94 Hill.—The Structure and Development of 
The structure of these organs is illustrated by Figs. 12, 13, 
and 14, PL VII. 
The piliferous layer is well marked, the cells being generally 
oblong in shape, whilst those which have grown out into 
root-hairs are nearly square. This layer may become lignified 
in old roots. The root-hairs, on becoming functionless, do 
not necessarily die off, but may become thickened and persist. 
According to Van Tieghem ( 14 £), the root-hairs are some¬ 
times formed very near the end of the root, just behind 
the calyptra. 
Immediately beneath the piliferous layer is a well-marked 
exodermis (Fig. 12 exo), normally one cell in thickness, but 
here and there forming a double layer. 
Beneath the exodermis are two or three layers of rounded 
thickened cells, with no intercellular spaces ; these, however, 
occur between the innermost row and the subjacent cortical 
layers. 
The cells of the inner cortex are arranged in a very regular 
radial manner, especially in the younger regions of the roots. 
They are rounded in shape, with lozenge-shaped schizogenous 
air-spaces between them ; these cells and their accompanying 
air-spaces gradually grow smaller in passing from the peri¬ 
phery to the central cylinder. 
The endodermis is extremely well marked, having much 
the same appearance as the endodermis of the rhizome, 
the radial and inner walls being very strongly thickened 
(Fig- 13)- 
In the older regions of the roots large lacunae are found 
in the outer parts of the inner cortex; these air-containing 
spaces show some signs of lysigenous development. Van 
Tieghem draws attention to this. 
The vascular cylinder may be pentarch, hexarch, or 
heptarch. Van Tieghem ( 14 #) states that the radiating 
vascular strands do not meet in the centre, being separated 
by parenchymatous cells; the material examined did not 
bear out this statement, for it was found that in every case 
the xylem-strands did meet in the centre. 
