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| | [[Image:Stem_nodes.jpg|right|thumb|350px|Stem showing internode and nodes plus leaf petiole and new stem rising from node.]] | | [[Image:Stem_nodes.jpg|right|thumb|350px|Stem showing internode and nodes plus leaf petiole and new stem rising from node.]] |
| − | A '''stem''' is one of two main structural axes of a [[vascular plant]]. The stem is normally divided into nodes and internodes, the nodes hold [[bud]]s which grow into one or more [[leaf|leaves]], [[inflorescence]] (flowers), [[cone]]s or other stems etc. The internodes act as spaces that distance one node from another. The term [[shoot]]s is often confused with stems, shoots generally refer to new fresh plant growth and does include stems but also to other structures like leaves or flowers. The other main structural axis of plants is the [[root]]. In most plants stems are located above the soil surface but some plants have [[underground stems]]. <br />
| + | The main [[axis]] of a plant; [[leaf]]-bearing and [[flower]]- bearing as distinguished from the [[root]]-bearing axis.{{SCH}} |
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| − | Stems have four main functions which are:<br />
| + | ==Gallery== |
| − | * Support for and the elevation of leaves, [[flower]]s and [[fruit]]s. The stems keep the leaves in the light and provide a place for the plant to keep its flowers and fruits.
| + | <gallery> |
| − | * Transport of fluids between the roots and the shoots in the [[xylem]] and [[phloem]].
| + | Image:Plant_nodes_B.jpg|Stem showing internode and nodes plus leaf petioles |
| − | * Storage of nutrients.
| + | Image:Labeledstemforposter_copy.jpg|[[Flax]] stem cross-section, showing locations of underlying tissues. Ep = [[epidermis]]; C = [[cortex]]; BF = bast fibres; P = [[phloem]]; X = [[xylem]]; Pi = [[pith]] |
| − | * The production of new living tissue. The normal life span of plant cells is one to three years. Stems have cells called [[meristem]]s that annually generate new living tissue.<br />
| + | image:Ferns.jpg|Tasmanian tree fern |
| | + | Image:Asparagus produce-1.jpg|White and green [[asparagus]] - crispy stems are edible parts of this vegetable |
| | + | </gallery> |
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| − | == Specialized terms for stems ==
| + | {{glossary}} |
| − | [[Image:Plant_nodes_B.jpg|right|thumb|300px|Stem showing internode and nodes plus leaf petioles.]]
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| − | Stems are often specialized for storage, asexual reproduction, protection or photosynthesis, including the following:
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| − | * [[Acaulescent]] - plants with very short stems that appear to have no stems. The leaves appear to rise out of the ground. Some viola.
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| − | * [[Arborescent]] - tree like with woody stems normally with a single trunk.
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| − | * [[Bud]] - an embryonic shoot with immature stem tip.
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| − | * [[Bulb]] - a short vertical underground stem with fleshy storage leaves attached, e.g. [[onion]], [[daffodil]], [[tulip]]. 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.
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| − | * [[Caespitose]] - when stems grow in a tangled mass or clump or in low growing mats.
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| − | * [[Cladophyll]] - a flattened stem that appears leaf like and is specialized for photosynthesis, e.g. [[asparagus]], [[cactus]] pads.
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| − | * [[Climbing]] - stems that cling or wrap around other plants or structures.
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| − | * [[Corm]] - a short enlarged underground, storage stem, e.g. [[taro]], [[crocus]], [[gladiolus]]. Corms form many small cormlets.
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| − | * [[Decumbent]] - stems that lay flat on the ground and turn upwards at the ends.
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| − | * [[Fruticose]] - stems that grow shrub like with woody like habit.
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| − | * [[Herbaceous]] - non woody, they die at the end of the growing season.
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| − | * [[Rhizome]] - a horizontal underground stem that functions mainly in reproduction but also in storage, e.g. most [[fern]]s, [[iris]]
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| − | * [[Runner (plant part)]] - a horizontally growing stem that grows on top of the ground and roots at the nodes.
