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Plant

Plant

A plant is any organism in the kingdom Plantae. Kingdoms are the main divisions into which scientists classify all living things on Earth. The other kingdoms are: Monera (single-celled organisms without nuclei), Protista (single-celled organisms with a nucleus), Fungi, and Animalia (animals). The scientific study of plants is called botany.

A general definition of a plant is any organism that contains chlorophyll (a green pigment contained in a specialized cell called a chloroplast) and can manufacture its own food. Another characteristic of plants is that their rigid cell walls are composed mainly of cellulose, a complex carbohydrate that is insoluble (cannot be dissolved) in water. Because of the vast number of plants that exist, cellulose is the most abundant organic compound on Earth. Biologists have identified about 500,000 species of plants, although there are many undiscovered species, especially in tropical rain forests.

Plant structure

Those plants that produce seeds are the dominant and most studied group of plants on the planet. The leaves of these plants are all covered with a cuticle, a waxy layer that inhibits water loss. The leaves have stomata, microscopic pores, that open during the day to take in carbon dioxide and release oxygen during photosynthesis (process by which sunlight is used to form carbohydrates from carbon dioxide and water, releasing oxygen as a by-product).

Words to Know

Carbohydrate: A compound consisting of carbon, hydrogen, and oxygen found in plants and used as a food by humans and other animals.

Chlorophyll: Green pigment found in chloroplasts that absorbs sunlight, providing the energy used in photosynthesis.

Chloroplasts: Small structures in plant cells that contain chlorophyll and in which the process of photosynthesis takes place.

Meristem: Special plant tissues that contain actively growing and dividing cells.

Phloem: Plant tissue consisting of elongated cells that transport carbohydrates and other nutrients.

Photosynthesis: Process by which sunlight is used by plants to form carbohydrates from carbon dioxide and water, releasing oxygen as a by-product.

Stomata: Pores in the surface of leaves.

Transpiration: Evaporation of water in the form of water vapor from the stomata.

Xylem: Plant tissue consisting of elongated cells that transport water and mineral nutrients.

Leaves are connected to the stem by veins, which transport water and nutrients throughout the plant. There are two special types of cells in this vascular system (the vessels that carry water and nutrients): xylem and phloem. Xylem (pronounced ZEYE-lem) are mainly responsible for the movement of water and minerals from the roots to the stems and leaves. Phloem (pronounced FLOW-em) are mainly responsible for the transport of food, principally carbohydrates produced by photosynthesis, from the leaves throughout the plant. The vascular system of plants differs from the circulatory system of animals in that water (in the form of vapor) evaporates out of a plant's stomata (a process called transpiration), whereas an animal's blood is recirculated throughout the body.

The roots of a plant take up water and minerals from the soil, and also anchor the plant. Most plants have a dense, fibrous network of roots, and this provides a large surface area for the uptake of water and minerals.

Plant development

As a plant grows, it undergoes developmental changes. Most plants continually produce new sets of organs, such as leaves, flowers, and fruits. In contrast, animals typically develop their organs only once and these organs merely increase in size as the animal grows.

A plant begins its life as a seed. Various environmental cues such as sunlight, temperature changes, and the presence of nutrients signal a seed to germinate (grow). During early germination, the young seedling depends upon nutrients stored within the seed itself for growth. As the seedling grows, it begins to produce chlorophyll and turn green. Most plants become green only when exposed to sunlight because the production of chlorophyll is light-induced.

In contrast to animals, whose bodies grow all over as they develop, plants generally grow in specific regions, referred to as meristems. A

meristem is a special tissue that contains actively growing and dividing cells. Apical meristems are at the tips of shoots and roots and are responsible for elongation of a plant. Lateral meristems are located along the outer sides of the stem of a plant and are responsible for thickening of the plant.

Plant diseases

Plant diseases can be infectious (transmitted from plant to plant) or noninfectious. Noninfectious diseases are usually referred to as disorders. Common plant disorders are caused by a shortage of plant nutrients, by waterlogged or polluted soil, and by polluted air. Too little (or too much) water or improper nutrition can cause plants to grow poorly. Plants can also be stressed by weather that is too hot or too cold, by too little or too much light, and by heavy winds. Pollution from automobiles and industry and the excessive use of herbicides (to kill weeds) can also cause noninfectious plant disorders.

