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 phloem and it consists of sieve tubes and thin-walled cells. All round the xylem and the phloem 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=. [Illustration: Fig. 23.--Longitudinal section of a vascular bundle. x 250 1. Annular vessel; 2. spiral vessel; 3. pitted vessel; 4. sieve tubes or phloem; 5. sclerenchyma.] [Illustration: Fig. 24.--Transverse section of a portion of the stem of Rottboellia exaltata. x 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.