40 THE WHEAT PLANT 



Lining the hair is a delicate peripheral layer of cytoplasm which 

 increases somewhat in thickness near the tip. The nucleus of the cell 

 from which the root-hair arises frequently migrates into the hair and 

 travels along the lumen towards the apex, at the same time becoming 

 elongated up to 40-50 p, (Fig. 28) : ultimately it becomes disorganised 

 when the root-hair shrivels and dies. 



The zone of cortex which measures about 200 p. across consists usually 

 of 6-8 layers of parenchymatous cells with small intercellular spaces, the 

 individual cells being from 20 to 40 p. in diameter. 



The endodermis is a single continuous layer of closely fitting cells ; 

 its outer walls are thin, but the inner walls in older roots are strongly 

 cuticularised and exhibit lines of stratification ; the radial walls are 



thickened also to some extent, especi- 

 ally where they join the inner surface 

 of the cells (e, Fig. 29). 



The pericycle is a single layer. 

 Its component cells, whose radial 

 diameters are somewhat larger than 

 their breadths, are of fairly uniform 

 dimensions except at the points 

 opposite the protoxylem, where they 

 are very small. After a time their 

 walls become more or less sclerotised 

 and lignified. 



The xylem strands are usually 7 

 or 8 in number in the seminal roots. 



ric. 28. Root-hairs ( x 260). n, Nucleus. , 



One or two very narrow vessels, 5- 



10 p, in diameter, with closely arranged annular or spiral thickening, con- 

 stitute the protoxylem of each bundle ; the centripetal vessels formed 

 later are pitted and wider, being 15-20 p. in diameter. In most primary 

 roots and some secondary ones the centre of the vascular cylinder is 

 occupied by a single large pitted vessel about 50 p, in diameter ; in other 

 roots there is no central vessel, but two or more large vessels are found 

 distributed irregularly in the conjunctive tissue. 



The phloem bundles are arranged alternately with the xylem strands ; 

 each generally consists of three cells (b, Figs. 26, 27). 



The conjunctive parenchyma of the cylinder has moderately thick 

 cell walls. 



The piliferous layer soon ceases its absorptive function ; the layer of 

 cortical cells immediately within it then becomes transformed into the 

 exodermis by suberisation of its walls, and acts for a time as a protective 

 covering to the root. 



Later, much of the cortical tissue shrivels, turns brown, and dies, but 