Infectious plant diseases are caused by living microorganisms that infect a plant and rob it of nutrients. Bacteria, fungi, and viruses are the living agents that cause plant diseases. None of these microorganisms are visible to the naked eye, but the diseases they cause can be detected by the symptoms of wilting, yellowing, stunting, and abnormal growth patterns.

Some plant diseases are caused by rod-shaped bacteria. The bacteria enter the plant through natural openings, like the stomata of the leaves, or through wounds in the plant tissue. Once inside, the bacteria plug up the plant's vascular system and cause the plant to wilt. Other common symptoms of bacterial disease include rotting and swollen plant tissues. Bacteria can be spread by water, insects, infected soil, or contaminated tools.

About 80 percent of plant diseases can be traced to fungi, which can grow on living or dead plant tissue. They can penetrate plant tissue or grow on the plant's surface. Fungal spores, which act like seeds, are spread by wind, water, soil, and animals to other plants. Warm, humid conditions promote fungal growth.

Viruses are the hardest pathogens (disease-causing organisms) to control. Destroying the infected plants to prevent spreading to healthy plants is usually the best control method. While more than 300 plant viruses have been identified, new strains continually appear because these organisms are capable of mutating (changing their genetic makeup). Viruses are spread by contaminated seeds and sucking insects (aphids, leafhoppers, thrips) that act as carriers of the virus. The symptoms of viral infection include yellowing, stunted growth in some part of the plant. Leaf rolls and narrow leaf growth are other indications of viral infection. The mosaic viruses can infect many plants. Plants infected with this virus have mottled or streaked leaves.

Scientists complete first planet genetic sequence

In the nineteenth century, Austrian botanist Gregor Mendel (18221884) started the science of genetics when he studied the genetic characteristics

of pea plants. Over 100 years later, in late 2000, scientists from the United States, Europe, and Japan determined the first complete genetic sequence of a plant. Fellow scientists hailed the accomplishment, saying it would deepen understanding of plant biology and provide new ways to engineer crops genetically to increase food production and improve nutrition. The planet, commonly called thale cress, is a small weed that is related to the mustard plant. It is worthless as a crop. However, like a laboratory mouse, it is being studied for insights that can be applied to virtually all other plants. As a matter of fact, scientists are testing genes found in the plant to make other plants flower more quickly, to keep fruits from ripening too early, and to produce healthier vegetable oils. Scientists have already identified 100 genes in the thale cress that can be used to design new herbicides.

[See also Cellulose; Flower; Photosynthesis; Phototropism; Seeds ]

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plant

plant, any organism of the plant kingdom, as opposed to one of the animal kingdom or of the kingdoms Fungi, Protista, or Monera in the five-kingdom system of classification. (A more recent system, suggested by genetic sequencing studies, places plants with animals and some other forms in an overarching group, the eukarya, to distinguish them from the prokaryotic bacteria and archaea, or ancient bacteria.) A plant may be microscopic in size and simple in structure, as are certain one-celled algae, or a gigantic, many-celled complex system, such as a tree.

Plants are generally distinguished from animals in that they possess chlorophyll, are usually fixed in one place, have no nervous system or sensory organs and hence respond slowly to stimuli, and have rigid supporting cell walls containing cellulose. In addition, plants grow continually throughout life and have no maximum size or characteristic form in the adult, as do animals. In higher plants the meristem tissues in the root and stem tips, in the buds, and in the cambium are areas of active growth. Plants also differ from animals in the internal structure of the cell and in certain details of reproduction (see mitosis).

There are exceptions to these basic differences: some unicellular plants (e.g., Euglena) and plant reproductive cells are motile; certain plants (e.g., Mimosa pudica, the sensitive plant) respond quickly to stimuli; and some lower plants do not have cellulose cell walls, while the animal tunicates (e.g., the sea squirt) do produce a celluloselike substance.

