A Handbook of Some South Indian Grasses. Rangachari K.
rudimentary. Usually the glumes are rather close together on the rachilla so that the internodes are very short; but in some grasses, as in Dinebra arabica, the glumes are rather distant and so the internodes are somewhat longer and conspicuous. In some species of Panicum the rachilla is jointed to the pedicel below the empty glumes, whereas it is articulated just above these glumes in Chloris barbata. Sometimes the rachilla is articulated between the flowers. This is the case in the spikelet of Dinebra arabica.
Pollination in most grasses is brought about by wind, though in a few cases self-pollination occurs. The terminal position of the inflorescence, its protrusion far above the level of the foliage leaves, the swinging and dangling anthers, the abundance of non-sticking pollen and the plumose stigmas are all intended to facilitate pollination by wind. Furthermore the stamens and the stigmas do not mature at the same time. In some grasses the stamens mature earlier, (protandry) while in others the stigmas protrude long before the stamens (protogyny). As the result of the pollination the ovary developes into a dry 1-seeded indehiscent fruit. The seed fills the cavity fully and the pericarp fuses with the seed-coat and so they are inseparable. Such a fruit is termed a caryopsis or grain. Though in the vast majority of grasses the pericarp is inseparable, in a few cases it is free from the seed-coat as in Sporobolus indicus and Eleusine indica.
Fig. 20.—Longitudinal section of a portion of the grain of Andropogon Sorghum. × 280
P. Pericarp; Sc. seed-coat; A. aleurone layer; E. endosperm; S. scutellum; Rs. root-sheath; Rc. root-cap; R. radicle; Pl. plumule; G. growing point.
Fig. 21.—A portion of the section of the grain of Andropogon Sorghum. × 500
P. pericarp; I. seed-coat; A. aleurone layer; S. starch.
The caryopsis consists of an embryo on one side at the base and the endosperm occupies the remaining portion. The embryo can be made out on the side of the grain facing the glume, as it is outlined as an oval area. On the other face of the grain which is towards the palea, the hilum is seen at the base. The grain varies in shape considerably. It may be rounded, oval, ellipsoidal, narrow and cylindrical, oblong terete or furrowed. There is considerable variation as regards the colour also.
The embryo consists of an axis and a scutellum. The axis, which is differentiated into the plumule directed upward and the radicle downward, is small and straight and it is covered more or less by the edges of the scutellum. The scutellum is attached to the axis at about its middle and its outer surface is in contact with the endosperm. This is an important organ as its function is to absorb nourishment from the endosperm during germination. The scutellum is considered to represent the first leaf or cotyledon. The endosperm consists mostly of starch. Just outside the endosperm and within the epidermis lies a layer of cells containing much proteid substance. This layer is called the aleurone layer. (See fig. 21.) As an illustration of the caryopsis, the grain of Andropogon Sorghum may be studied. All the structural details are shown in fig. 20 which is a longitudinal section of the grain.
The primary axis of the embryo is enclosed by a closed sheath both above and below. The sheath which envelopes the radicle is called coleorhiza and that of the plumule, pileole or germ-sheath.
CHAPTER IV.
HISTOLOGY OF THE VEGETATIVE ORGANS.
The shoots and roots of grasses conform in their internal structure to the monocotyledonous type. In all grasses numerous threads are found running longitudinally within the stem and some of these pass into the leaves, at the nodes, and run as nerves in the blades of the leaves. These threads are the vascular bundles. The rest of the tissue of the stem and leaves consists of thin-walled parenchymatous cells of different sorts.
The general structure of these bundles is more or less the same in all grasses. A vascular bundle consists of only xylem and phloëm, without the cambium, and so no secondary thickening can take place in the stems of grasses. Such bundles as these are called closed vascular bundles to distinguish them from the dicotyledonous type of vascular bundles which are called open vascular bundles on account of the existence of the cambium.
Fig. 22.—Transverse section of a vascular bundle. × 250
1. Annular vessel; 2. spiral vessel; 3. pitted vessel; 4. phloëm or sieve tubes; 5. sclerenchyma.
The component parts and elements of which the vascular bundles in grasses are composed may be learnt by studying the transverse and longitudinal sections of these bundles in any grass. The cross and longitudinal sections of a vascular bundle of the stem of Pennisetum cenchroides, are shown in figs. 22 and 23. In the figure of the transverse section the two large cavities indicated by the number 3 and the two small circular cavities with thick walls lying between the larger ones and indicated by the numbers 1 and 2 are the chief elements of the xylem.
By looking at the longitudinal section it is obvious that these elements are really vessels, the larger being pitted and the smaller annular and spiral vessels. These vessels together with the numerous small thick-walled cells lying between the pitted vessels constitute the xylem. Just above the xylem there is a group of large and small thin-walled cells. This is the phloëm and it consists of sieve tubes and thin-walled cells. All round the xylem and the phloëm there are many thick-walled cells. These are really fibres forming the bundle-sheath. On account of this bundle-sheath the bundles are called fibro-vascular bundles.
Fig. 23.—Longitudinal section of a vascular bundle. × 250
1. Annular vessel; 2. spiral vessel; 3. pitted vessel; 4. sieve tubes or phloëm; 5. sclerenchyma.
Fig. 24.—Transverse section of a portion of the stem of Rottboellia exaltata. × 70
1. Epidermis; 2. sclerenchyma; 3. vascular bundle.
Structure of the stem.—The stem of a grass consists of a mass of parenchymatous cells with a number of fibro-vascular bundles imbedded in it, and it is covered externally by a protective layer of cells, the epidermis. The stem is usually solid in all grasses in the young stage, but as it matures the internodes become hollow in many grasses and they remain solid in a few. In the internodes the fibro-vascular bundles run longitudinally and are parallel, but in the nodes they run in all directions and form a net work from which emerge a few bundles to enter the leaves. So far as the broad general features are concerned, the stems of many grasses are more or less similar in structure. However, when we take into consideration the arrangement of bundles, the development and arrangement of sclerenchyma, every species of grass has its own special characteristics. And these are so striking and constant that it may be possible to identify the species from these characters alone.
We may take as a type the stem of Rottboellia exaltata. This stem is somewhat semi-circular in transverse section and it is almost straight and flat in the front (the side towards the axillary bud). The peripheral portion of the stem becomes somewhat rigid and thick due to the aggregation of vascular bundles, some small and others large. The outermost series of bundles consisting of small and larger bundles are in contact with the layers of the cells lying just beneath the epidermis and these cells are also thick-walled. A few are away from these being separated by three or four layers of cells from the peripheral bundles. In all these vascular bundles the bundle-sheath