STEMS, ROOTS, AND LEAVES 



91 



The epidermis, in most plant stems, consists of a single layer 

 of uniform cells, which serve to protect the young stem until 

 the cork layers appear. As soon as the cork is formed the epi- 

 dermis is sloughed off and the cork functions in its place. When 

 present the epidermal cells may be seen to be of uniform size 



FIG. 61. Microphotographs of sections of the stem and wood of the 

 alder (Alnus mollis) 



a, transverse section o'f a stem of Alnus three years old, showing spring and summer 

 wood, pith, and bark (microphotograph by Dr. E. C.Jeffrey) ; 6, transverse section 

 of the wood of Alnus ; c, tangential section ; d, radial .section. Note the large light 

 water ducts in all sections and the difference in the appearance of the wood rays in 

 the tangential and radial sections 



and structure, with their cell walls greatly thickened on the 

 outer exposed surface. This outer cell wall is usually waxy 

 and impervious to water. 



* The cork layer is composed of thin-walled cells which have 

 the properties of ordinary bottle cork. In young stems they 

 form a thin layer of brown bark, which increases in thickness 

 as the stem matures. 



