876 



A TEXTBOOK OF THEORETICAL BOTANY 



phellogen is a true secondary cambium formed by the rejuvenation of tissues 

 already fully differentiated, which may be situated anywhere between the 

 secondary phloem and the epidermis itself, although in most cases the first 

 phellogen lies in the cortex near to the outer surface (Fig. 863). It is a single- 

 layered meristem, and by tangential divisions it cuts oft" a succession of cells 

 outwards, which are arranged in radial rows, corresponding to the cells of the 

 phellogen from which they have arisen. There are a few examples of 

 intraxylary phellogens, e.g., Artemisia, where cork formation begins in the 

 wood of the current year, forming a sleeve over the wood and ending upwards 



■>i^\ 



iA' 



«^. 



■7-5' 







^,^,.»,,.« .»•••».,•? ;.*. .^^ 



***^»^* , 



Cortex 



Phellem 

 Phellogen 

 Per icy de 

 Phloem 



Xylem 



pith 



Fig. 863. — Ribes rubrutii. Transverse section of a woody stem 

 in its second year showing phellogen formed in contact with 

 the pericycle. 



in a dome in the pith near the stem apex, thus isolating all new growth from 

 the old. This may be related to the desert conditions under which the plants 

 concerned chiefly live. 



Priestley has shown an association between phellogen formation and the 

 existence of some anatomical barrier to outward sap diffusion. When a 

 functional endodermis is present this acts as such a barrier and the phellogen 

 commences in the pericycle. In the absence of such an endodermis there 

 may be no corresponding barrier until the hypodermis or the epidermis is 

 reached, and it is there that phellogen appears. Intermediate cases may be 

 accounted for by the presence of an internal cuticle at some level in the cortex 

 or by the existence of an impermeable sclerotic layer. According to Priestley's 

 view the accumulation of sap on the inner side of a diffusion barrier provides 

 the physiological stimulus to increased cell division. As the cell sap always 



