HISTOLOGY OF THE VEGETATIVE ORGANS.
The shoots and roots of grasses conform in their internal structure to
the monocotyledonous type. In all grasses numerous threads are found
running longitudinally within the stem and some of these pass into the
leaves, at the nodes, and run as nerves in the blades of the leaves.
These threads are the vascular bundles. The rest of the tissue of the
stem and leaves consists of thin-walled parenchymatous cells of
different sorts.
The general structure of these bundles is more or less the same in all
grasses. A vascular bundle consists of only xylem and phloem, without
the cambium, and so no secondary thickening can take place in the stems
of grasses. Such bundles as these are called =closed vascular bundles=
to distinguish them from the dicotyledonous type of vascular bundles
which are called =open vascular bundles= on account of the existence of
the cambium.
[Illustration: Fig. 22.--Transverse section of a vascular bundle. x 250
1. Annular vessel; 2. spiral vessel; 3. pitted vessel; 4. phloem or
sieve tubes; 5. sclerenchyma.]
The component parts and elements of which the vascular bundles in
grasses are composed may be learnt by studying the transverse and
longitudinal sections of these bundles in any grass. The cross and
longitudinal sections of a vascular bundle of the stem of _Pennisetum
cenchroides_, are shown in figs. 22 and 23. In the figure of the
transverse section the two large cavities indicated by the number 3 and
the two small circular cavities with thick walls lying between the
larger ones and indicated by the numbers 1 and 2 are the chief elements
of the xylem.
By looking at the longitudinal section it is obvious that these elements
are really vessels, the larger being pitted and the smaller annular and
spiral vessels. These vessels together with the numerous small
thick-walled cells lying between the pitted vessels constitute the
xylem. Just above the xylem there is a group of large and small
thin-walled cells. This is the phloem and it consists of sieve tubes and
thin-walled cells. All round the xylem and the phloem there are many
thick-walled cells. These are really fibres forming the =bundle-sheath=.
On account of this bundle-sheath the bundles are called =fibro-vascular
bundles=.
[Illustration: Fig. 23.--Longitudinal section of a vascular bundle. x
250
1. Annular vessel; 2. spiral vessel; 3. pitted vessel; 4. sieve tubes or
phloem; 5. sclerenchyma.]
[Illustration: Fig. 24.--Transverse section of a portion of the stem of
Rottboellia exaltata. x 70
1. Epidermis; 2. sclerenchyma; 3. vascular bundle.]
=Structure of the stem.=--The stem of a grass consists of a mass of
parenchymatous cells with a number of fibro-vascular bundles imbedded in
it, and it is covered externally by a protective layer of cells, the
epidermis. The stem is usually solid in all grasses in the young stage,
but as it 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 cenchroides. x 70
1. Epidermis; 2. sclerenchyma; 3. vascular bundle.]
The stem of _Pennisetum cenchroides_ is somewhat round in outline in the
transverse section with a slight curvature in the front. The vascular
bundles are rather numerous and irregularly scattered all over the
ground tissue. The peripheral bundles are not so close to the periphery
of the stem as in _Rottboellia exaltata_. These are separated from the
epidermis by several layers of parenchymatous cells. Further, these
peripheral bundles are all imbedded in a continuous sclerenchymatous
band which runs round the stem in the form of a ring. The epidermal
cells as well as the layer of cells in immediate contact with it are
thick-walled. In the vascular bundles of the ground tissue the
bundle-sheath is rather prominent and the phloem portion is well
developed. (See figs. 25 and 26.)
[Illustration: Fig. 27.--Transverse section of the stem of Eriochloa
polystachya. x 25]
In the stem of _Eriochloa polystachya_, all the vascular bundles are
more or less peripheral in position leaving a wide area of
parenchymatous cells in the centre. The outline of the stem in cross
section is rotund or ovate-rotund with the front side somewhat flattened
and straight. The epidermal cells alone are thickened. A well developed
continuous ring of sclerenchyma is present and this is connected with
the epidermal layer at short intervals by means of short
sclerenchymatous bands. So the parenchymatous cells of the cortex lying
outside the sclerenchymatous ring are divided into small isolated areas.
There are three series of vascular bundles.
