NATURE OF PLANTS 



77 



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 found in a year's growth are cut off from the 

 cambium zone early in the spring, the first cells, and in fact the 

 majority of all cells formed, being xylem cells and later a few 

 cells are added to the phloem. Consequently the 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 the bands or annual rings (Fig. 55). 

 This is due to the fact that the first cells formed in the spring 

 have thinner walls and often contain a great many ducts, where- 

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

 vided with thicker walls. So there is a sharp contrast between 

 the small thick walled cells of the summer wood and the thinner 

 walls and larger cells of the spring wood (Fig. 56). This ap- 

 pears to the eye as a band, the annual ring (Fig. 55). The 



Fig. 55. 



Fig. 56. 



Fig. 55. Diagram of a cross-section of a stem of black oak four years 

 old: p, pith; 1, 2, 3, 4, annual rings of xylem; c, cambium zone; ph, 

 phloem ; cr, cortex ; ck, cork ; m, medullary rays. 



Fig. 56. Magnified view of a portion of one of the bands of black oak 

 in Fig. 55, showing the thick-walled summer wood succeeded by the 

 thinner-walled cells and vessels. This association of cells causes the banded 

 appearance of the annual rings of growth, m, medullary ray; v, vessels 

 in the spring wood. 