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| − | * [[Scape]] - a stem that holds flowers that comes out of the ground and has no normal leaves. Hosta, Lily, Iris.
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| − | * [[Stolon]]s or runner - is an above ground, horizontal stem that produces rooted plantlets at its nodes, e.g. [[strawberry]], [[spider plant]].
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| − | * [[Tree]] - a woody stem that is longer than 5 meters with a main [[trunk (botany)|trunk]].
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| − | * [[Spine (botany)|Thorns]] - a reduced stem with a sharp point and rounded shape. e.g. [[honeylocust]], [[Crataegus|hawthorn]].
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| − | * [[Tuber]] - a swollen, underground storage stem adapted for storage and reproduction, e.g. [[potato]].
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| − | * [[Woody]] - hard textured stems with secondary xylum.
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| − | == Stem structure ==
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| − | [[Image:Labeledstemforposter_copy.jpg|thumb|right|300px|[[Flax]] stem cross-section, showing locations of underlying tissues. Ep = [[epidermis]]; C = [[cortex]]; BF = bast fibres; P = [[phloem]]; X = [[xylem]]; Pi = [[pith]]]]
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| − | Stem usually consist of three tissues, [[Epidermis_%28botany%29|dermal tissue]], [[ground tissue]] and [[vascular tissue]]. The dermal tissue covers the outer surface of the stem and usually functions to waterproof, protect and control gas exchange. The ground tissue usually consists mainly of parenchyma cells and fills in around the vascular tissue. It sometimes functions in photosynthesis. Vascular tissue provides long distance transport and structural support. Most or all ground tissue may be lost in woody stems. The dermal tissue of aquatic plants stems may lack the waterproofing found in aerial stems. The arrangement of the vascular tissues varies widely among plant species.
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| − | === Dicot stems ===
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| − | [[Dicot]] stems with primary growth have a pith in the center with vascular bundles in a distinct ring visible in cross section. The outside of the stem is covered with an epidermis, which is covered by a waterproof cuticle. The epidermis also may contain [[stomata]] for gas exchange and hairs. A cortex of parenchyma cells lies between the epidermis and vascular bundles.
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| − | Woody dicots and many nonwoody dicots have [[secondary growth]] originating from their lateral or secondary meristems: the [[vascular cambium]] and the [[cork 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 produce [[secondary xylem]] to the inside and [[secondary 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 the [[periderm]], which replaces the epidermis in function. Areas of loosely-packed cells in the periderm that function in gas exchange are called lenticels.
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| − | Secondary [[xylem]] 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 of [[dendrochronology]], 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 adult [[tree]] is called a [[trunk (botany)|trunk]]. The dead, usually darker inner wood of a large diameter trunk is termed the [[heartwood]]. The outer, living wood is termed the [[sapwood]].
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| − | === Monocot stems ===
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| − | Vascular bundles are present throughout the [[monocot]] stem, although concentrated towards the outside. This differs from the monocot root that has a ring of vascular bundles and often none in the center. The shoot 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 produce secondary growth and are therefore seldom woody. However, many monocot stems increase in diameter via anamolous [[secondary growth]].
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| − | === Gymnosperm stems ===
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| − | All [[gymnosperms]] are woody plants. Their stems are similar in structure to woody dicots except that most gymnosperms produce only [[tracheid]]s in their xylem, not the vessels found in dicots. Gymnosperm wood also often contains [[resin]] ducts. Woody dicots are called hardwoods, e.g. [[oak]], [[maple]] and [[walnut]]. In contrast, softwoods are gymnosperms, such as [[pine]], [[spruce]] and [[fir]].
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| − | [[image:Ferns.jpg|thumb|250px|right|Tasmanian tree fern]]
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| − | === Fern stems ===
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| − | Most [[fern]]s have [[rhizome]]s with no vertical stem. The exception is [[Cyatheales|tree ferns]], with vertical stems up to about 15 meters. Stem anatomy of ferns is more complicated that dicots because fern stems often have one or more leaf gaps in cross section. A leaf gap is where the vascular tissue branches off to a [[frond]]. In cross section, the vascular tissue does not form a complete cylinder where a leaf gap occurs. Fern stems may have [[Stele (biology)|solenosteles]] or [[Stele (biology)|dictyosteles]] or variations of them. Many fern st
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| − | ems have phloem tissue on both sides of the xylem in cross-section.
