92 



PKACTICAL BOTANY 



similar result may be obtained on treatment with strong 

 chromic acid. 



By comparing sections of twigs of various ages, 

 starting from such as have just escaped from the bud, 

 the following facts may be established 



i. The cork-cambium appears in the layer of cortical 

 cells immediately below the epidermis. 



ii. These cells divide parallel to the external surface 

 of the stem. 



iii. The result of successive divisions in this direction 



FIG. 7. A, B. Diagrams illustrating the formation of periclerm in the layer of cells 

 (2) directly below the epidermis (1). A shows the first periclinal division of 

 the hypodermal layer (2). B shows as the result of repeated periclinal divisions 

 a radial row of cells, of which the outer portion (a) is cork ; the inmost portion 

 (c) is the phelloderm : these are separated by a single cell (b), which represents 

 the cork cambium. 



is the formation of secondary tissues, which develop 

 externally as cork, internally as phelloderm. 



iv. The true cork-cambium consists of only a single 

 cell in each radial row, from which, by successive divi- 

 sion, all these secondary tissues are derived : compare 

 cambium of vascular bundles (see below p. 106, &c.). 



v. The cells of the cork-cambium occasionally divide 

 radially. 



The diagram (Fig. 7) will help to make this plain 