[Illustration: Fig. 28.--Transverse section of a portion of the stem of
Eriochloa polystachya. x 70
1. Epidermis; 2. sclerenchyma; 3. vascular bundle.]
One series consists of small bundles lying inside the sclerenchyma ring
at the base of each of the connecting bands. The second series is made
up of large vascular bundles imbedded in the ring so as to bulge out
inside the ring. The vascular bundles of the third series are found just
away from the ring and separated from it by a few layers of
parenchymatous cells. (See figs. 27 and 28.)
[Illustration: Fig. 29.--Transverse section of the stem of Setaria
glauca. x 15]
[Illustration: Fig. 30.--Transverse section of a portion of the stem of
Setaria glauca. x 50
1. Epidermis; 2. sclerenchyma; 3. vascular bundle.]
Another stem in which the vascular bundles are more or less peripheral
in position and enclosing a wide parenchyma is that of _Setaria glauca_.
In the transverse section of the stem the outline is ovate, laterally
compressed, obtusely keeled at the back and somewhat concave in the
front. The sclerenchymatous band is narrow and continuous and very close
to the epidermis, being separated from it only by two or three layers of
thin-walled cells. The epidermal cells alone are thickened. As to the
vascular bundles there are three sets. One set of bundles lying just
outside the sclerenchymatous ring consists of small ones connecting the
ring with the epidermis. Just inside the sclerenchymatous ring lies a
series of bundles which are connected with it. Still inside, at some
distance from the sclerenchymatous band, are seen vascular bundles
forming a row and enclosing a large space of the ground tissue
consisting of only parenchyma. (See figs. 29 and 30.)
[Illustration: Fig. 31.--Transverse section of the stem of Panicum
ramosum x 24]
[Illustration: Fig. 32.--Transverse section of a portion of the stem of
Panicum ramosum. x 75
1. Epidermis; 2. sclerenchyma; 3. vascular bundle.]
[Illustration: Fig. 33.--Transverse section of the stem of Andropogon
caricosus. x 25]
The stem of _Panicum ramosum_ is semi-circular and somewhat flat on one
side. The epidermal cells alone are thickened. There is a broad well
developed continuous band of sclerenchyma, which is connected at regular
intervals with the epidermis by small vascular bundles. Another row of
vascular bundles lies just inside the sclerenchymatous ring and each of
these bundles is in contact with the band. Away from the ring lie a
number of bundles forming a series disposed in two irregular rings
around a broad portion of the ground tissue. (See figs. 31 and 32.)
[Illustration: Fig. 34.--Transverse section of a portion of the stem of
Andropogon caricosus. x 75
1. Epidermis; 2. sclerenchyma; 3. vascular bundle.]
[Illustration: Fig. 35.--Transverse section of a portion of the stem of
Panicum Isachne. x 100
1. Epidermis; 2. sclerenchyma; 3. vascular bundle.]
The stem of the grass _Andropogon caricosus_ is oval in outline, the
front being flat. The epidermal cells and those below and in contact
with them are thick-walled. The sclerenchymatous ring though present is
very narrow and not very conspicuous. It consists of one or two layers
of cells connecting a few vascular bundles forming the outermost set.
There is a series of vascular bundles inside the ring which surrounds a
large area of the ground tissue. Two isolated bundles, one in front and
another at the back of the ground tissue, are found. The cells of the
ground tissue lying just inside the vascular bundles are all very much
thickened. (See figs. 33 and 34.)
[Illustration: Fig. 36.--Transverse section of the stem of Eragrostis
interrupta. x 25]
[Illustration: Fig. 37.--Transverse section of the stem of Panicum
flavidum. x 15]
[Illustration: Fig. 38.--Transverse section of a portion of the stem of
Panicum flavidum. x 70
1. Epidermis; 2. sclerenchyma; 3. vascular bundle.]
The stems of _Panicum Isachne_ and _Eragrostis interrupta_ are hollow.
The stem of the former is circular in outline in cross section, though
wavy. There is a sclerenchymatous ring close to the epidermis but
separated from it by a few layers of parenchyma. One set of bundles is
imbedded in the band, and another set just touches the inner border of
it. A third series is disposed around a fairly large amount of ground
tissue, which may or may not have a cavity in the centre. The stem of
_Eragrostis_ _interrupta_ has more or less the same structure, but the
cortex has air spaces here and there. Other minor differences may be
seen on referring to figs. 35 and 36.
