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Ch. IV, 3] AXATO^IY OF STE^IS 133 



crystal?, which have 

 significance ah-eady 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 

 fiat, continuous, broT^Tiish ^^ 



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 r 



cork which cut off the areas 



of bark (Fig. SO). The ^ - °" Sjf 



wood shows clearly the ,r- - *-*?;-S 



various cells of the xylem 

 and medullary rays Fig. 

 87), as like'n-ise 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. S". — 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 (Ughter) with sieve and paren- 

 chjina elements : cambium ; sj'lem. showing three annual rings, with large 

 ducts, wood cells, and (on the sides') medullan.' rays : and pith. The ^iew 

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

 a wall-chart by L. Kny.) 





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