56o GROWTH, ASSIMILATION, AND ACCUMULATION 



white pine they may attain lengths up to 4000 fi. Although differing struc- 

 turally from the cells of other meristems, physiologically the properties of 

 cambium initials are essentially similar to those of other 

 meristematic tissues. Cambium cells are vacuolated and 

 often prominently so (Bailey, 1930). They often show 

 protoplasmic streaming. 



The cambium usually becomes active in producing new 

 cells before primary growth has entirely ceased in all of 

 the tissues at the corresponding level of the stem. 



Successive radial divisions of the cells of the cambium 

 layer result in the development of secondary xylem on its 

 inner face and secondary phloem on its outer face and 

 cause increase in the diameter of the axis. Increase in 

 length of the vascular rays also occurs as a result of cam- 

 bial activity. The secondary xylem and secondary phloem 

 lie between the primary xylem and primary phloem as the 

 conductive tissues produced during primary growth are 

 termed. All increase in the diameter of stems resulting 

 from cambial activity is due to the production of additional 

 layers of phloem and xylem within the body of the stem. 

 In annual plants or in perennial species in which the stems 

 die down to the ground at the end of each growing season 

 secondary growth of the cambium never continues beyond 

 the current season. In species with woody stems new layers 

 of both xylem and phloem are developed during each period 

 of cambial activity, so that such species exhibit an annual 

 increase in diameter. In some woody perennials the cam- 

 bium may continue to develop secondary tissues for hun- 

 dreds of years, cambial activity being resumed periodically 

 with the advent of each growing season. Since in such 

 species the older phloem is first converted into bark and is 

 eventually sloughed off, the bulk of the structure of all 

 older stems and roots is composed of secondary xylem. 



Cells originating from the cambium pass through the 

 same three morphological stages of growth as do those de- 

 veloped from apical meristems. Formation of a new cell 

 in the xylem is initiated by division of one of the cells of 

 the cambium layer, the new cell wall developing midway of 

 the cell in the tangential plane (Fig. 55). The outer one 

 of the two daughter cells remains a cambium cell, but the inner one enlarges. 





m if 



Fig. 120. Per- 

 spective view of 

 typical fusiform 

 cambium cell. 



