96 



GENERAL BOTANY 



The development of the tissue cells of the phloem and xylem 

 follows very closely the stages of cell growth and differentiation 

 already traced in the root tip. The cell-division stage takes place 

 very largely in the cambium layer (Fig. 54), corresponding to the 

 meristem of the root tip. This cell-division stage is followed by 

 a period of growth in diameter and by changes in cytoplasm and 



Permanent tissue Growing Cell division Growing ^ Permanent tissue 

 and maturing an d maturing 



FIG. 54. The development of sieve tubes, fibers, ducts, and parenchyma from 

 the cambium. Diagrammatic 



Each of the above elements of the phloem and xylem is represented in three stages, 

 namely, as a part of the cambium, as a growing and maturing cell or cell chain, and 

 as a permanent element of either the xylem or the phloem. The cambium is regarded 

 as a cylindrical meristem layer, which, like the apical meristems, produces new 

 cells by division. Consult the text discussion of this figure 



cell walls which may be designated as the growth and maturing 

 stages for both the phloem and the xylem elements. The growth 

 and maturing stages are followed in each tissue cell or cell 

 complex by the permanent tissue elements. 



We may now consider in greater detail the changes which 

 occur during the growth and maturing stages of the living cell 

 elements, the fibrous elements, and the ducts. 



Xylem elements. In the case of the cells which will ultimately 

 form the living storage parenchyma cells very little change takes 

 place during the growth and maturing stage, except a slight 



