n the upper surface.

The epidermal cells of this leaf are large and uniformly round. (See
figs. 66 and 67.)

[Illustration: Fig. 66.--Transverse section of a leaf of Panicum
flavidum. x 20

1. Vascular bundle; 2. sclerenchyma; 3. motor-cells.]

[Illustration: Fig. 67.--Transverse section of the leaf of Panicum
flavidum. x 150

1. Motor-cells; 2. stomata; 3. sclerenchyma; 4. chlorophyllous layer.]

In the case of most grasses the motor-cells are found in groups of
three, four or five between the vascular bundles. The central motor-cell
is usually the largest and it is somewhat obovate in shape in a
transverse section of the leaf. In the leaves of _Panicum javanicum_ and
_Eriochloa polystachya_ there are three or four motor cells in the group
and the group consists of four, five or rarely six motor cells in the
leaves of _Eragrostis Willdenoviana_. When there are distinct furrows
between ridges these cells lie in the furrows and they are many in
number. In the leaves of _Panicum repens_ there are five to seven
motor-cells in the furrows and the single row of cells stretched between
the motor-cells and the lower epidermis in the furrow consists of more
or less clear cells with sparsely scattered small chlorophyll grains.
(See fig. 52.) The motor-cells occupying the furrows in the leaves of
_Aristida setacea_ are more in number than in _Panicum repens_ and are
of a different shape. All the cells lying in the furrow between the
motor-cells and the sclerenchyma are clear cells free from chlorophyll
grains.

Although the motor-cells differ in shape from the ordinary epidermal
cells in most grasses, there are, however, a few grasses in which the
motor-cells do not differ very much from the epidermal cells except in
size. For example, in the leaves of _Panicum colonum_ the motor-cells
are just like the ordinary epidermal cells in shape but are larger. (See
fig. 64.)

Motor-cells are usually confined to the upper epidermis, but they may
also be found in the lower epidermis. In the leaves of _Pennisetum
cenchroides_ motor-cells are found in both the upper and the lower
epidermis, the group in the upper epidermis alternating with that in the
lower.

[Illustration: Fig. 68.--Transverse section of a portion of the leaf of
Pennisetum cenchroides. x 100

1. Motor-cells; 2. stomata; 3. sclerenchyma; 4. chlorophyllous layer.]

CHAPTER V.

CLASSIFICATION.

The family Gramineae is usually divided into two series