Fig. 50.—Lower epidermis of the leaf of Panicum javanicum. × 300
The lower epidermis of the leaf in the transverse section is even and consists of small and large round cells. The upper epidermis is slightly wavy and it is made up of some small round cells alternating with groups of larger cells. The epidermal cells lying over sclerenchyma and the smaller vascular bundles are small and round, while those lying over the furrows between the vascular bundles are large and are called motor or bulliform cells. The presence of motor cells is a characteristic feature of the leaves of many grasses.
The continuity of both the upper and the lower epidermis is interrupted by the stomata. Air-cavities are seen below these stomata. The arrangement of the stomata, the shape of the guard cells and the characteristics of the epidermal cells become clear on examining a piece of epidermis. (See figs. 49 and 50.)
The structure of the leaf of Panicum javanicum may be taken as typical of the structure of the leaves of most grasses. The leaves of Eriochloa polystachya, Cynodon and Paspalums are very much like the leaves of Panicum javanicum in their internal structure.
Considerable amount of variation, however, occurs in the leaves of grasses especially as regards the arrangement of fibres and motor cells.
Fig. 51.—A portion of the transverse section of the leaf of Eriochloa polystachya × 120
1. Motor cell; 2. stomata; 3. sclerenchyma; 4. chlorophyllous layer.
Every large primary vascular bundle in the leaves of many grasses possesses sclerenchymatous bands both above and below. The other vascular bundles may have bands of sclerenchyma on both sides or on one side only or none. For example, in the leaves of Panicum repens both the primary and secondary bundles are provided with sclerenchyma on both the sides, while those of the third order may have it on one side or not. The hyaline margin of this leaf and of the leaves of other grasses consists entirely of sclerenchyma. (See fig. 53.)