222 



A TEXTBOOK OF BOTANY 



[Ch. V, 3 



description, the walls being everywhere continuous. This 

 absence of stomata is perfectly 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 layer of the 

 thick cortex, called the 



ENDODERMIS (Fig. 163), 



the exact morphological 

 equivalent of the starch 

 sheath of stems (page 130). 

 In the roots, however, the 

 walls of this layer are 

 partially cutinized, espe- 

 cially on the radial parts, 

 for reasons not yet under- 

 stood. Also there appears 

 a notable difference in the 



Fig. 163. — Cross section of a root of 

 a Bean, Vicia Faba, just behind the hair 

 zone ; X 80. 



The four strands of xylem meet in the arrangement of the yOUllg 

 center, obliterating the pith, while the 

 strands of phloem stand separately be- 

 tween the arms of the cross thus formed. 

 Between xylem and phloem can be seen 

 the developing cambium, which presently 

 begins to form new xylem inside of the 

 phloem, thus originating bundles of the 

 ordinary stem type. Surrounding the 

 fibro-vascular system is the endodermis, 

 and outside thereof the very wide cortex. 

 (Fibro-vascular system after L. Kny, the 

 remainder drawn from nature.) 



fibro-vascular bundles as 

 compared with the stem. 

 The xylem, recognizable 

 by the very large size 

 of the ducts, and the 

 phloem, distinguished by 

 the smaller angular form 

 of the sieve tubes, do not 



