A Handbook of Some South Indian Grasses. Rangachari K.
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.
Fig. 60.—Transverse section of a leaf of Spinifex squarrosus. × 10
1. Vascular bundle; 2. sclerenchyma.
Fig. 61.—A portion of the transverse section of the leaf of Spinifex squarrosus. × 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.
Fig. 62.—Upper epidermis of the leaf Panicum Isachne. × 200
1. Cells overlying the veins; 2. cells overlying the furrows; 3. stomata.
Fig. 63.—Lower epidermis of the leaf Panicum Isachne. × 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 papillæ.
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.
Fig. 64.—Transverse section of a portion of the leaf of Panicum colonum. × 120
1. Motor cells; 2. stomata; 3. sclerenchyma; 4. chlorophyllous layer.
Fig. 65.—Transverse section of a portion of the leaf of Panicum fluitans. × 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.)
Fig. 66.—Transverse section of a leaf of Panicum flavidum. × 20
1. Vascular bundle; 2. sclerenchyma; 3. motor-cells.
Fig. 67.—Transverse section of the leaf of Panicum flavidum. × 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.
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