Stem

Stem, the ascending portion of the axis of a plant, bearing leaves as lateral appendages. In the Thallophyta there cannot be said to be either stem or leaf, and in higher plants, as in the case of the liverwort, or even the duckweeds (Lemna), the stem is sometimes represented by a flat, green, cellular, and very unstem-like expansion. Stems differ from roots (i) in growing generally upward, away - that is, from the centre of gravity; (ii) in growing at the apex without any cap of dead cells, though this growing-point is generally overlapped by rudimentary leaves forming a terminal bud; (iii) in bearing leaves as lateral appendages differing from themselves in structure; and (iv) in giving off their branches exogenously - i.e. from more superficial layers of tissue than roots do. Stems may grow horizontally, but even then commonly bend upward at their growing points, whilst even drooping branches generally take an ascending direction as they first leave the stem.

The functions of a stem are: (i) to support the leaves and flowers, and connect the former with the roots, the other main organs for taking in food; (ii) when still young and green, to assimilate, like the leaves; and (iii), more especially in perennials, to act as a food reservoir. The physiologioal utility of branching stems would seem to be the exposure of leaves and flowers to as much air and light as possible; and it is in stem-structures that many of the processes not only of food-transfer from assimilating organs to growing parts take place, but also those of food "elaboration."

In the higher plants the stem originates in the plumule of the embryo, and as the stem elongates it remains terminated by a similar bud, as long as growth continues. The lower leaves of the bud, as it now develops into a shoot, become separated in succession, the points at which they spring from the stem being termed nodes, and the lengths of stem between them, internodes. Some plants, such as the primrose, are misleadingly called acaulescent, or stemless, because their stems are so short and the internodes so little developed that the leaves are crowded together in a rosette.

The stems of the great groups of the higher plants differ markedly in internal structure. Those of mosses are mainly cellular, with an ill-defined epidermis without stomata, and the mere rudiment of a central vascular axis. Those of ferns, known as acrogenous, have very short internodes, and are covered with the scars of fallen leaves: they are generally cylindric, crowned with a circle of leaves and seldom branched; and, in addition to cortical and other sclerenchyma, have a circle of closed fibro-vascular bundles surrounding a soft mass of fundamental tissue, and sending out branch bundles into the leaves. Monocotyledons have stems also mostly cylindric and unbranched, with no separable bark, and hardest near the outside. They may have a distinct epidermis and primary cortex, and contain numerous' scattered closed bundles which bend inwards from the roots and outwards into the leaves. There are, however, several modifications of this type. Grasses, of which the Bamboos are gigantic tropical representatives, have mostly hollow internodes with straight bundles, which intercross at the nodes. Such a stem is called fistular, and sometimes a culm. The unbranched stem of a palm or a tree-fern is called a caudex. Most Monocotyledons have a considerable amount of sclerenchymatous conjunctive tissue round their bundles. Some few arborescent Liliaceae, such as Aloe and Dracaena, develop additional bundles in a merismatic pericycle.

All Dicotyledons have at first, as annuals always retain, a succulent herbaceous stem, with a distinct epidermis, and generally a green hypoderm. In woody plants, mostly perennial, the epidermis is soon thrown off and replaced by the periderm, beneath which will be the phellogen and phelloderm, the central medulla shrivels, and the bulk of the stem is made up of the ring of open fibrovascular bundles, separated by the medullary rays, and made up of annual rings of xylem within and of phloem outside the line of cambium.

If a woody plant has one main stem at least ten or twelve feet high it is called a tree; whilst if it branches freely near the ground it is a shrub.

Though usually round in section, the stem is sometimes angular; being triangular, for example, in sedges, and square in many Labiatae. In other cases the leaves are, so to speak, continued down the sides of the stem, which is then termed winged, as in some thistles. In surface the stem may be smooth, or, technically, glabrous; furrowed, hairy, downy, bristly or setose, that is, having stiff hairs; or prickly, as in the rose.

In duration stems may be annual, biennial, or, perennial; but a large number of plants known as herbaceous perennials have perennial underground stems, but send up branches above ground that are annual, dying down each winter.

Other forms of aerial stem, such as the runner, the offset, the sucker, the thorn, some tendrils, and the phylloclade, together with most forms of underground stem, such as the rhizome, corm, bulb, and tuber, have mostly been separately described.