A Handbook of Some South Indian Grasses. Rangachari K.
The vascular bundles are strengthened by fibres, on both the sides in the case of larger bundles and on only one side in small bundles.
Fig. 47.—Transverse section of the stele portion of the root of Andropogon Sorghum. × 150
Co. Cortex; en. endodermis; pe. pericycle; ph. phloëm; pr. protoxylem; xy. xylem vessel.
Fig. 48.—A portion of the transverse section of the leaf of Panicum javanicum. × 100
Ep. c. An ordinary epidermal cell; st. stomata; sc. sclerenchyma; ph. phloëm; chl c. chlorenchyma; m. motor cells; xy. xylem.
For a detailed study of the structure of the leaves of grasses the leaf of the grass Panicum javanicum may be chosen. In a transverse section of this leaf, the vascular bundles are very conspicuous. The larger bundles are normal in every way, while in the smaller ones the xylem elements are considerably reduced. Around every one of the vascular bundles there is a single row of large cells containing large chlorophyll grains (the chlorophyllous layer). In a well developed large vascular bundle the chlorophyllous layer is open below just close to the sclerenchymatous band. On both sides of the larger vascular bundle there are bands of sclerenchyma. In the case of smaller bundles some are strengthened by sclerenchyma on the lower side and others have none. The spaces between the bundles are occupied by thin-walled parenchymatous cells containing small chlorophyll grains.
Fig. 49.—Upper epidermis of the leaf of Panicum javanicum. × 300
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.)
Fig. 52.—Transverse section of a portion of the leaf of Panicum repens. × 120
1. Motor cells; 2. stomata; 3. sclerenchyma; 4. chlorophyllous layer.
All the vascular bundles in the leaves of Aristida setacea have broad sclerenchymatous bands on both the sides. Besides these bands arranged like a girder above and below each bundle, there are on the lower side bands of sclerenchyma. So the sclerenchyma becomes almost continuous on the lower side.
Fig. 53.—Transverse section of the leaf margin of Panicum repens. × 180
1. Sclerenchyma; 2. stomata; 3. chlorophyllous layer.
Fig. 54.—Transverse section of a part of the leaf of Aristida setacea. × 30.
1. Vascular bundle; 2. sclerenchyma; 3. motor cells.
Fig. 55.—Transverse section of a portion of the leaf of Aristida setacea. × 120
1. Motor cells; 2. stomata; 3. sclerenchyma; 4. epidermis; 5. cutin layer.
The sclerenchyma lying on the lower side of the primary bundles are contiguous with the bundle, while those above are separated from the bundle by the chlorophyllous layer. (See fig. 55.) In the case of secondary and tertiary bundles the sclerenchymatous bands lying on the lower side are in contact with the chlorophyllous layer, whereas the upper bands are either in contact with this layer or separated from it by a few parenchymatous cells.
All the vascular bundles in the leaves of Eragrostis Willdenoviana are provided with sclerenchyma on both the sides. The lower band of the primary vascular bundles is continuous with the vascular bundle, the chlorophyllous layer being open below. The upper bands of the primary and the lower bands of the secondary vascular bundles just touch the chlorophyllous layer. In the secondary bundles the sclerenchyma band above is separated from the chlorophyllous layer by two layers of parenchyma. In the case of the leaves of Panicum flavidum, P. colonum, P. fluitans and Pennisetum cenchroides the sclerenchyma is separated from the chlorophyllous layer by layers of parenchyma.
Fig. 56.—Transverse section of a part of the leaf of Eragrostis Willdenoviana. × 30
1. Vascular bundle; 2. sclerenchyma; 3. motor cells.
Fig. 57.—Transverse section of a portion of the leaf of Eragrostis Willdenoviana. × 150
1. Motor cells; 2. stomata; 3. sclerenchyma; 4. chlorophyllous layer; 5. vascular strand cut through; 6. hair.
Fig. 58.—Transverse section of a part of the leaf of Panicum colonum. × 30
1. Vascular bundle; 2. sclerenchyma; 3. motor cells.
Even from the few examples dealt with above, it is obvious that the range of variation of sclerenchyma in leaves is very great. In the leaves of Aristida setacea there is a considerable amount of sclerenchyma whilst in some leaves such as those of Panicum colonum, P. flavidum and Panicum fluitans the sclerenchyma is reduced to its minimum.
Fig. 59.—Transverse section of a part of the leaf of Panicum fluitans. × 30
1. Vascular bundle; 2. sclerenchyma.