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

 each year is generally indicated by 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- 



