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.)
[Ill.u.s.tration: Fig. 27.--Transverse section of the stem of Eriochloa polystachya. 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.
[Ill.u.s.tration: Fig. 28.--Transverse section of a portion of the stem of Eriochloa polystachya. 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.)
[Ill.u.s.tration: Fig. 29.--Transverse section of the stem of Setaria glauca. 15]
[Ill.u.s.tration: Fig. 30.--Transverse section of a portion of the stem of Setaria glauca. 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 s.p.a.ce of the ground tissue consisting of only parenchyma. (See figs. 29 and 30.)
[Ill.u.s.tration: Fig. 31.--Transverse section of the stem of Panic.u.m ramosum 24]
[Ill.u.s.tration: Fig. 32.--Transverse section of a portion of the stem of Panic.u.m ramosum. 75
1. Epidermis; 2. sclerenchyma; 3. vascular bundle.]
[Ill.u.s.tration: Fig. 33.--Transverse section of the stem of Andropogon caricosus. 25]
The stem of _Panic.u.m 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.)
[Ill.u.s.tration: Fig. 34.--Transverse section of a portion of the stem of Andropogon caricosus. 75
1. Epidermis; 2. sclerenchyma; 3. vascular bundle.]
[Ill.u.s.tration: Fig. 35.--Transverse section of a portion of the stem of Panic.u.m Isachne. 100
1. Epidermis; 2. sclerenchyma; 3. vascular bundle.]
The stem of the gra.s.s _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.)
[Ill.u.s.tration: Fig. 36.--Transverse section of the stem of Eragrostis interrupta. 25]
[Ill.u.s.tration: Fig. 37.--Transverse section of the stem of Panic.u.m flavidum. 15]
[Ill.u.s.tration: Fig. 38.--Transverse section of a portion of the stem of Panic.u.m flavidum. 70
1. Epidermis; 2. sclerenchyma; 3. vascular bundle.]
The stems of _Panic.u.m 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 s.p.a.ces here and there. Other minor differences may be seen on referring to figs. 35 and 36.
[Ill.u.s.tration: Fig. 39.--Transverse section of a portion of the stem of Panic.u.m colonum. 70
1. Epidermis; 2. sclerenchyma; 3. vascular bundle.]
The stems of gra.s.ses growing in wet or marshy situations differ in structure from those detailed above. As examples the stems of _Panic.u.m flavidum_, _Panic.u.m colonum_, _Panic.u.m Crus-galli_ and _Panic.u.m fluitans_ may be considered. The stem of _Panic.u.m 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 _Panic.u.m colonum_, _Panic.u.m stagninum_ and _Panic.u.m 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 _Panic.u.m Crus-galli_ three or four bundles are met with amidst the stellate cells.
[Ill.u.s.tration: Fig. 40.--Transverse section of a portion of the stem of Panic.u.m Crus-galli. 70
1. Epidermis; 2. sclerenchyma; 3. vascular bundle.]
[Ill.u.s.tration: Fig. 41.--Transverse section of the stem of Panic.u.m stagninum. 10]
The cortex outside the band of sclerenchyma is full of air-cavities, small and large. In _Panic.u.m 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 _Panic.u.m 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 _Panic.u.m fluitans_ and _Panic.u.m Crus-galli_, but in the stems of _Panic.u.m colonum_ and _Panic.u.m flavidum_ the epidermis alone is thickened. In the cortical portion outside the sclerenchymatous band, small vascular bundles occur in the stems of _Panic.u.m colonum_, _Panic.u.m Crus-galli_ and _Panic.u.m fluitans_.
(See figs. 39, 40, 42 and 43.)
[Ill.u.s.tration: Fig. 42.--Transverse section of the stem of Panic.u.m fluitans. 15]
[Ill.u.s.tration: Fig. 43.--Transverse section of a portion of the stem of Panic.u.m fluitans. 60
1. Epidermis; 2. sclerenchyma; 3. vascular bundle; 4. air-cavity; 5.
diaphragm.]
The stem of _Panic.u.m 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 gra.s.ses 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 gra.s.ses 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=.
[Ill.u.s.tration: Fig. 44.--Transverse section of a part of the root of Pennisetum cenchroides. 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 gra.s.ses 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 const.i.tute 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.)
[Ill.u.s.tration: Fig. 45.--Transverse section of the entire root of Andropogon Sorghum. 25]
[Ill.u.s.tration: Fig. 46.--Transverse section of the cortical portion of the root of Andropogon Sorghum. 150
r.h. Root-hair; p.l. piliferous layer; ex. exodermis; co. cortex.]