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 



(Figs 26, 27). 



The piliferous layer 



consists of thin - walled 



elongated cells usually 



from 170 to 350 n long and 



25-30 [i 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 

 seen to be long tubular 

 structures closed at the 

 apex, each from 8 to 12 n 

 in diameter, and reaching 

 a length of 1-1-5 mm - m 

 a damp atmosphere. The 

 wall of the root-hair is 



FIG. 27. Vascular cylinder of a heptarch seminal root Composed of Cellulose 

 (xaio). p x , Protoxylem ; other letters as in the pre- which allows the absorp- 

 vious figure. . - .. . . 



tion or soil solutions to 



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

 ginous and particles of soil adhere to it. 



6 



FIG. 26. Transverse section of a seminal root ( x 100). 

 a, Piliferous layer ; c, cortex ; e, endodermis ; p, peri- 

 cycle ; p' small cells of pericycle opposite protoxylem ; 

 /, protoxylem ; b, phloem ; x, large central vessel. 



