stem

General characteristics

While most stems are erect, aerial structures, some remain underground, others creep over or lie prostrate on the surface of the ground, and still others are so short and inconspicuous that the plants are said to be stemless, or acaulescent. When stems lie flattened immediately above but not on the ground, with tips curved upward, they are said to be decumbent, as in juniper. If stems lie flat on the ground but do not root at the nodes (joints), the stem is called procumbent or prostrate, as in purslane. If a stem creeps along the ground, rooting at the nodes, it is said to be repent or creeping, as in ground ivy.

Most stems are cylindrical and tapering, appearing circular in cross section; others may be quadrangular or triangular.

Herbaceous stems (annuals and herbaceous perennials) die to the ground after blooming or at the end of the growing season. They usually contain little woody tissue. Woody stems (perennials) have considerable woody supporting tissue and live from year to year. A woody plant with no main stem or trunk, but usually with several stems developed from a common base at or near the ground, is known as a shrub.

External features

A shoot or branch usually consists of a stem, or axis, and leafy appendages. Stems have several distinguishing features. They arise either from the epicotyl of the embryo in a seed or from buds. The stem bears both leaves and buds at nodes, which are separated by leafless regions or internodes, and sometimes roots and flowers (see illustration).

Winter woody twig (horse chestnut) showing apical dominance

The nodes are the regions of the primary stem where leaves and buds arise. The number of leaves at a node is usually specific for each plant species. In deciduous plants which are leafless during winter, the place of former attachment of a leaf is marked by the leaf scar. The scar is formed in part by the abscission zone formed at the base of the leaf petiole. The stem regions between nodes are called internodes. Internode length varies greatly among species, in different parts of the same stem, and under different growing conditions.

Lenticels are small, slightly raised or ridged regions of the stem surface that are composed of loosely arranged masses of cells in the bark. Their intercellular spaces are continuous with those in the interior of the stem, therefore permitting gas exchange similar to the stomata that are present before bark initiation.

There are three major types of stem branching: dichoto-mous, monopodial, and sympodial. Dichotomy occurs by a division of the apical meristem to form two new axes. If the terminal bud of an axis continues to grow and lateral buds grow out as branches, the branching is called monopodial. If the apical bud terminates growth in a flower or dies back and one or more axillary buds grow out, the branching is called sympodial. Often only one bud develops so that what appears to be single axis is in fact composed of a series of lateral branches arranged in linear sequence.

The large and conspicuous stems of trees and shrubs assume a wide variety of distinctive forms. Columnar stems are basically unbranched and form a terminal leaf cluster, as in palms, or lack obvious leaves, as in cacti. Branching stems have been classified either as excurrent, when there is a central trunk and a conical leaf crown, as in firs and other conifers, or as decur-rent (or deliquescent), when the trunk quickly divides up into many separate axes so that the crown lacks a central trunk, as in elm. See Tree

Internal features

The stem is composed of the three fundamental tissue systems that are found also in all other plant organs: the dermal (skin) system, consisting of epidermis in young stems and peridem in older stems of many species; the vascular (conducting) system, consisting of xylem (water conduction) and phloem (food conduction); and the fundamental or ground tissue system, consisting of parenchyma and sclerenchyma tissues in which the vascular tissues are embedded. The arrangement of the vascular tissues varies in stems of different groups of plants, but frequently these tissues form a hollow cylinder enclosing a region of ground tissue called pith and separated from the dermal tissue by another region of ground tissue called cortex. See Cortex (plant), Epidermis (plant), Phloem, Pith, Sclerenchyma, Xylem

Part of the growth of the stem results from the activity of the apical meristem located at the tip of the shoot. The derivatives of this meristem are the primary tissues; epidermis, primary vascular tissues, and the ground tissues of the cortex and pith. In many species, especially those having woody stems, secondary tissues are added to the primary. These tissues are derived from the lateral meristems, oriented parallel with the sides of the stem: cork cambium (phellogen), which gives rise to the secondary protective tissue periderm, which consists of phellum (cork), phellogen (cork cambium), and phelloderm (secondary cortex) and which replaces the epidermis; and vascular cambium, which is inserted between the primary xylem and phloem and forms secondary xylem (wood) and phloem. See Apical meristem, Lateral meristem

