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.