CH. IV, 3] 



ANATOMY OF STEMS 



133 



crystals, which have the 

 significance already ex- 

 plained for those of the 

 leaf (page 111). 



Woody stems exhibit 

 their cell structure very 

 clearly in sections (Fig. 87). 

 In the bark can be seen the 

 flat, continuous, brownish 

 cells of the cork, made by 

 a special cork-cambium just 

 beneath them. The first 

 cork is usually formed just 

 beneath the epidermis, 

 which it replaces ; but later 

 the cork-cambium forms 

 anew each year at some 

 distance from the surface, 

 thus building the layers of 

 cork which cut off the areas 

 of bark (Fig. 80). The 

 wood shows clearly the 

 various cells of the xylem 

 and medullary rays (Fig. 

 87), as likewise the cellular 

 construction of the annual 

 rings, with the contrast 

 between the loose open cells 

 of the spring wood and the 

 compact growth of the pre- 

 ceding autumn. 



FIG. 87. A segment, in cross section, 



of a stem of Linden. From without inward are epidermis (here unusu- 

 ally persistent) ; cork ; cortex (the starch sheath not shown) ; phloem, con- 

 sisting of alternating layers of bast fibers (lighter) with sieve and paren- 

 chyma elements ; cambium ; xylem, showing three annual rings, with large 

 ducts, wood cells, and (on the sides) medullary rays ; and pith. The view 

 shows one complete fibro-vascular bundle, three years old. (Drawn from 

 a wall-chart by L. Kny.) 