The vascular tissues and the closely associated ground tissues—pericycle (on the outer boundary of vascular region), interfascicular regions (medullary or pith rays), and frequently also the pith—may be treated as a unit called the stele. The variations in the arrangement of the vascular tissues serve as a basis for distinguishing the stelar types. The word stele means column and thus characterizes the system of vascular and associated ground tissues as a column. This column is enclosed within the cortex, which is not part of the stele.

in linguistics, the part of a word that bears the lexical meaning of that word. A stem does not include inflectional affixes such as endings. It may consist of one root, such as dom (“house”); of a root with one or more derivational suffixes, such as dom-ik (“house” [diminutive]), kras-n-yi (“red”; -yi is an ending), and kras-n-en’k-ii (“red” [affectionate diminutive]; -ii is an ending); of a root and a prefix, as in pri-gorod (“suburb”); or of a root, a prefix, and a suffix, such as s-del-a-t’(“to do” [perfective]; -t’ is the infinitive suffix indicating the function of the verb in its clause and is not part of the stem). In some languages the stem may also include infixes.

in higher plants, the axial organ that together with leaves constitutes a shoot. Stems transport water and other substances from the roots to the leaves. The branching of the stem and the ordered arrangement of leaves, flowers, and fruits increase the assimilative surface of a plant. Stems often participate in the storage of water and reserve nutrients and in photosynthesis. The stem regions from which lateral organs, including branches and leaves, depart are called nodes, and the areas between nodes are called internodes.

Stems may be herbaceous or woody. The main stem of woody plants is called a trunk. Stems are generally cylindrical in shape, although they may also be triangular (in sedges), quadrangular (in mints), polyhedral, or flattened (in cacti). Their habit may be erect, prostrate, creeping, or climbing. Some are aerial structures, whereas others remain underground.

Stems range in length from 1–1.5 mm (in freshwater duckweeds) to 200–300 m (in tropical rattan palms). The diameter may range from fractions of a millimeter (in mosses) to 10–11 m (in baobabs and sequoias). A stem increases in length as a result of the activity of the shoot’s apical meristem, which constitutes the growing point. Besides apical growth, grasses and other plants are characterized by intercalary growth at the bases of the internodes.

A stem is composed of an epidermis externally and a central cylinder, or stele, internally. Between the epidermis and stele is the primary cortex, whose internal parenchymatous layer is converted into the endodermis. The primary cortex borders on the peripheral part of the stele—the pericycle—which is represented by parenchymatous or mechanical tissue. The pericycle is absent in some plants.

The stele consists mostly of vascular tissues; the phloem is outside the xylem. In the center of the stem of leafy mosses there is a vascular bundle, the elements of which are only externally similar to the conducting elements of the phloem and xylem. In vascular plants the development of the procambium precedes the formation of vascular tissues. In club mosses there is no pith, and the xylem is divided into ribbonlike strands surrounded by phloem. In horsetails closed collateral bundles, with carinal cavities instead of xylem, are arranged around a central hollow cavity. In ferns vascular tissues form a ring around the pith.

The stems of seed plants have fascicular and continuous vascular systems divided by parenchymatous medullary rays that extend outward. The external part of the procambium becomes differentiated into the primary phloem, on whose periphery mechanical fibers often develop; the internal part of the procambium becomes differentiated into the primary xylem. A layer of cells forming the cambium is found between the vascular tissues. The cambium deposits elements of the secondary phloem externally and elements of the secondary xylem internally, resulting in the thickening of the stele.

Unlike the nodal part of the stem, the central part of the inter-nodes lacks leaf and branch gaps (lacunae). Trilacunar nodes are common in dicotyledons, for example, in apples. Less common are monolacunar and multilacunar nodes; the former are typical of lilacs, and the latter of elders. The most active secondary thickening is characteristic of perennial woody plants, in whose secondary xylem and, sometimes, phloem annual, or growth, rings may be observed. With age, as a result of the development of the periderm, the primary cortex and, later, the outer part of the phloem atrophy and form the cortex.

Most monocotyledons have the type of closed collateral bundles characteristic of palms; as a result the bundles are arranged diffusely in transverse sections. There is a tendency toward a circular arrangement of bundles only in a few grasses with culms, in spiderworts, and in yams. Secondary thickening is characteristic only of arborescent Liliaceae, for example, aloes and dracaena palms; the meristem, which is formed in the pericycle or primary cortex, gives rise to concentric bundles and interfascicular, often lignifying, parenchyma.