80 Growth 



meristematic cell. In woody plants, the root and stem axes continue to 

 grow not only in length but in diameter. This is accomplished chiefly by 

 the activity of another meristem, quite different in character, the vascu- 

 lar cambium. The tissues that such meristems produce are termed 



secondary. 



The vascular cambium is a sheath of embryonic cells extending from 

 the beginning of secondary growth in the shoot tip to a corresponding 

 position in the root. It arises between the xylem and the phloem of the 

 primary vascular bundles and forms xylem on its inner face and phloem 

 on its outer one. Each cambium cell produces a radial row of daughter 

 cells on either side. The tissues thus formed can usually be recognized 

 by this cell pattern, though it may sometimes be altered by the marked 

 increase in diameter of certain cells, notably the vessels and sieve tubes. 

 Cells of the apical meristem are relatively uniform, varied though their 

 products may be. Cambium cells, on the other hand, from the beginning 

 are differentiated into two quite dissimilar types of cells, corresponding 

 to the longitudinal and transverse cellular systems in vascular tissue. 

 Those cambium cells that produce tracheids, fibers, sieve cells, and other 

 elements of the longitudinal system are termed fusiform initials and are 

 usually much elongated in the dimension parallel to the axis. The ray 

 initials are much smaller and essentially isodiametric and produce the 

 rays in wood and phloem. 



The fusiform initials, especially those destined to form tracheids and 

 fibers, may be from 50 to 100 times as long as wide. Often they do not 

 differ greatly in length from the mature cells that they produce, though 

 in some cases the fibers of the summer wood may become much 

 longer than their initials (Bosshatd, 1951). Most of the divisions of these 

 initials must be longitudinal and tangential since only in this way can 

 additions be made to the width of the axis. The division of such an 

 elongate cell violates Hofmeister's law. It is a remarkable process and 

 was first clearly described by I. W. Bailey (1920a and b; Fig. 4-19). 

 The nucleus, usually in the center of the cell, divides mitotically. Be- 

 tween the two daughter nuclei the cell plate is then laid down by the 

 phragmoplast, an extension of the system of fibers at telophase. This ap- 

 pears in longitudinal section as two sets of fibers (sometimes called 

 kinoplasmosomes) connected by the cell plate, one moving upward and 

 one downward until the basis of the new division wall has been com- 

 pleted. An account of division in cambial cells was also given by 

 Kleinmann ( 1923 ) in a paper written during the war and without a 

 knowledge of Bailey's work. 



Since the circumference of the axis continually increases, it is necessary, 

 if the cambium cells are not to enlarge in tangential diameter, that they 

 increase in number by occasional radial divisions. In storied cambia, 



