stem, supporting structure of a plant, serving also to conduct and to store food materials. The stems of herbaceous and of woody plants differ: those of herbaceous plants are usually green and pliant and are covered by a thin epidermis instead of by the bark of woody plants. There is relatively more pith in herbaceous stems, and the cambium cambium (kăm`bēəm), thin layer of generative tissue lying between the bark and the wood of a stem, most active in woody plants.
..... Click the link for more information. , which increases the diameter of woody stems, is usually almost inactive; it is therefore characteristic of herbaceous stems that, although they increase in height, their increase in diameter is small. Most herbaceous plants are annuals; some have specialized underground stems (see bulb bulb, thickened, fleshy plant bud, usually formed under the surface of the soil, which carries the plant over from one blooming season to another. It may have many fleshy layers (as in the onion and hyacinth) or thin dry scales (as in some lilies)—both of which
..... Click the link for more information. , corm corm, short, thickened underground stem, usually covered with papery leaves. A corm grows vertically, producing buds at the upper nodes and roots from the lower surface. Corms serve as organs of food storage and in some plants (e.g.
..... Click the link for more information. , rhizome rhizome (rī`zōm) or rootstock,
..... Click the link for more information. , and tuber tuber, enlarged tip of a rhizome (underground stem) that stores food. Although much modified in structure, the tuber contains all the usual stem parts—bark, wood, pith, nodes, and internodes.
..... Click the link for more information. ) that store food and enable the plant to survive unfavorable growing conditions. Aerial stems may be specialized as tendrils, thorns, or runners (stolons); another specialization is the fleshy, moisture-retaining stem of many arid-land plants (such as most cacti and other succulents). Aerial stems are usually erect; however, in the climbing plants they require support and in others (e.g., melons) they are prostrate. The vascular system in the stem consists chiefly of xylem (upward-conducting) and phloem (downward-conducting) tissue, usually in vascular bundles arranged concentrically on either side of the cambium—the xylem (wood wood, botanically, the xylem tissue that forms the bulk of the stem of a woody plant. Xylem conducts sap upward from the roots to the leaves, stores food in the form of complex carbohydrates, and provides support; it is made up of various types of cells specialized
..... Click the link for more information. ) inside, the phloem outside. In monocotyledonous plants, which generally lack cambium, the bundles are scattered throughout the stem tissue. The sap ducts are formed of elongated cells joined end to end; in the xylem the cell ends dissolve away completely to form continuous tubes and in the phloem they develop perforations and are called sieve plates. Herbaceous stems are marked externally by leaf and bud nodes; woody stems also bear lenticels (pores for transpiration), scars where leaves, twigs, and fruits have dropped off, and bud scars. The annual extension growth of a woody stem develops from a terminal bud usually protected by bud scales or stipules; when the scales fall away, a characteristic bud scar remains. The sap sap, fluid in plants consisting of water and dissolved substances. Cell sap refers to this fluid present in the large vacuole, or cell cavity, that occupies most of the central portion of mature plant cells.
..... Click the link for more information. of certain stems contain gums, latexes, and resins used commercially; many are the source of wood of great economic importance.

stem

Plant axis that emerges from the roots, supports the branches, bears buds and shoots with leaves, and contains the vascular (conducting) tissues (xylem and phloem) that transport water, minerals, and food to other parts of the plant. The pith (a central core of spongy tissue) is surrounded by strands (in dicots; see cotyledon) or bundles (in monocots) of conducting xylem and phloem, then by the cortex and outermost epidermis, or bark. The cambium (an area of actively dividing cells) lies just below the bark. Lateral buds and leaves grow out of the stem at intervals called nodes; the intervals on the stem between the nodes are called internodes. In flowering plants, various stem modifications (rhizome, corm, tuber, bulb, stolon) let the plant survive dormantly for years, store food, or sprout asexually. All green stems perform photosynthesis, as do leaves; in plants such as the cacti (see cactus) and asparagus, the stem is the chief site of photosynthesis.

stem¹

1. the main axis of a plant, which bears the leaves, axillary buds, and flowers and contains a hollow cylinder of vascular tissue

2. any similar subsidiary structure in such plants that bears a flower, fruit, or leaf

3. a corresponding structure in algae and fungi

4. a banana stalk with several bunches attached

5. a round pin in some locks on which a socket in the end of a key fits and about which it rotates

6. any projecting feature of a component: a shank or cylindrical pin or rod, such as the pin that carries the winding knob on a watch

7. Electronics the tubular glass section projecting from the base of a light bulb or electronic valve, on which the filament or electrodes are mounted

a. the main upright timber or structure at the bow of a vessel

b. the very forward end of a vessel (esp in the phrase from stem to stern)

stem²

Skiing a technique in which the heel of one ski or both skis is forced outwards from the direction of movement in order to slow down or turn

Stem

The organ of vascular plants that usually develops branches and bears leaves and flowers. On woody stems a branch that is the current season's growth from a bud is called a twig. The stems of some species produce adventitious roots. See Root (botany)

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.