76 



Growth 



In many forms growth seems to be centered in a small group of cells. 

 Brumfield (1943) produced specific chromosomal changes in root-tip 

 cells of Crepis and Vicia by X radiation which could be recognized in 

 the descendants of these cells farther back along the root. They were 

 found to form wedge-shaped sectors of the entire root, including parts 

 of the root cap, epidermis, cortex, and vascular cylinder (p. 268). Such a 

 sector usually occupied about a third of the area of the root cross section, 

 and Brumfield concluded that there were about three cells at the tip from 



Fig. 4-16. Longitudinal diagram of root development as observed by Williams. The 

 tissues all arise from a small group of cells at the very tip. 1, epidermis; 2, hypo- 

 dermis; 3, endodermis; 4, pericycle; 5, mitotic figure; 6, young cortical cells; 7, stelar 

 initials; 8, dermatogen; 9, metaxylem initial. B, C, D, transverse diagrams showing 

 origin of cortical cells from a single cell of the endodermis. ( From B. C. Williams. ) 



which all the tissues of the root were derived, though these cells could 

 not be recognized in sections of the root apex. Von Guttenberg ( 1947 ) 

 later presented evidence from a considerable variety of dicotyledonous 

 plants that there is a single apical cell that gives rise to the whole root 

 and which thus is comparable to the apical cell of lower vascular plants, 

 but this has received little confirmation. 



Popham ( 1955a ) found in Pisum sativum a transverse row of meri- 

 stematic initials across the root apex that gives rise to all the tissues of 

 the root and the cap, and Clowes (1954) observed a somewhat similar 



