Stem showing internode and nodes plus leafpetiolesThis above-ground stem ofPolygonum has lost its leaves, but is producingadventitious roots from the nodes.Xylem and Phloem
Astem is one of two main structural axes of avascular plant, the other being theroot. It supportsleaves,flowers andfruits, transports water and dissolved substances between the roots and the shoots in thexylem andphloem, engages in photosynthesis, stores nutrients, and produces new living tissue.[1] The stem can also be called theculm,halm,haulm,stalk, orthyrsus.
The stem is normally divided into nodes and internodes:[2]
The nodes are the points of attachment for leaves and can hold one or more leaves. There are sometimes axillarybuds between the stem and leaf which can grow into branches (withleaves,conifer cones, orflowers).[2]Adventitious roots (e.g.brace roots) may also be produced from the nodes.Vines may producetendrils from nodes.
The term "shoots" is often confused with "stems"; "shoots" generally refers to new fresh plant growth, including both stems and other structures like leaves or flowers.[2]
In most plants, stems are located above the soil surface, but some plants haveunderground stems.
Support for and the elevation of leaves, flowers, and fruits. The stems keep the leaves in the light and provide a place for the plant to keep its flowers and fruits.
Transport of fluids between the roots and the shoots in the xylem and phloem.
Storage of nutrients.
Production of new living tissue. The normal lifespan of plant cells is one to three years. Stems have cells calledmeristems that annually generate new living tissue.
Photosynthesis.
Stems have two pipe-like tissues calledxylem andphloem. The xylem tissue arises from the cell facing inside and transports water by the action oftranspiration pull,capillary action, androot pressure. The phloem tissue arises from the cell facing outside and consists of sieve tubes and their companion cells. The function of phloem tissue is to distribute food from photosynthetic tissue to other tissues. The two tissues are separated bycambium, a tissue that divides to form xylem or phloem cells.
Acaulescent: Used to describe stems in plants that appear to be stemless. Actually these stems are just extremely short, the leaves appearing to rise directly out of the ground, e.g. someViola species.
Arborescent: Tree with woody stems normally with a single trunk.
Axillary bud: A bud which grows at the point of attachment of an older leaf with the stem. It potentially gives rise to a shoot.
Branched: Aerial stems are described as being branched or unbranched.
Bulb: A short vertical underground stem with fleshy storage leaves attached, e.g.onion,daffodil, andtulip. Bulbs often function in reproduction by splitting to form new bulbs or producing small new bulbs termed bulblets. Bulbs are a combination of stem and leaves so may better be considered as leaves because the leaves make up the greater part.
Caespitose: When stems grow in a tangled mass or clump or in low growing mats.
Cladode (includingphylloclade): A flattened stem that appears leaf-like and is specialized for photosynthesis,[4] e.g.cactus pads.
Climbing: Stems that cling or wrap around other plants or structures.
Decumbent: A stem that lies flat on the ground and turns upwards at the ends.
Fruticose: Stems that grow shrublike with woody like habit.
Herbaceous: Non woody stems which die at the end of the growing season.
Internode: An interval between two successive nodes. It possesses the ability to elongate, either from its base or from its extremity depending on the species.
Node: A point of attachment of a leaf or atwig on the stem in seed plants. A node is a very small growth zone.
Dermal tissue covers the outer surface of the stem and usually functions to protect the stem tissue, and controlgas exchange. The predominant cells of dermal tissue areepidermal cells.[6]
Ground tissue usually consists mainly ofparenchyma,collenchyma andsclerenchyma cells, and they surround vascular tissue. Ground tissue is important in aiding metabolic activities (eg.respiration,photosynthesis, transport, storage) as well as acting as structural support and forming newmeristems.[7] Most or all ground tissue may be lost inwoody stems.
Dicot stems with primary growth havepith in the center, with vascular bundles forming a distinct ring visible when the stem is viewed in cross section. The outside of the stem is covered with an epidermis, which is covered by a waterproof cuticle. The epidermis also may containstomata for gas exchange and multicellular stem hairs calledtrichomes. A cortex consisting ofhypodermis (collenchyma cells) andendodermis (starch containing cells) is present above thepericycle and vascular bundles.
