54 PRACTICAL BOTANY 



b. At least one cell with very small radial diameter, and with 

 protoplasmic contents, and thin cellulose walls : this is the 

 cork-cambium, or phellogen. 



c. Cells with thick cellulose walls, and protoplasmic contents 

 with chlorophyll : no intercellular spaces : this is the phello- 

 derm, which is also derived from the cork-cambium. 



Treat a thin section with concentrated sulphuric acid : the 

 walls of all the tissues will swell, and gradually lose their 

 sharpness of outline, with the exception of the cuticularized 

 outer wall of the epidermis, and the cork, both of which resist 

 the action of the acid. A 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 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 division, all these 

 secondary tissues are derived : compare cambium of vascular 

 bundles (see below, p. 63-65, c.). 



v. The cells of the cork-cambium occasionally divide 

 radially. 



The diagram (Fig. 5) will help to make this plain. 



Examine points where a lenticel has been cut through, or 

 make median sections through a lenticel. 



Note that here the cork layer widens out laterally so as to 

 form a hemispherical mass (semicircular in section), which is 

 covered by the extended epidermis ; if the section be median, 

 a stoma will usually be seen at the apex of the lenticel : the 

 whole mass of tissue consists of cells of a corky nature, with 

 intercellular spaces. 



3. The cortical tissue is a broad band consisting of 



