84 



Growth 



The cambial region is obviously a much more active and plastic one 

 than early workers regarded it. New initials are being produced and 

 others are disappearing. Rays are being formed, fusing and dividing. 

 Changes and rearrangements are continually taking place among the 

 initials. The two sides of the cambium are forming quite different types 

 of cells, and further differentiation within the xylem and the phloem is 

 beginning. All these changes, however, are so well coordinated and regu- 

 lated that a specific pattern of structure, constant enough for taxonomic 

 purposes, is produced and maintained. There are few places in the plant 

 where histological differentiation can be so well studied as in the prod- 

 ucts of the vascular cambium. 



ii i 



I 



Fig. 4-22. Radial sections of wood of Ginkgo (A) and Amentotaxus (B and C),- show- 

 ing relation of young ray to ends of wood cells. ( From Barghoorn. ) 



The cambium proper consists of a single row of cells, though on the 

 xylem side there are usually several rows of mother cells, developed from 

 it, which by their division produce the xylem. Relative activity in xylem 

 and phloem production differs considerably in different forms. In Thuja, 

 Bannan (1955) found that phloem began to develop later than xylem but 

 then continued at a steady rate. In larch, however, Knudson ( 1913 ) had 

 reported that phloem development preceded that of xylem, though the 

 most rapid growth of each took place at the same time. In Acer, Cocker- 

 ham (1930) and Elliott (1935) observed that the first cambial divisions 

 formed the spring type of sieve tubes. This was followed by xylem 

 growth, during which no new phloem was formed. As xylem production 

 ceased, a second phase of phloem development occurred in which the 



