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.
Fig. 68.—Transverse section of a portion of the leaf of Pennisetum cenchroides. × 100
1. Motor-cells; 2. stomata; 3. sclerenchyma; 4. chlorophyllous layer.