[Illustration: Fig. 39.--Transverse section of a portion of the stem of
Panicum colonum. x 70
1. Epidermis; 2. sclerenchyma; 3. vascular bundle.]
The stems of grasses growing in wet or marshy situations differ in
structure from those detailed above. As examples the stems of _Panicum
flavidum_, _Panicum colonum_, _Panicum Crus-galli_ and _Panicum
fluitans_ may be considered. The stem of _Panicum flavidum_ is broadly
ovate in cross section with a flat front and is more or less solid,
though occasionally the parenchymatous cells in the centre get broken.
Two rows of vascular bundles surround a fairly large amount of
parenchymatous cells of the ground tissue. There is a continuous ring of
sclerenchyma separated from the epidermis by a fairly broad cortex. The
cortex has a number of fairly large air-cavities separated by bands of
parenchymatous cells. Within the sclerenchymatous band lie small
vascular bundles at regular intervals just towards the cortex. A few
isolated bundles are in contact with the inner border. (See figs. 37 and
38.)
The stems of _Panicum colonum_, _Panicum stagninum_ and _Panicum
Crus-galli_ have in their centre in the ground tissue 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 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
periphery there is a single layer of cells called the =pericycle=. The
arrangement of the xylem and the phloem is different from that of the
stem. They lie side by side on different radii, and not one behind the
other on the same radius as in the stem. The number of xylem groups is
fairly large and the development of the xylem is from the pericycle
towards the centre of the stele. (See figs. 44 and 45.) The
parenchymatous cells in the centre of the stele become thick-walled in
older roots.
=Structure of the leaf.=--The structure of the leaf of grasses is quite
characteristic of the family. In every leaf a number of vascular
bundles, some small and others large, pass from the base to the apex.
Externally the leaf is covered on both the sides by the epidermis. The
spaces existing between the vascular bundles and the epidermis are
filled with parenchymatous cells. The larger vascular bundles consist of
xylem and phloem surrounded by a bundle sheath of a single layer of
cells. In the smaller bundles the xylem is very much reduced. Around
every vascular bundle there is a single row of somewhat large cells
densely packed with large chloroplasts, the =chlorophyllous layer=. The
vascular bundles are strengthened by fibres, on both the sides in the
case of larger bundles and on only one side in small bundles.
[Illustration: Fig. 47.--Transverse section of the stele portion of the
root of Andropogon Sorghum. x 150
Co. Cortex; en. endodermis; pe. pericycle; ph. phloem; pr. protoxylem;
xy. xylem vessel.]
[Illustration: Fig. 48.--A portion of the transverse section of the leaf
of Panicum javanicum. x 100
Ep. c. An ordinary epidermal cell; st. stomata; sc. sclerenchyma; ph.
phloem; chl c. chlorenchyma; m. motor cells; xy. xylem.]
For a detailed study of the structure of the leaves of grasses the leaf
of the grass _Panicum javanicum_ may be chosen. In a transverse section
of this leaf, the vascular bundles are very conspicuous. The larger
bundles are normal in every way, while in the smaller ones the xylem
elements are considerably reduced. Around every one of the vascular
bundles there is a single row of large cells containing large
chlorophyll grains (the chlorophyllous layer). In a well developed large
vascular bundle the chlorophyllous layer is open below just close to the
sclerenchymatous band. On both sides of the larger vascular bundle there
are bands of sclerenchyma. In the case of smaller bundles some are
strengthened by sclerenchyma on the lower side and others have none. The
spaces between the bundles are occupied by thin-walled parenchymatous
cells containing small chlorophyll grains.
[Illustration: Fig. 49.--Upper epidermis of the leaf of Panicum
javanicum. x 300]
[Illustration: Fig. 50.--Lower epidermis of the leaf of Panicum
javanicum. x 300]
The lower epidermis of the leaf in the transverse section is even and
consists of small and large round cells. The upper epidermis is slightly
wavy and it is made up of some small round cells alternating with groups
of larger cells. The epidermal cells lying over sclerenchyma and the
smaller vascular bundles are small and round, while those lying over the
furrows between the vascular bundles are large and are called =motor= or
=bulliform cells=. The presence of motor cells is a characteristic
feature of the leaves of many grasses.
The continuity of both the upper and the lower epidermis is interrupted
by the stomata. Air-cavities are seen below these stomata. The
arrangement of the stomata, the shape of the guard cells and the
characteristics of the epidermal cells become clear on examining a piece
of epidermis. (See figs. 49 and 50.)
