THE ROLE OF THE CAMBIUM IN GROWTH 561 



usually in length as well as in cross-sectional area, and generally develops 

 directly into one of the xylem elements. Often, however, the inner of the 

 two cambium derivatives may divide one or more times before maturation of 

 the cells ensues. This usually happens in the formation of wood parenchyma 

 cells during which the xylem mother cell is cut into a vertical series of cells 

 by transverse divisions. Tracheids, fibers, vessels, wood parenchyma and wood 

 ray cells are developed in the xylem from the cambial derivatives 

 (Chap. XV). 



A new phloem cell is initiated in a similar manner from the outer of 

 the two daughter cells after division of a cambium cell (Fig. 55). These 

 outer cells may develop directly into mature phloem elements, or may first 

 divide before maturation ensues. In general, division of the phloem mother 

 cells before maturation appears to be of more frequent occurrence than division 

 of the corresponding xylem mother cells. Maturation of the cambial deriva- 

 tives formed on the outer face of the cambium results in the development of 

 sieve tubes, companion cells, phloem parenchyma, phloem ray cells, and phloem 

 fiber cells (Chap. XXVIII). 



Continuation of secondary growth from the cambium results during each 

 growing season in the development of a zone of secondary xylem cells inside 

 of the cambium, and a zone of secondary phloem exterior to it. The enlarge- 

 ment of the xylem cells produced from the cambium initials results in outward 

 movement of the cambium and all of the cells lying outside of this layer, 

 necessarily resulting in an increase in the girth of the cambium cylinder. 

 Enlargement of the immature phloem cells developed from the cambium, on 

 the other hand, results in outward movement only of the phloem and tissues 

 external to it. Generally several times as many new xylem elements as phloem 

 elements are produced by the cambium during a period of growth activity. 



Division of the cambium cells results not only in the formation of sec- 

 ondary phloem and xylem, but also, as the stem grows in diameter, in an 

 increase in the girth of the cambium layer. Some increase in the circumference 

 of the cambium cylinder results from a lengthening of the cambium initials 

 along their tangential axis as seen in cross section, but mostly this is brought 

 about by an increase in the number of cells around the cylinder as the stem 

 grows older. According to Bailey (1923) two principal methods by which 

 this occurs can be recognized. New cambium cells may result from radial 

 divisions of older cambium initials (Fig. 121, //) or by "pseudo-transverse"' 

 divisions followed by a sliding of the derivatives past each other (Fig. 121, B). 

 Each of these methods of multiplication of cambium cells is characteristic of 

 certain species. 



