CHAPTER XI 



Cork-formation, etc. 



One result of secondary thickening is a marked enlargement of 

 the periphery of stem or root, in consequence of which the outer 

 tissues are subjected to increasing tension (cf. p. 120). These, 

 the epidermis and cortex, ordinarily have but a limited power 

 of stretching, and, as soon as this limit is reached, they rupture 

 and no longer form an effective covering for the underlying tissues. 

 This function is henceforth fulfilled by a secondary protective 

 tissue, the cork, formed by the active division of another cambium, 

 the phellogen or cork-cambium, which arises somewhere in the 

 cortex. In a few cases [e.g. in some cultivated Maples, viz. Acer 

 striatum) the epidermal and cortical cells are capable of limited 

 growth and division, and here the formation of a cork-cambium 

 is correspondingh' delayed. 



Cork-formation takes place in essentially the same way in both 

 stem and root. The phellogen invariably arises hy the develop- 

 ment of two successive tangential walls, in the case of the stem, 

 most commonly in the cells of the subepidermal layer (Fig. 65). 

 The cells are thus each divided into three segments, of which the 

 central constitute the actual cork-cambium [c.c), whilst the outer 

 form the first layer of cork and the inner the first, and often the 

 onl}', layer of a tissue known as plicUodcrm [Ph.] . WTiilst the outer 

 and inner segments undergo no further division, the cells of the 

 phellogen divide again and again, one of the two products of 

 each division becoming differentiated as cork or phelloderm, 

 while the other remains as the cell of the cork-cambium. In 

 the majoritv of cases, however, these divisions of the phellogen 

 lead to the cutting-off of cells on the outside only, so that no 

 further formation of phelloderm takes place. 



The activity of the cambium results in the development of 



i.3p 



