A Handbook of Some South Indian Grasses. Rangachari K.
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.
Fig. 39.—Transverse section of a portion of the stem of Panicum colonum. × 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.
Fig. 40.—Transverse section of a portion of the stem of Panicum Crus-galli. × 70
1. Epidermis; 2. sclerenchyma; 3. vascular bundle.
Fig. 41.—Transverse section of the stem of Panicum stagninum. × 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.)
Fig. 42.—Transverse section of the stem of Panicum fluitans. × 15
Fig. 43.—Transverse section of a portion of the stem of Panicum fluitans. × 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.
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. phloëm.
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.)
Fig. 45.—Transverse section of the entire root of Andropogon Sorghum. × 25
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.
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 phloëm 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 phloëm 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,