across the air-cavities. Later as the stem matures these
disappear and the cavities become conspicuous. (See figs. 42 and 43.)

=Structure of the root.=--As already stated, the roots of grasses
conform to the monocotyledonous type, but the variations met with in
their structure are not so great as in the case of the stem. The
root-tips are protected by root-caps, and the actual tip of the root is
very distinct in the roots of all grasses and it can be seen very
clearly in a longitudinal section of the root. The actual tip of the
root is sharply distinct from the root-cap as there are two distinct
sets of cells, one giving rise to the root-tip and the other to the
root-cap.

The young root-tips are always free from root-hairs, and they are
confined to the portions behind the root-tips. The extent of the
root-hair region will vary according to the vigour and development of
the roots and the nature of the soil. The root-hairs are mere
protrusions of the cells of the outermost layer of the cortex of the
root and this layer is called the =piliferous layer=.

[Illustration: Fig. 44.--Transverse section of a part of the root of
Pennisetum cenchroides. x 100

1. Root-hair; 2. piliferous layer; 3. cortex; 4. endodermis; 5.
pericycle; 6. xylem; 7. phloem.]

To learn the structure of the roots of grasses we may select as types
the roots of _Pennisetum cenchroides_ and _Andropogon Sorghum_ and
consider their structural details. In the transverse sections of these
roots we find a fairly broad cortex consisting of thin-walled
parenchymatous cells more or less regularly arranged. (See figs. 44 and
45.) Just below the piliferous layer two or three layers of thick-walled
cells are seen. In the roots of _Andropogon Sorghum_ these thick-walled
cells are very conspicuous as they consist of several layers. These
layers of thick-walled cells constitute the =exodermis=. (See fig. 46.)
The innermost layer of cells of the cortex is called the =endodermis=
and it becomes conspicuous on account of the thickening in the lateral
and inner walls of the cells of this layer. (See figs. 44 and 47.)

[Illustration: Fig. 45.--Transverse section of the entire root of
Andropogon Sorghum. x 25]

[Illustration: Fig. 46.--Transverse section of the cortical portion of
the root of Andropogon Sorghum. x 150

r.h. Root-hair; p.l. piliferous layer; ex. exodermis; co. cortex.]

The rest of the root forming the central core is the stele and at its
periph