matures the internodes become hollow in many grasses and they
remain solid in a few. In the internodes the fibro-vascular bundles run
longitudinally and are parallel, but in the nodes they run in all
directions and form a net work from which emerge a few bundles to enter
the leaves. So far as the broad general features are concerned, the
stems of many grasses are more or less similar in structure. However,
when we take into consideration the arrangement of bundles, the
development and arrangement of sclerenchyma, every species of grass has
its own special characteristics. And these are so striking and constant
that it may be possible to identify the species from these characters
alone.

We may take as a type the stem of _Rottboellia exaltata_. This stem is
somewhat semi-circular in transverse section and it is almost straight
and flat in the front (the side towards the axillary bud). The
peripheral portion of the stem becomes somewhat rigid and thick due to
the aggregation of vascular bundles, some small and others large. The
outermost series of bundles consisting of small and larger bundles are
in contact with the layers of the cells lying just beneath the epidermis
and these cells are also thick-walled. A few are away from these being
separated by three or four layers of cells from the peripheral bundles.
In all these vascular bundles the bundle-sheath is very strongly
developed all round and is very much developed especially at the sides.
It is this great development of sclerenchyma that makes the outer
portion of the cortex hard. Within the ground tissue are found a number
of vascular bundles scattered more or less uniformly. These bundles have
no continuous bundle-sheaths but have instead groups of fibres at the
sides and in front of the phloem. The cavities near the annular vessels
are somewhat larger and conspicuous in these bundles.

[Illustration: Fig. 25.--Transverse section of the stem of Pennisetum
cenchroides. x 20]

The epidermal cells are all thickened very much and the outer layer is
cutinized and impregnated with silica. This is the case in the epidermis
of the stems and leaves of most grasses. (See fig. 24.)

In order to give a general idea of the variations in the structure of
the stem in grasses a few examples are chosen and the details of the
structure of the stems of these grasses are dealt with here.

[Illustration: Fig. 26.--Transverse section of a portion of the stem of
Pennisetum cenchroi