The structure of the leaf of _Panicum javanicum_ may be taken as typical
of the structure of the leaves of most grasses. The leaves of _Eriochloa
polystachya_, Cynodon and Paspalums are very much like the leaves of
_Panicum javanicum_ in their internal structure.
Considerable amount of variation, however, occurs in the leaves of
grasses especially as regards the arrangement of fibres and motor cells.
[Illustration: Fig. 51.--A portion of the transverse section of the leaf
of Eriochloa polystachya x 120
1. Motor cell; 2. stomata; 3. sclerenchyma; 4. chlorophyllous layer.]
Every large primary vascular bundle in the leaves of many grasses
possesses sclerenchymatous bands both above and below. The other
vascular bundles may have bands of sclerenchyma on both sides or on one
side only or none. For example, in the leaves of _Panicum repens_ both
the primary and secondary bundles are provided with sclerenchyma on both
the sides, while those of the third order may have it on one side or
not. The hyaline margin of this leaf and of the leaves of other grasses
consists entirely of sclerenchyma. (See fig. 53.)
[Illustration: Fig. 52.--Transverse section of a portion of the leaf of
Panicum repens. x 120
1. Motor cells; 2. stomata; 3. sclerenchyma; 4. chlorophyllous layer.]
All the vascular bundles in the leaves of _Aristida setacea_ have broad
sclerenchymatous bands on both the sides. Besides these bands arranged
like a girder above and below each bundle, there are on the lower side
bands of sclerenchyma. So the sclerenchyma becomes almost continuous on
the lower side.
[Illustration: Fig. 53.--Transverse section of the leaf margin of
Panicum repens. x 180
1. Sclerenchyma; 2. stomata; 3. chlorophyllous layer.]
[Illustration: Fig. 54.--Transverse section of a part of the leaf of
Aristida setacea. x 30.
1. Vascular bundle; 2. sclerenchyma; 3. motor cells.]
[Illustration: Fig. 55.--Transverse section of a portion of the leaf of
Aristida setacea. x 120
1. Motor cells; 2. stomata; 3. sclerenchyma; 4. epidermis; 5. cutin
layer.]
The sclerenchyma lying on the lower side of the primary bundles are
contiguous with the bundle, while those above are separated from the
bundle by the chlorophyllous layer. (See fig. 55.) In the case of
secondary and tertiary bundles the sclerenchymatous bands lying on the
lower side are in contact with the chlorophyllous layer, whereas the
upper bands are either in contact with this layer or separated from it
by a few parenchymatous cells.
All the vascular bundles in the leaves of _Eragrostis Willdenoviana_ are
provided with sclerenchyma on both the sides. The lower band of the
primary vascular bundles is continuous with the vascular bundle, the
chlorophyllous layer being open below. The upper bands of the primary
and the lower bands of the secondary vascular bundles just touch the
chlorophyllous layer. In the secondary bundles the sclerenchyma band
above is separated from the chlorophyllous layer by two layers of
parenchyma. In the case of the leaves of _Panicum flavidum_, _P.
colonum_, _P. fluitans_ and _Pennisetum cenchroides_ the sclerenchyma is
separated from the chlorophyllous layer by layers of parenchyma.
[Illustration: Fig. 56.--Transverse section of a part of the leaf of
Eragrostis Willdenoviana. x 30
1. Vascular bundle; 2. sclerenchyma; 3. motor cells.]
[Illustration: Fig. 57.--Transverse section of a portion of the leaf of
Eragrostis Willdenoviana. x 150
1. Motor cells; 2. stomata; 3. sclerenchyma; 4. chlorophyllous layer; 5.
vascular strand cut through; 6. hair.]
[Illustration: Fig. 58.--Transverse section of a part of the leaf of
Panicum colonum. x 30
1. Vascular bundle; 2. sclerenchyma; 3. motor cells.]
Even from the few examples dealt with above, it is obvious that the
range of variation of sclerenchyma in leaves is very great. In the
leaves of _Aristida setacea_ there is a considerable amount of
sclerenchyma whilst in some leaves such as those of _Panicum_ _colonum_,
_P. flavidum_ and _Panicum fluitans_ the sclerenchyma is reduced to its
minimum.
