ue stellate cells
with air-cavities. This part is surrounded by a fairly broad portion of
parenchymatous cells in which are imbedded two rows of bundles. Outside
these bundles runs round the stem a narrow sclerenchymatous band with a
few bundles in it of which some touch it inside and others outside. Two
bundles are found by themselves in the tissue of stellate cells. In
_Panicum Crus-galli_ three or four bundles are met with amidst the
stellate cells.

[Illustration: Fig. 40.--Transverse section of a portion of the stem of
Panicum Crus-galli. x 70

1. Epidermis; 2. sclerenchyma; 3. vascular bundle.]

[Illustration: Fig. 41.--Transverse section of the stem of Panicum
stagninum. x 10]

The cortex outside the band of sclerenchyma is full of air-cavities,
small and large. In _Panicum colonum_ the outline of the stem is
ellipsoidal with the front quite flat, and the cortex is narrow at the
sides and very broad in front and at the back. The sclerenchymatous ring
is circular in outline. The stem of _Panicum Crus-galli_ is broadly
ovoid and the cortex is uniformly broad. The epidermal cells as well as
the lower cells are thickened in the stems of _Panicum fluitans_ and
_Panicum Crus-galli_, but in the stems of _Panicum colonum_ and _Panicum
flavidum_ the epidermis alone is thickened. In the cortical portion
outside the sclerenchymatous band, small vascular bundles occur in the
stems of _Panicum colonum_, _Panicum Crus-galli_ and _Panicum fluitans_.
(See figs. 39, 40, 42 and 43.)

[Illustration: Fig. 42.--Transverse section of the stem of Panicum
fluitans. x 15]

[Illustration: Fig. 43.--Transverse section of a portion of the stem of
Panicum fluitans. x 60

1. Epidermis; 2. sclerenchyma; 3. vascular bundle; 4. air-cavity; 5.
diaphragm.]

The stem of _Panicum fluitans_ is round in outline in the transverse
section and has a large cavity. Just close to the cavity and separated
from it by only one or two parenchymatous cells are found vascular
bundles forming a series. Outside this series of bundles lies a
sclerenchymatous band which is wavy, following the lower edges of the
large air-cavities. One series of bundles is connected with this
sclerenchymatous ring. The air-cavities are large and uniform and are
separated by bands of parenchymatous cells. In each of these bands lies
a vascular bundle on the upper side near the periphery. Sometimes we
find, especially in young stages, diaphragms of stellulate cells
stretched