ELM. STEM. 71 



Treat a thin section with concentrated sulphuric 

 acid : the walls of all the tissues will swell, and gradu- 

 ally lose their sharpness of outline, with exception of 

 the cuticularised outer wall of the epidermis, and the 

 cork. 



N.B. The cork is sometimes developed to an extraordinary ex- 

 tent on the twigs of the Elm, so that it appears externally as 

 thick radial plates of tissue. 



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 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 (cf. cambium of vascular bundles). 



v. The cells of the cork-cambium occasionally divide radially. 



As stems grow older, layers of cork appear successively further 

 and further from the external surface : not only the cortex but 

 also the outer and older portions of the phloem are thus cut off 

 from physiological connection with the inner tissue ; the term 

 Bark is applied to tissues thus cut off, together with the cork 

 which forms the physiological boundary. As a good example of 

 such successive layers of cork may be mentioned the stem of 

 Vitis. 



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, there will usually be seen a 



