162 BOTANY 



of them in the cortex and bast, may also be developed as scleren- 

 chymatous cells during the process of dilatation. 



The epidermis may continue to expand for years in some species of Rose, 

 Acacia, Holly and Maple, and in the Mistletoe. The outer walls of the cells are 

 usually strongly thickened, and when ruptured on the surface become reinforced by 

 new layers of thickening deposited within. 



2. Periderm. As a rule, however, the epidermis does not take 

 part in the dilatation but is passively stretched and ultimately 

 ruptured. A new limiting tissue is thus required to protect the 

 underlying tissues from drying up. This arises as the CORK by the 

 activity of a special secondary meristem, situated at the periphery 

 of the organ (Fig. 18,5). 



This CORK-CAMBIUM or PHELLOGEN is usually formed in the first 

 season, soon after, or even before, the commencement of secondary 

 growth. It may arise from the epidermis by tangential division of 

 its cells. More usually, however, it is formed from the layer of 

 cortex just below the epidermis, less commonly from a deeper layer 

 of the cortex or from the pericycle. The last case is the rule for 

 roots (Fig. 171 B, k). The meristem and all the products of its 

 activity are known collectively as the PERIDERM. The cells cut off 

 to the outer side become CORK-CELLS ; those developed to the inner 

 side become unsuberised cells with abundant chlorophyll, which round 

 off and are added to the cortex. With the formation of the periderm 

 the surface of the stem appears brown. 



The cells formed on the inner side by the phellogen are termed collectively the 



I'HELLODEKM. 



The cork-cambium is as a rule a typical initial cambium (cf. p. 46), at least 

 when it forms both cork and phelloderm. An initial layer may, however, be wanting, 

 e.g. in many Monocotyledons ; in this case the permanent cells from which the 

 cork cambium proceeds divide into a number of cells which become cork-cells, and 

 the process is repeated in adjacent cells of the permanent tissue. 



Periderm formation takes place at a later period in those plants in which the 

 epidermis continues to expand for years ; it is wanting only in the species of 

 Mistletoe. 



True cork is wanting in Cryptogams, even in the Pteridophy tes. When protec- 

 tion is required its place may be taken by the impregnation of the cell walls with 

 a very resistant brown substance or by the addition of suberised lamellae to the 

 walls, that is the transformation of certain layers of cells into a cutis tissue ( 82 ). 



As the result of the activity of the cork -cambium thick fissured 

 incrustations of cork may arise as in the Cork Oak from which bottle 

 cork is obtained. The stratification which this exhibits marks the 

 annual increments. In other cases a corky layer with a smooth outer 

 surface only a few layers of cells thick is formed (Figs. 59, 185). 

 This may allow of the secondary growth in thickness of the stem 

 continuing for a long time before it ultimately becomes torn and 

 is shed. 