Stems of twoRoystonea regia palms showing characteristic bulge, leaf scars and fibrous roots,Kolkata, India
Woody dicots and many nonwoody dicots havesecondary growth originating from their lateral or secondary meristems: thevascular cambium and thecork cambium or phellogen. The vascular cambium forms between the xylem and phloem in the vascular bundles and connects to form a continuous cylinder. The vascular cambium cells divide to producesecondary xylem to the inside andsecondary phloem to the outside. As the stem increases in diameter due to production of secondary xylem and secondary phloem, the cortex and epidermis are eventually destroyed. Before the cortex is destroyed, a cork cambium develops there. The cork cambium divides to produce waterproof cork cells externally and sometimes phelloderm cells internally. Those three tissues form theperiderm, which replaces the epidermis in function. Areas of loosely packed cells in the periderm that function in gas exchange are called lenticels.
Secondaryxylem is commercially important as wood. The seasonal variation in growth from the vascular cambium is what creates yearly tree rings in temperate climates. Tree rings are the basis ofdendrochronology, which dates wooden objects and associated artifacts.Dendroclimatology is the use of tree rings as a record of past climates. The aerial stem of an adulttree is called atrunk. The dead, usually darker inner wood of a large diameter trunk is termed theheartwood and is the result oftylosis. The outer, living wood is termed the sapwood.
Vascular bundles are present throughout themonocot stem, although concentrated towards the outside. This differs from the dicot stem that has a ring of vascular bundles and often none in the center. Theshoot apex in monocot stems is more elongated. Leaf sheathes grow up around it, protecting it. This is true to some extent of almost all monocots. Monocots rarely producesecondary growth and are therefore seldom woody, withpalms andbamboo being notable exceptions. However, many monocot stems increase in diameter via anomalous secondary growth.
Allgymnosperms are woody plants. Their stems are similar in structure to woody dicots except that most gymnosperms produce onlytracheids in their xylem, not the vessels found in dicots. Gymnosperm wood also often containsresin ducts. Woody dicots are called hardwoods, e.g.oak,maple andwalnut. In contrast, softwoods are gymnosperms, such aspine,spruce andfir.
Mostferns have rhizomes with no vertical stem. The exception istree ferns, which have vertical stems that can grow up to about 20 metres. The stem anatomy of ferns is more complicated than that of dicots because fern stems often have one or moreleaf gaps in cross section. A leaf gap is where the vascular tissue branches off to afrond. In cross section, the vascular tissue does not form a complete cylinder where a leaf gap occurs. Fern stems may havesolenosteles ordictyosteles or variations of them. Many fern stems have phloem tissue on both sides of the xylem in cross-section.
Medicines obtained from stems includequinine from the bark ofcinchona trees,camphor distilled from wood of a tree in the same genus that providescinnamon, and the muscle relaxantcurare from the bark of tropical vines.
Tannins used for tanningleather are obtained from the wood of certain trees, such asquebracho.Cork is obtained from the bark of thecork oak.Rubber is obtained from the trunks ofHevea brasiliensis.Rattan, used for furniture and baskets, is made from the stems of tropical vining palms.Bast fibers for textiles and rope are obtained from stems of plants likeflax,hemp,jute andramie. The earliest known paper was obtained from the stems ofpapyrus by the ancient Egyptians.
Amber isfossilized sap from tree trunks; it is used forjewelry and may contain preserved animals. Resins from conifer wood are used to produceturpentine androsin. Tree bark is often used as amulch and in growing media for container plants. It also can become the naturalhabitat oflichens.
Some ornamental plants are grown mainly for their attractive stems, e.g.:
^C. Michael Hogan. 2010."Abiotic factor".Encyclopedia of Earth. Emily Monosson and C. Cleveland, eds. National Council for Science and the EnvironmentArchived 2013-06-08 at theWayback Machine. Washington, D.C.
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