The Plant Kingdom

The systems of classification of the plant kingdom vary in naming and placing the larger categories (even the divisions) because there is little reliable fossil evidence, as there is in the case of animals, to establish the true evolutionary relationships of and distances between these groups. However, comparisons of nucleic acid sequences in plants are now serving to clarify such relationships among plants as well as other organisms.

A widely held view of plant evolution is that the ancestors of land plants were primitive algae that made their way from the ocean to freshwater, where they inhabited alternately wet-and-dry shoreline environments, eventually giving rise to such later forms as the liverworts and mosses. From some remote fern ancestor, in turn, arose the seed plants.

The plant kingdom traditionally was divided into two large groups, or subkingdoms, based chiefly on reproductive structure. These are the thallophytes (subkingdom Thallobionta), which do not form embryos, and the embryophytes (subkingdom Embryobionta), which do. All embryophytes and most thallophytes have a life cycle in which there are two alternating generations (see reproduction). The plant form of the thallophytes is an undifferentiated thallus lacking true roots, stems, and leaves. The subkingdom Thallobionta is composed of more than 10 divisions of algae and fungi (once considered plants). The subkingdom Embryobionta is composed of two groups: the bryophytes (liverwort and moss), division Bryophyta, which have no vascular tissues, and a group consisting of seven divisions of plants that do have vascular tissues. The Bryophyta, like other nonvascular plants, are simple in structure and lack true roots, stems, and leaves; they therefore usually live in moist places or in water.

The vascular plants have true roots, stems, and leaves and a well-developed vascular system composed of xylem and phloem for transporting water and food throughout the plant; they are therefore able to inhabit land. Three of the divisions of the vascular plants are currently represented by only a very few species. They are the Psilotophyta, with only three living species; the Lycopodiophyta (club mosses); and the Equisetophyta (horsetails). All the plants of a fourth subdivision, the Rhyniophyta, are extinct. The remaining divisions include the dominant vegetation of the earth today: the ferns (see Polypodiophyta), the cone-bearing gymnosperms (see Pinophyta), and the angiosperms, or true flowering plants (see Magnoliophyta). The latter two classes, because they both bear seeds, are often collectively called spermatophytes, or seed plants.

The gymnosperms are all woody perennial plants and include several orders, of which most important are the conifer, the ginkgo, and the cycad. The angiosperms are separated into the monocotyledonous plants—usually with one cotyledon per seed, scattered vascular bundles in the stem, little or no cambium, and parallel veins in the leaf—and the dicotyledonous plants—which as a rule have two cotyledons per seed, cylindrical vascular bundles in a regular pattern, a cambium, and net-veined leaves. There are some 50,000 species of monocotyledon, including the grasses (e.g., bamboo and such cereals as corn, rice, and wheat), cattails, lilies, bananas, and orchids. The dicotyledons contain nearly 200,000 species of plant, from tiny herbs to great trees; this enormously varied group includes the majority of plants cultivated as ornamentals and for vegetables and fruit.

Importance of Plants

Plants are essential to the balance of nature and in people's lives. Green plants, i.e., those possessing chlorophyll, manufacture their own food and give off oxygen in the process called photosynthesis, in which water and carbon dioxide are combined by the energy of light. Plants are the ultimate source of food and metabolic energy for nearly all animals, which cannot manufacture their own food. Besides foods (e.g., grains, fruits, and vegetables), plant products vital to humans include wood and wood products, fibers, drugs, oils, latex, pigments, and resins. Coal and petroleum are fossil substances of plant origin. Thus plants provide people not only sustenance but shelter, clothing, medicines, fuels, and the raw materials from which innumerable other products are made.

Plant Studies

The scientific study of plants is called botany; the study of their relationship to their environment and of their distribution is plant ecology. The cultivation of plants for food and for decoration is horticulture. For specific approaches to the study of plants and animals, see biology.

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Plant

Plant

Plants (of the kingdom Plantae) are multicellular, eukaryotic organisms that develop from an embryo and that have cell walls and chloroplasts. Plants are distinguished from algae (from which they are descended) by a higher degree of multicellular complexity and from fungi by the ability to photosynthesize (those few plants that have lost this ability evolved from others that could).

