The rest of the root forming the central core is the stele and at its periphery there is a single layer of cells called the =pericycle=. The arrangement of the xylem and the phloem is different from that of the stem. They lie side by side on different radii, and not one behind the other on the same radius as in the stem. The number of xylem groups is fairly large and the development of the xylem is from the pericycle towards the centre of the stele. (See figs. 44 and 45.) The parenchymatous cells in the centre of the stele become thick-walled in older roots.
=Structure of the leaf.=--The structure of the leaf of gra.s.ses is quite characteristic of the family. In every leaf a number of vascular bundles, some small and others large, pa.s.s from the base to the apex.
Externally the leaf is covered on both the sides by the epidermis. The s.p.a.ces existing between the vascular bundles and the epidermis are filled with parenchymatous cells. The larger vascular bundles consist of xylem and phloem surrounded by a bundle sheath of a single layer of cells. In the smaller bundles the xylem is very much reduced. Around every vascular bundle there is a single row of somewhat large cells densely packed with large chloroplasts, the =chlorophyllous layer=. 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.
[Ill.u.s.tration: Fig. 47.--Transverse section of the stele portion of the root of Andropogon Sorghum. 150
Co. Cortex; en. endodermis; pe. pericycle; ph. phloem; pr. protoxylem; xy. xylem vessel.]
[Ill.u.s.tration: Fig. 48.--A portion of the transverse section of the leaf of Panic.u.m javanic.u.m. 100
Ep. c. An ordinary epidermal cell; st. stomata; sc. sclerenchyma; ph.
phloem; chl c. chlorenchyma; m. motor cells; xy. xylem.]
For a detailed study of the structure of the leaves of gra.s.ses the leaf of the gra.s.s _Panic.u.m javanic.u.m_ 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 s.p.a.ces between the bundles are occupied by thin-walled parenchymatous cells containing small chlorophyll grains.
[Ill.u.s.tration: Fig. 49.--Upper epidermis of the leaf of Panic.u.m javanic.u.m. 300]
[Ill.u.s.tration: Fig. 50.--Lower epidermis of the leaf of Panic.u.m javanic.u.m. 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 gra.s.ses.
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 _Panic.u.m javanic.u.m_ may be taken as typical of the structure of the leaves of most gra.s.ses. The leaves of _Eriochloa polystachya_, Cynodon and Paspalums are very much like the leaves of _Panic.u.m javanic.u.m_ in their internal structure.
Considerable amount of variation, however, occurs in the leaves of gra.s.ses especially as regards the arrangement of fibres and motor cells.
[Ill.u.s.tration: 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 gra.s.ses 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 _Panic.u.m 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 gra.s.ses consists entirely of sclerenchyma. (See fig. 53.)
[Ill.u.s.tration: Fig. 52.--Transverse section of a portion of the leaf of Panic.u.m 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.
[Ill.u.s.tration: Fig. 53.--Transverse section of the leaf margin of Panic.u.m repens. 180
1. Sclerenchyma; 2. stomata; 3. chlorophyllous layer.]
[Ill.u.s.tration: Fig. 54.--Transverse section of a part of the leaf of Aristida setacea. 30.
1. Vascular bundle; 2. sclerenchyma; 3. motor cells.]
[Ill.u.s.tration: 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 _Panic.u.m flavidum_, _P.
colonum_, _P. fluitans_ and _Pennisetum cenchroides_ the sclerenchyma is separated from the chlorophyllous layer by layers of parenchyma.
[Ill.u.s.tration: Fig. 56.--Transverse section of a part of the leaf of Eragrostis Willdenoviana. 30
1. Vascular bundle; 2. sclerenchyma; 3. motor cells.]
[Ill.u.s.tration: 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.]
[Ill.u.s.tration: Fig. 58.--Transverse section of a part of the leaf of Panic.u.m 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 _Panic.u.m_ _colonum_, _P. flavidum_ and _Panic.u.m fluitans_ the sclerenchyma is reduced to its minimum.
[Ill.u.s.tration: Fig. 59.--Transverse section of a part of the leaf of Panic.u.m fluitans. 30
1. Vascular bundle; 2. sclerenchyma.]
In the leaves of gra.s.ses growing in dry situations the development of sclerenchyma is generally very considerable. The gra.s.s _Aristida setacea_ is a good example of a xerophytic gra.s.s. The sea-sh.o.r.e gra.s.s _Spinifex squarrosus_ is another example of the same kind. But in the leaves of this gra.s.s, 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.
[Ill.u.s.tration: Fig. 60.--Transverse section of a leaf of Spinifex squarrosus. 10
1. Vascular bundle; 2. sclerenchyma.]
[Ill.u.s.tration: 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 gra.s.ses are more or less even, but in the case of a few gra.s.ses the upper surface consists of ridges and furrows, instead of being even. In the leaves of _Panic.u.m 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 _Panic.u.m 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 _Panic.u.m fluitans_.
[Ill.u.s.tration: Fig. 62.--Upper epidermis of the leaf Panic.u.m Isachne.
200
1. Cells overlying the veins; 2. cells overlying the furrows; 3.
stomata.]
[Ill.u.s.tration: Fig. 63.--Lower epidermis of the leaf Panic.u.m Isachne.
200
1. Cells overlying the veins; 2. cells overlying the furrows; 3.
stomata.]