STEMS, ROOTS, AND LEAVES 95 



THE CAMBIUM LAYER AND ANNUAL GROWTH IN THICKNESS 



We have already become familiar with the structure of the 

 cambium and of the annual layers of phloem and xylem which 

 are formed by it each season. We may now proceed to discuss 

 more in detail the formation and development of the tissue ele- 

 ments which comprise the phloem and xylem. 



The cambium is a cylindrical layer of actively dividing cells, 

 which functions much like the apical meristems of the root and 

 the stem. The cells of the cambium layer divide repeatedly 

 during the spring and summer months, and the products of this 

 division are gradually transformed into the permanent sieve 

 tubes, ducts, and other tissue elements of the annual layers of 

 the phloem and xylem. 



The cambium in Fig. 52 is only a few cells in width, owing 

 to the fact that the section from which the drawing was made was 

 cut from a twig gathered late in the autumn. The cambium is 

 bounded on its inner side by the regular rows of cells comprising 

 the tissue elements of the xylem, and on the outside by the sieve 

 tubes and accompanying cells of the phloem. 



When active growth begins in the spring, the cells of this nar- 

 row cambium zone increase greatly by cell division, so that the 

 cylinder becomes several cells in thickness. The outer layers of 

 new cells thus produced then become gradually transformed into 

 the sieve tubes and the living and fibrous elements of the phloem, 

 while the inner layers, in a similar manner, become transformed 

 into the* ducts and other tissue elements of the xylem. Since, 

 however, many more layers of cambial cells become transformed 

 into xylem elements than into phloem during one growing season, 

 the rings of xylem are much wider than those of the phloem. 

 This process, repeated each year, finally produces, in old trees, a 

 thick cylinder of wood covered by a comparatively narrow layer 

 of bark. The usual period for the production of wood and phloem 

 by the cambium seems to range, in temperate climates, from about 

 the fifteenth of April to the fifteenth of August or the first of 

 September, although phloem may be formed after these dates in 

 some trees. 



