A Handbook of Some South Indian Grasses. Rangachari K.
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 phloëm. The cavities near the annular vessels are somewhat larger and conspicuous in these bundles.
Fig. 25.—Transverse section of the stem of Pennisetum cenchroides. × 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.
Fig. 26.—Transverse section of a portion of the stem of Pennisetum cenchroides. × 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 phloëm portion is well developed. (See figs. 25 and 26.)
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.
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.)
Fig. 29.—Transverse section of the stem of Setaria glauca. × 15
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 space of the ground tissue consisting of only parenchyma. (See figs. 29 and 30.)
Fig. 31.—Transverse section of the stem of Panicum ramosum × 24
Fig. 32.—Transverse section of a portion of the stem of Panicum ramosum. × 75
1. Epidermis; 2. sclerenchyma; 3. vascular bundle.
Fig. 33.—Transverse section of the stem of Andropogon caricosus. × 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.)
Fig. 34.—Transverse section of a portion of the stem of Andropogon caricosus. × 75
1. Epidermis; 2. sclerenchyma; 3. vascular bundle.
Fig. 35.—Transverse section of a portion of the stem of Panicum Isachne. × 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.)
Fig. 36.—Transverse section of the stem of Eragrostis interrupta. × 25
Fig. 37.—Transverse section of the stem of Panicum flavidum. × 15
Fig. 38.—Transverse section of a portion of the stem of Panicum flavidum. × 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