[Illustration: Fig. 59.--Transverse section of a part of the leaf of
Panicum fluitans. x 30
1. Vascular bundle; 2. sclerenchyma.]
In the leaves of grasses growing in dry situations the development of
sclerenchyma is generally very considerable. The grass _Aristida
setacea_ is a good example of a xerophytic grass. The sea-shore grass
_Spinifex squarrosus_ is another example of the same kind. But in the
leaves of this grass, the development of sclerenchyma is not very
considerable, but there is a great development of parenchymatous cells
free from chlorophyll within the leaf, the chlorophyll bearing cells
being confined to the upper and the lower surfaces of the leaves.
[Illustration: Fig. 60.--Transverse section of a leaf of Spinifex
squarrosus. x 10
1. Vascular bundle; 2. sclerenchyma.]
[Illustration: Fig. 61.--A portion of the transverse section of the leaf
of Spinifex squarrosus. x 60
1. Sclerenchyma; 2. vascular bundle; 3. epidermis; 4. stomata.]
The upper and the lower surfaces of the leaves of many grasses are more
or less even, but in the case of a few grasses the upper surface
consists of ridges and furrows, instead of being even. In the leaves of
_Panicum repens_ and _Eragrostis Willdenoviana_ the upper surface is
wavy and consists of shallow furrows and slightly raised ridges. But in
the leaves of _Aristida setacea_ and _Panicum fluitans_ the furrows are
deeper and the ridges are more prominent. In _Aristida setacea_ the
ridges are flat-topped and they are rounded with broad furrows in
_Panicum fluitans_.
[Illustration: Fig. 62.--Upper epidermis of the leaf Panicum Isachne. x
200
1. Cells overlying the veins; 2. cells overlying the furrows; 3.
stomata.]
[Illustration: Fig. 63.--Lower epidermis of the leaf Panicum Isachne. x
200
1. Cells overlying the veins; 2. cells overlying the furrows; 3.
stomata.]
The epidermis covering the leaves consists of elongated cells with plane
or sinuous walls, various kinds of short cells intercalated between the
ends of long cells, motor-cells and stomata. Hairs of different sorts
occur as outgrowths of the epidermis. The roughness of the surface of
the leaves of grasses is due to the presence of very minute short hairs
borne by the epidermis. In most cases these short hairs are found in
regular rows. Although the epidermis is more or less even in the leaves
of several grasses such as _Panicum repens_, _P. flavidum_ and
_Eriochloa polystachya_, it is wavy or undulating in the leaves of a few
grasses. For example, the upper epidermis in the leaves of _Panicum
fluitans_ is undulating as it follows the contour of the ridges and
furrows.
The epidermal cells have even surfaces in the leaves of most grasses but
in some they bulge out. In the leaves of _Panicum flavidum_ the cells of
the lower epidermis are quite even, whilst those of the upper epidermis
bulge out. The cells of both the upper and the lower epidermis are
distinctly bulging out in the leaves of _Panicum colonum_. In _Panicum
fluitans_ the cells of the upper epidermis bulge out so much as to form
distinct papillae.
The free surface of the epidermis is more or less cutinised in the
leaves of all grasses. In some leaves the cuticle is very thick and even
papillate as in the leaves of _Aristida setacea_ and _Panicum repens_
whilst in others it is very thin, as in the leaves of _Panicum colonum_
and _P. fluitans_. Cutinisation is rather prominent in the leaves of
grasses growing under dry conditions and it is less pronounced in
mesophytic grasses.
[Illustration: Fig. 64.--Transverse section of a portion of the leaf of
Panicum colonum. x 120
1. Motor cells; 2. stomata; 3. sclerenchyma; 4. chlorophyllous layer.]
[Illustration: Fig. 65.--Transverse section of a portion of the leaf of
Panicum fluitans. x 120
1. Motor cells; 2. stomata; 3. sclerenchyma; 4. chlorophyllous layer.]
As regards size, the epidermal cells overlying the sclerenchyma are
small and those lying over parenchyma are larger. Amongst the larger
cells some may be motor-cells. The stomata occur in regular rows between
the vascular bundles and they are quite characteristic of grasses. They
are more or less similar in structure in all grasses. In the leaves of
many grasses stomata are found in both the upper and the lower epidermis
and they are confined to the lower epidermis in a few grasses only.
The motor-cells vary very much both as regards their shape and position.
In some leaves as in the leaves of the grass _Panicum flavidum_ the
motor-cells are confined to the midrib on 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.]