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 is very strongly developed all round and is very much developed especially at the sides. It is this great development of sclerenchyma that makes the outer portion of the cortex hard. Within the ground tissue are found a number of vascular bundles scattered more or less uniformly. These bundles have no continuous bundle-sheaths but have instead groups of fibres at the sides and in front of the phloëm. The cavities near the annular vessels are somewhat larger and conspicuous in these bundles.