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| − | == Economic importance ==
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| − | [[Image:Asparagus produce-1.jpg|right|thumb|250px|White and green [[asparagus]] - crispy stems are edible parts of this vegetable]]
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| − | There are thousands of species whose stems have economic uses. Stems provide a few major staple crops such as [[potato]] and [[taro]]. [[Sugar cane]] stems are a major source of sugar. [[Maple sugar]] is obtained from trunks of [[maple]] trees. [[Vegetable]]s from stems are [[asparagus]], [[bamboo shoots]], cactus pads or [[nopalito]]s, [[kohlrabi]], and [[Eleocharis dulcis|water chestnut]]. The spice, [[cinnamon]] is bark from a tree trunk. Cellulose from tree trunks is a food additive in bread, grated Parmesan cheese, and other processed foods. [[Gum arabic]] is an important food additive obtained from the trunks of ''[[Acacia senegal]]'' trees. [[Chicle]], the main ingredient in [[chewing gum]], is obtained from trunks of the chicle tree.
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| − | Medicines obtained from stems include [[quinine]] from the bark of [[cinchona]] trees, [[camphor]] distilled from wood of a tree in the same genus that provides cinnamon, and the muscle relaxant [[curare]] from the bark of tropical vines.
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| − | [[Wood]] is a used in thousands of ways, e.g. [[building material|buildings]], [[furniture]], [[boat]]s, [[airplane]]s, [[wagon]]s, [[car]] parts, [[musical instrument]]s, [[sports equipment]], [[railroad tie]]s, [[utility pole]]s, fence posts, [[pile|pilings]], [[toothpick]]s, [[match]]es, [[plywood]], [[coffin]]s, [[Shake (shingle)|shingle]]s, [[barrel]] staves, [[toy]]s, [[tool]] handles, [[picture frame]]s, [[wood veneer|veneer]], [[charcoal]] and [[firewood]]. [[Wood pulp]] is widely used to make [[paper]], [[cardboard]], [[cellulose]] sponges, [[cellophane]] and some important [[plastic]]s and [[textile]]s, such as [[cellulose acetate]] and [[rayon]]. [[Bamboo]] stems also have hundreds of uses, including paper, buildings, furniture, boats, musical instruments, [[fishing pole]]s, [[water pipe]]s, plant stakes, and [[scaffolding]]. Trunks of [[palm trees]] and [[tree fern]]s are often used for building. [[Phragmites|Reed]] stems are also important building materials in some areas.
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| − | [[Tannin]]s used for tanning [[leather]] are obtained from the wood of certain trees, such as [[quebracho]]. [[Cork (material)|Cork]] is obtained from the bark of the [[cork oak]]. [[Rubber]] is obtained from the trunks of ''[[Hevea 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 include [[flax]], [[hemp]], [[jute]] and [[ramie]]. The earliest paper was obtained from the stems of [[papyrus]] by the ancient Egyptians.
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| − | [[Amber]] is fossilized sap from tree trunks; it is used for [[jewelry]] and may contain ancient animals. Resins from conifer wood are used to produce [[turpentine]] and [[rosin]]. Tree bark is often used as a [[mulch]] and in growing media for container plants.
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| − | Some ornamental plants are grown mainly for their attractive stems, e.g.:
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| − | * White bark of [[paper birch]]
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| − | * Twisted branches of [[Corkscrew Willow|corkscrew willow]] and [[hazel|Harry Lauder's walking stick]] (''Corylus avellana'' 'Contorta')
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| − | * Red, peeling bark of [[paperbark maple]]
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| − | [[Category:Plant anatomy]]
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| − | [[Category:Plant morphology]]
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