THE ROOTS 



39 



ANATOMY OF THE ROOTS 



In a transverse section through a young seminal root about a centi- 

 metre from its apex the following tissues are readily recognised: (i) an 



outer piliferous layer, (2) 



a broad zone of cortex 



limited internally by a 



well-defined endodermis, 



and (3) a polyarch cen- 

 tral vascular cylinder with 



its surrounding pericycle 



(Kg* 26, 27). 



The piliferous layer 



consists of thin - walled 



elongated cells usually 



from 170 to 350/1 long and 



25-30 p. in diameter. 



From many of these are 



developed root-hairs 



which are produced by the 



bulging out and growth 



of a small portion of the 



cell wall, the point of 



origin in each case being usually between the middle and the transverse 



septum nearest the root- 

 tip. These hairs first ap- 

 pear as very short papillae 

 about i 5 to 2 millimetres 

 behind the apex of the 

 root : further back, where 

 fully developed, they are 

 p' seen to be long tubular 

 structures closed at the 

 apex, each from 8 to 12 /t 

 in diameter, and reaching 

 a length of 1-1 > mm. in 

 a damp atmosphere. The 

 wall of the root-hair is 



III.. 27. VaM.ularcxlmder .,! a heptarch seminal root Composed of Cellulose 

 (X2IO). />'. I'rotoxylcm ; other letters as in the pro which allows the absorp- 

 vious figure. 



tion ol soil solutions to 



proceed freely by osmosis ; the cell wall becomes more or less mucila- 

 ginous and particles of soil adhere to it. 



FIG. 26. Tniiuvcrvr wction of a eminal root ( * too). 

 a. Piliferous layer ; r . cortex ; r, endodernm ; />, pen- 

 cycle ; f> unall cell* of pericycle oppo*tte protoxyiem ; 

 /, protoxyiem ; ft. phloem ; \, Urge central ve*el. 