Characteristics of Plants

Almost all plants live on land and have adapted to the conditions on land through the development of a waxy cuticle to prevent drying out, structures to absorb and transport water throughout their bodies (the bryophytes are an exception), and rigid internal support to remain erect without the buoyancy available in water. This rigidity is provided in large part by the cell wall, which is composed of cellulose , a complex carbohydrate , and lignin , a phenolic compound that stiffens the cellulose fibers.

The plant life cycle has two distinct multicellular phases: a haploid phase (in which chromosomes are present only as single copies) and a diploid phase (in which chromosomes are present in pairs). The haploid organism produces gametes that fuse to form an embryo, which develops into the diploid organism. The diploid organism produces haploid spores that germinate to form the haploid organism. This "alternation of generations" is found only in plants and some algae.

Almost all plants photosynthesize, using the sun's energy to power the production of sugar from carbon dioxide and water. Photosynthesis occurs in chloroplasts, membrane-bound organelles that contain the green pigment chlorophyll. Chloroplasts are descended from free-living photosynthetic bacteria that became symbiotic partners of ancient single-celled plant ancestors. Evidence of the chloroplast's bacterial origin is found in the presence of deoxyribonucleic acid (DNA) within it, as well as its size and structure.

The photosynthetic production of sugars by plants is the basis for all terrestrial food chains. Photosynthesis also produces oxygen, needed by animals, fungi, and other organisms (including plants themselves) to release the stored energy in those sugars.

Diversity

Plants are classified into twelve phyla (sometimes called divisions) in two major groups. The bryophytes are the most primitive group, lacking vascular tissues for the transport of water. There are three phyla of bryophytesthe mosses, liverworts, and hornwortsthat together comprise about 24,000 species. In contrast, plants in the second group, the tracheophytes, have well-developed vascular systems. The tracheophytes contain nine phyla and are divided into two groups: those without seeds and those with them. Ferns, which reproduce without seeds, contain approximately 13,000 species. Three other phyla of seedless vascular plants (Psilophyta, Lycopodophyta, and Equisetophyta) together include just over 1,000 species.

Seeds are structures that contain an embryo and food reserves wrapped in a protective seed coat. In the gymnosperms , the seed develops on structures exposed to the environment. Gymnosperms include Ginkophyta, which contains only one species, Ginkgo biloba ; Cycadophyta (220 species); Gnetophyta (68 species); and Coniferophyta (588 species). Conifers bear seeds in cones and include many familiar needle-bearing evergreens, such as pine, spruce, and fir. Anthophyta, or angiosperms, enclose their seeds within ovaries. The angiosperms are the flowering plants and are the most diverse of all plant phyla, with about 235,000 species.

see also Algae; Alternation of Generations; Angiosperms; Biodiversity; Bryophytes; Fungi; Gymnosperms; Pteridophytes; Seedless Vascular Plants

Richard Robinson

Bibliography

Raven, Peter, Ray F. Evert, and Susan E. Eichhorn. Biology of Plants, 6th ed. New York: W. H. Freeman and Company, 1999.

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plant

plant / plant/ • n. 1. a living organism of the kind exemplified by trees, shrubs, herbs, grasses, ferns, and mosses, typically growing in a permanent site, absorbing water and inorganic substances through its roots, and synthesizing nutrients in its leaves by photosynthesis using the green pigment chlorophyll. ∎  a small organism of this kind, as distinct from a shrub or tree: garden plants. 2. a place where an industrial or manufacturing process takes place: the company has 30 plants in Mexico. ∎  machinery used in an industrial or manufacturing process: inadequate investment in new plant. ∎  any system that is analyzed and controlled, e.g., the dynamic equations of an aircraft or the equations governing chemical processes. 3. a person placed in a group as a spy or informer: we thought he was a CIA plant spreading disinformation. ∎  a thing put among someone's belongings to incriminate or compromise them: he insisted that the cocaine in the glove compartment was a plant. • v. [tr.] 1. place (a seed, bulb, or plant) in the ground so that it can grow. ∎  place a seed, bulb, or plant in (a place) to grow: the garden is planted with herbs. ∎ inf. bury (someone). 2. [tr.] place or fix in a specified position: she planted a kiss on his cheek. ∎  (plant oneself) position oneself: she planted herself on the arm of his chair. ∎  establish (an idea) in someone's mind: the seed of doubt is planted in his mind. ∎  secretly place (a bomb that is set to go off at a later time). ∎  put or hide (something) among someone's belongings to compromise or incriminate the owner: he planted drugs on him to extort a bribe. ∎  send (someone) to join a group or organization to act as a spy or informer. ∎  found or establish (a colony, city, or community). ∎  deposit (young fish, spawn, oysters, etc.) in a river or lake. PHRASES: have (or keep) one's feet firmly planted on the ground be (or remain) level-headed and sensible.DERIVATIVES: plant·a·ble adj. plant·let / -lit/ n. plant·like / -ˌlīk/ adj.

