CORK OF SAMBUCUS. 



183 



externally by sclerenchyma fibres. The cortex is from ten to 



fifteen cells thick. The projecting ridges of the stem show a 



strong hypodermal sheath of collenchyma, which in the furrows 



is reduced to a layer two or three cells thick. Under the stornata 



the collenchyma sheath is interrupted by the green cortical paren- 



chyma, which here extends to the epidermis. In parts of the 



stem which have begun to appear grey on the surface the forma- 



tion of the cork layer commences, by tangential division of the 



outermost collenchyma cells immediately subtending the epi- 



dermis. The inner of the sister cells thus produced again divides, 



and it is then the middle cell of the three radially-disposed cells, 



which further functions as a cork-cambium cell. It is easy to 



recognise, even after the periderm has become many layers thick 



(Fig. 73, ph). Outermost in each radial row lies the outer, while 



innermost lies the inner 



segment of the original 



collenchyma cell (cl) ; the 



flattened cell (ph), bounding 



the inner segment externally, 



is the cork cambium or 



phellogen cell. At first the 



phellogen gives rise only to 



cork cells on its outer side ; 



soon, however, it begins, 



though comparatively but 



sparely, to cut off cells on 



its inner side also, which con- 



tain chlorophyll grains, and 



take part in the thickening of 



the cortex, which the growth 



in thickness of the stem has 



cstrpfphprl TVii sprrmrtarv 

 Ondary 



cortex developed from the 



. , 

 cork cambium, is known as 



the phellogenic cortex, or phelloderm. The whole of the pro- 

 ducts of the activity of the cork cambium are collected together 

 under the name periderm, 



Lenticels. In fortunate cross-sections we can, moreover, 

 determine that the formation of a continuous cork layer is pre- 

 ceded by a peculiar process, which commences under the stomata. 

 The primary cortical cells which surround the air-chamber com- 



FlG - 73. Cross-section through the surface 

 fayoungsteinofStoifo*cin^ro. Epidermis; 

 Ph, phellogen ; cl and cl, outer and inner parts 

 of the original colleuchyma cell (x 240). 



