222 



A TEXTBOOK OF BOTANY 



[Ch. V, 3 



description, the walls being eve 

 absence of stomata is perfectly 



Fig. 16.3. — f'ros.s section of :i root of 

 a Beau, Vicia Faha. just loehiiicl the liair 

 zone ; X 80. 



The four strand.s of xj'leni meet in the 

 center, obliteratint; the- pith, wliile the 

 strand.s of phloem stand separately Ik-- 

 tween the arms of the cross thus formed. 

 Between xyleni and phloem can be seen 

 the developing cambium, which i:)resently 

 begins to form riSw xj-lem inside of the 

 phloem, thus originating bundles of the 

 ordinary stem tj'pe. Surrounding tlie 

 fibro-vascular system is the endodenriis, 

 and outside thereof the vwy w ide cortex. 

 (Fibro-vascular system ;ifter L. Kn,\', the 

 remainder drawn from nature.) 



■ry where continuous. This 

 explained by the habits of 

 roots, which have no 

 chlorophyll and need no 

 cutinized epidermis. The 

 oxygen used in the respira- 

 tion of the roots passes in 

 solution directly through 

 these walls, which are 

 uncutinized. 



Cross sections bring out 

 several additional features 

 (Fig. 163). Here can be 

 seen more distinctly the 

 innermost laj^er of the 

 thick cortex, called the 



ENDODERMIS fFig. 16.3), 



the exact morphological 

 equivalent of the starch 

 sheath of stems (page 130). 

 In the roots, however, the 

 w^alls of this layer are 

 partially cutinized, espe- 

 cialljf on the radial parts, 

 for reasons not yet under- 

 stood. Also there appears 

 a notable difference in the 

 arrangement of the young 

 fil:>ro-vascular bundles as 

 compared with the stem. 

 The xylem, recognizable 

 l:)y the very large size 

 of the ducts, and the 

 phloem, distinguished by 

 the smaller angular form 

 of the sieve tubes, do not 