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plant

plant Multicellular organism whose cells have cellulose cell walls and contain chloroplasts or similar structures (plastids). They develop from diploid embryos, and have a regular alternation of haploid and diploid generations in their life cycle. Most plants are green and make their own food by photosynthesis. A few are colourless parasites or saprophytes. Simple plants reproduce by means of spores, while more advanced plants produce seeds and fruits. Plants show a wide range of biochemistry; some produce chemicals such as alkaloids, narcotics, and even cyanide; others secrete substances into the soil to prevent other plants growing near them. Many of these chemicals form the bases for the development of drugs. Plants are classified on the basis of their morphology (shape and structure). The most important phyla are: the Bryophyta (bryophytes), which include the mosses and liverworts; Lycopodophyta, or club mosses; Sphenophyta (horsetails); Filicinophyta (ferns); Cycadophyta (cycads); Ginkgophyta (ginkgo); Coniferophyta (conifers); and Angiospermophyta (angiosperms). See also alternation of generations

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plant

plant Any living organism of the kingdom Plantae. Plants are distinguished from other organisms by their life cycles, in which haploid male and female organisms develop from spores produced by meiosis in the adult diploid organisms and produce gametes by mitosis; fertilization results in a diploid embryo that undergoes its early development in the haploid female. Most plants manufacture carbohydrates by photosynthesis, in which simple inorganic substances are built up into organic compounds. The radiant energy needed for this process is absorbed by chlorophyll, a complex pigment not found in animals. Plants also differ from animals in the possession of cell walls (usually composed of cellulose). Plants are immobile, as there is no necessity to search for food, and they respond slowly to external stimuli. For a classification of the plant kingdom, see see plant kingdom.

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"plant." A Dictionary of Biology. . Encyclopedia.com. 21 Sep. 2017 <http://www.encyclopedia.com>.

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plant

plant young tree or herb newly planted or intended for planting (OE.), XIV; member of the vegetable kingdom XVI. OE. plante, if it survived, coalesced in ME. with — (O)F. plante :- Rom. use of L. planta shoot for planting (whence the OE. word), prob. f. plantāre, perh. orig. thrust in with the sole of the foot (planta), whence plant vb. OE. plantian, reinforced in ME. from (O)F. planter.
So plantation XV. — F. or L.

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plant

plantant, Brabant, Brandt, brant, cant, enceinte, extant, gallant, Kant, levant, pant, pointe, pointes, rant, scant •confidant • commandant • hierophant •Rembrandt • Amirante •gallivant •aren't, aslant, aunt, can't, chant, courante, détente, enchant, entente, grant, implant, Nantes, plant, shan't, slant, supplant, transplant, underplant •plainchant • ashplant • eggplant •house plant • restaurant •debutant, debutante •absent, accent, anent, ascent, assent, augment, bent, cement, cent, circumvent, consent, content, dent, event, extent, ferment, foment, forewent, forwent, frequent, gent, Ghent, Gwent, lament, leant, lent, meant, misrepresent, misspent, outwent, pent, percent, pigment, rent, scent, segment, sent, spent, stent, Stoke-on-Trent, Tashkent, tent, torment, Trent, underspent, underwent, vent, went •orient • comment • portent •malcontent

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