Fig. 275.—Transverse Section of the Stem of a Monocotyledon
The monocotyledonous plants, then, are those in which the stem is more or less woody and cylindrical, without either true bark or pith; and the woody tissue is not arranged in concentric rings, but in isolated bundles, which first bend inwards, as they rise, towards the centre of the stem, and then curve outwards towards the surface, which is hardened by the formation of a layer of hard woody matter. As a rule the stem is unbranched, and its growth takes place by a single bud at the summit. In nearly all of them the leaves are long and narrow, with veins running parallel throughout their length; and the parts of the flower are arranged in whorls of three or six. The outer whorl of the flower is often a conspicuous white or coloured perianth (that portion of the flower which lies outside the anthers), but in some the perianth is absent, the flower being protected by scaly bracts. The seeds are produced in a case called the ovary, and are fertilised by pollen grains which are developed in the anthers. When the pollen grains are set free they alight on the adhesive stigma, and grow, sending their tubes down into the ovary. The term monocotyledon is applied to these plants because the embryo has only one cotyledon or seed-leaf.
Fig. 276.—Leaf of a Monocotyledon
The principal divisions of this group are the Glumaceous Monocotyledons, in which the flower has no perianth, but is enclosed in scaly bracts or husks called glumes; and the Petaloid Monocotyledons, distinguished generally by the presence of a more or less conspicuous white or coloured perianth. The first of these includes the rushes, sedges, and grasses; and the other contains the lilies and orchids, with their allies, together with certain aquatic and semi-aquatic plants.
Among the Grasses there are several species that show a preference for the immediate neighbourhood of the sea, some growing luxuriantly at the bases of the cliffs where the beach is sandy, and others thriving best in salt marshes; but before dealing with these individually we shall note the general characteristics of the order (Gramineæ) to which they belong.
Grasses are distinguished by their jointed stems, which are usually hollow, with a split sheath, and bearing alternately arranged narrow leaves. The flowers, which are disposed either in spikes (sessile flowers arranged along a common axis) or in panicles (flowers stalked and arranged as in fig. 281), consist of scale-like bracts enclosing the stamens and the pistil. The bracts are in two series, the outer usually consisting of two glumes, and the inner of two pales; the upper pale, however, has two ribs running through it, and is therefore usually looked upon as a combination of two. In some species both glumes and pales are absent; but the former, when present, enclose one or more flowers, among which may be some that are abortive. The stamens are generally three in number, attached to the base of the flower; and the ovary is superior or free, that is, it grows above the other parts of the flower, and contains but one seed.