Plant stem

• 1. Introduction

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  A plant stem holds a leaf up into the light, then quietly moves water from the roots below to the shoots above. It is one of two main structural axes of a vascular plant, the other being the root. The stem supports leaves, flowers, and fruits. It transports water and dissolved substances, engages in photosynthesis, stores nutrients, and produces new living tissue. Depending on the plant and the language used, the same organ might be called a culm, a halm, a haulm, a stalk, or a thyrsus. The woody stem of a tree carries a name everyone knows: the trunk. How does a single structure manage so many jobs at once? Why does the stem of a banana plant differ so completely from the stem of an oak, or a fern, or a potato hidden in the soil? And how did this one organ end up giving people sugar, paper, rubber, medicine, and jewelry? The answers begin with a simple grid of points and the gaps between them.

• 2. Nodes And Internodes

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  Nodes are the points where leaves attach, and a single node can hold one or more leaves. Between the stem and a leaf there are sometimes axillary buds, which can grow into branches bearing leaves, conifer cones, or flowers. Nodes can also produce adventitious roots, such as brace roots, and in vines they may sprout tendrils. Internodes are the intervals that separate one node from the next, spacing them apart along the stem. An internode can elongate, either from its base or from its tip, depending on the species. The word shoots is often confused with stems, but shoots generally means fresh new plant growth, including stems together with leaves or flowers. Most stems sit above the soil surface, yet some plants keep their stems underground. That hidden geometry sets up the next question: what flows through the stem, and along which channels?

• 3. Xylem And Phloem

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  Two pipe-like tissues run through every stem, and they pull in opposite directions. The xylem arises from the cell facing inside and carries water, moved by transpiration pull, capillary action, and root pressure. The phloem arises from the cell facing outside and is built from sieve tubes and their companion cells. Phloem distributes food made in photosynthetic tissue out to the rest of the plant. Between these two systems sits the cambium, a tissue that divides to form new xylem or phloem cells. The work these tissues support is constant, because the normal lifespan of plant cells is only one to three years. To keep going, stems carry meristems, cells that generate new living tissue every year. That annual renewal is also what lets a stem thicken, and thickening is where the deeper architecture of the stem reveals itself.

• 4. Three Tissues

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  Dermal tissue covers the outer surface of the stem, protecting it and controlling gas exchange, with epidermal cells as its predominant form. Ground tissue lies beneath, made mainly of parenchyma, collenchyma, and sclerenchyma cells, and it surrounds the vascular tissue. Ground tissue aids metabolic activities such as respiration, photosynthesis, transport, and storage, while also giving structural support and forming new meristems. In woody stems, most or all of that ground tissue may be lost. Vascular tissue, made of xylem, phloem, and cambium, handles long distance transport of water, minerals, and metabolites like sugars and amino acids. The arrangement of these vascular tissues varies widely from one plant species to another. That variation is sharpest when you compare the great divisions of plants, beginning with the dicots.

• 5. How Different Plants Build

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  Dicot stems with primary growth keep pith in the center, with vascular bundles forming a distinct ring visible in cross section. An epidermis covers the outside, sealed by a waterproof cuticle and dotted with stomata and multicellular hairs called trichomes. Woody dicots and many nonwoody dicots add secondary growth from two lateral meristems, the vascular cambium and the cork cambium, or phellogen. As the vascular cambium produces secondary xylem inward and secondary phloem outward, the cortex and epidermis are eventually destroyed, replaced by a periderm whose loosely packed gas-exchange zones are called lenticels. Monocot stems take a different path, scattering vascular bundles throughout the stem rather than in a ring, with the shoot apex more elongated and protected by leaf sheaths. Monocots rarely produce secondary growth and are seldom woody, with palms and bamboo as notable exceptions. Gymnosperms are all woody plants, though most produce only tracheids in their xylem rather than the vessels found in dicots, and their wood often contains resin ducts. Woody dicots like oak, maple, and walnut are called hardwoods, while softwoods are gymnosperms such as pine, spruce, and fir. Ferns break the pattern again: most have rhizomes with no vertical stem, the exception being tree ferns, whose vertical stems can grow up to about 20 metres. A fern stem often has one or more leaf gaps in cross section, where vascular tissue branches off to a frond and the cylinder stays incomplete. That incompleteness is part of why fern anatomy, with its solenosteles and dictyosteles, is more complicated than that of dicots.

• 6. Stems In Disguise

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  Acaulescent plants appear stemless, yet their stems are simply extremely short, so the leaves seem to rise straight from the ground, as in some Viola species. A bulb is a short vertical underground stem with fleshy storage leaves attached, seen in onion, daffodil, and tulip, often splitting to form new bulbs or small bulblets. A corm is a short enlarged underground storage stem, as in taro, crocus, and gladiolus, while a tuber is a swollen underground storage stem adapted for storage and reproduction, like the potato. A cladode, or phylloclade, is a flattened stem that looks leaf-like and does the work of photosynthesis, as in cactus pads. A rhizome runs horizontally underground for reproduction and storage, found in most ferns and in iris, while a stolon runs horizontally near the surface, producing rooted plantlets, and a runner is a stolon that roots at its nodes, as in the garden strawberry and Chlorophytum comosum. A pseudostem is a false stem made of rolled leaf bases that can stand 2 to 3 metres tall, as in banana. The stem can also turn defensive, sharpening its outer layers into a prickle, as with rose thorns, or forming a thorn, a modified stem with a sharpened point. Even the wood of a trunk divides into roles: sapwood is the outer secondary xylem still active in fluid transport with live parenchyma cells, while heartwood is the inner secondary xylem that no longer transports fluid, darker from deposited and oxidized secondary metabolites such as polyphenolic compounds. These disguises matter beyond botany, because some of these very stems became staples of the human diet and economy.

• 7. What Stems Give Us

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  Thousands of species have stems with economic uses, starting with food. Potato and taro are staple crops drawn from stems, sugarcane stems are a major source of sugar, and maple sugar comes from the trunks of maple trees. Vegetables harvested from stems include asparagus, bamboo shoots, cactus pads or nopalitos, kohlrabi, and water chestnut, while the spice cinnamon is bark taken from a tree trunk. Stems also supply medicine: quinine from the bark of cinchona trees, camphor distilled from the wood of a tree in the same genus that gives cinnamon, and the muscle relaxant curare from the bark of tropical vines. Wood from secondary xylem runs through daily life, from buildings, furniture, and boats to musical instruments, matches, toothpicks, and coffins, while wood pulp becomes paper, cellophane, and plastics and textiles like cellulose acetate and rayon. The natural world keeps its own records in these tissues, since the seasonal growth of the vascular cambium creates yearly tree rings in temperate climates. Those rings are the basis of dendrochronology, which dates wooden objects, and of dendroclimatology, which reads them as a record of past climates. From stems also come cork from the bark of the cork oak, rubber from the trunks of Hevea brasiliensis, and tannins for leather from woods such as quebracho. The earliest known paper was made from the stems of papyrus by the ancient Egyptians, and amber, fossilized sap from tree trunks, can still hold preserved animals inside it.