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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

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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Plant

Plant

Plant evolution and classification

Evolution of plants

Classification of plants

Plant structure

Plant development

Characteristics of plant cells

Cell structures and their functions

Importance to humans

Resources

A plant is an organism in the kingdom Plantae. According to the five-kingdom classification system used by most biologists, plants have the following characteristics: they are multicellular during part of their life; they are eukaryotic, in that their cells have nuclei; they reproduce sexually; they have chloroplasts with chlorophyll-a, chlorophyll-b and carotenoids as photosynthetic pigments; they have cell walls with cellulose, a complex carbohydrate; they have life cycles with an alternation of a sporophyte phase and

a gametophyte phase; they develop organs which become specialized for photosynthesis, reproduction, or mineral uptake; and most live on land during their life cycle.

Biologists have identified about 500,000 species of plants, although there are many undiscovered species in the tropics.

Plant evolution and classification

From the time of Aristotle until the 1950s, most people classified all organisms into the animal kingdom or the plant kingdom. Fungi and plant like, single-celled organisms were placed into the plant kingdom, in view of certain highly derived, but superficial characteristics of these organisms.

In 1959, Robert Whittaker advocated a five-kingdom classification system. According to a recent modification of that system, the five kingdoms are: Monera (single-celled, prokaryotic organisms, such as bacteria); Protista (various eukaryotic groups, such as algae and water molds); Fungi (spore-forming eukaryotes which lack flagella, such as mushrooms and various molds); Animalia (various multicellular eukaryotic groups, such as jellyfish and vertebrates); and Plantae, or plants.

Biologists now recognize an additional kingdom of prokaryotes, the Archaebacteria or ancient bacteria, which have unique characteristics that distinguish them from Eubacteria, or true bacteria in the kingdom Monera. The evolutionary relationships of Eukaryotes, Archaebacteria, and Eubacteria are uncertain at the present time. Undoubtedly, as our knowledge of evolution and biological diversity increases, Whittakers five kingdom classification system will require further modification.

Evolution of plants

There was little life on land 500 million years ago, although the oceans abounded with diverse photosynthetic organisms, as well as species in the Monera, Protoctista, and Animalia kingdoms. Land plants appear to have evolved from photosynthetic, aquatic ancestors about 500 million years ago, probably from the Chlorophyta, or green algae. Both groups use chlorophyll-a and chlorophyll-b as photosynthetic pigments, store their energy reserves as starch, and have cellulose in their cell walls.

The evolution of the terrestrial habit required special adaptations of reproductive and vegetative tissues for protection against desiccation. The most significant adaptation of the reproductive tissues is enclosure of the sex cells (egg and sperm) within specialized tissues, and retention of the fertilized egg as it develops into a multicellular embryo. The most significant adaptation of the vegetative tissue is development of a parenchymatous cell organization, in which unspecialized cells (parenchyma) are embedded in a dense matrix of cells. This reduces water loss by reducing the overall surface area of the plant per cell, and also provides the plant with a body matrix for differentiation of specialized tissues.

The life cycle of all plants consists of an alternation of generations, in which a haploid gametophyte (tissue in which each cell has one copy of each chromosome) alternates with a diploid sporophyte (tissue in which each cell has two copies of each chromosome). A major trend in plant evolution has been the increasing dominance of the sporophyte. Chlorophyta (green algae), the ancestors of land plants, have a dominant gametophyte and greatly reduced sporophyte. Bryophyta, the most primitive land plants, have a more elaborate sporophyte than Chlorophyta, although their gametophyte is still dominant. Free-sporing vascular plants (Filicinophyta, Lycopodophyta, and Sphenophyta) have a somewhat more dominant sporophyte phase than gametophyte phase. However, seed plants, the most advanced of the land plants, have a greatly reduced gametophyte, and a dominant sporophyte.

Classification of plants

All species are classified hierarchically. Related species are grouped into a genus; related genera into a family; related families into an order; related orders into a class; related classes into a phylum; and related phyla into a kingdom. Below, the most significant characteristics of the nine phyla of the kingdom Plantae are briefly considered.

Bryophyta is a phylum with three classes, the largest of which is the mosses, with about 15,000 species. The gametophyte phase is dominant, and in mosses this is the familiar, small, green, leafy plant. Bryophytes do not have true leaves, stems, or roots, and they lack a vascular system for transporting food and water. They reproduce by making spores, and are mostly found in bogs or moist woodlands, so their sperm can swim through water to reach the eggs. Mosses are particularly prominent in the northern boreal forest and arctic and alpine tundra.

The Lycopodophyta is a phylum with about 1,000 species. The sporophyte phase is dominant, and is the familiar, low-growing, green plant in many species which superficially resembles the branch of a pine. Their leaves are tiny structures, termed microphylls, and are arranged in whorls on the stem. The stems of lycopods and all subsequent phyla have vascular tissues for efficient transport of food and water. Like bryophytes, they reproduce by making spores, and are mostly found in wet areas so their sperm can swim to reach the eggs. Lycopods are most abundant in the tropics, although numerous species of Lycopodium (ground pine) grow in woodlands in the temperate zone.

The Sphenophyta has a single genus, Equisetum, with about 10 species. Equisetum is commonly called horsetail, because the dominant sporophyte phase of these plants superficially resembles a horses tail. It is an erect stem, with whorls of microphylls, and a spore-producing, cone-like structure, termed a strobilus, on top. Horsetails are mostly found in moist woodlands of the temperate zone, since their sperm must swim to reach the eggs.

The Filicinophyta has about 11,000 species, which are known commonly as ferns. The sporophyte phase is dominant, and is the more familiar form of ferns that is commonly seen in temperate-zone woodlands. Like the leaves of all subsequent phyla, those of ferns have a complex system of branched veins, and are referred to as megaphylls. Ferns reproduce by making spores, and they are mostly restricted to moist environments so their sperm can swim to reach the eggs. Most species occur in tropical and subtropical ecosystems.

The Cycadophyta has about 200 species, which are known commonly as cycads. Like all subsequent phyla, cycads are seed-producing plants. They are considered gymnosperms, because they bear their seeds naked on specialized leaves called sporophylls. The sporophyte phase is dominant, and appears rather like a shrublike palm in many species, although cycads are only distantly related to palms. Cycads have flagellated sperm which swim to fertilize the eggs, a characteristic of evolutionarily primitive, free-sporing plants (all phyla above), but not of other seed plants (except for Ginkgo, see below). Cycads grow in tropical and subtropical regions of the world.

The Ginkgophyta consists of a single species, Ginkgo biloba, a gymnosperm which bears its seeds in green, fruit-like structures. The sporophyte phase of Ginkgo is dominant, and is a tree with fan-shaped leaves that arise from spurs on the branches. Like the cycads, Ginkgo has flagellated sperm that swim to fertilize the eggs. Ginkgo only exists in cultivation, and is widely planted as an ornamental tree throughout the United States and other temperate countries.

The Coniferophyta has about 600 species, and includes familiar evergreen trees such as pines, spruces, and firs. The conifers are the best known and most abundant of the gymnosperms. The sporophyte phase is dominant, and is the familiar cone-bearing tree. Male reproductive structures produce pollen grains, or male gametophytes, which travel by wind to the female reproductive structures. The pollen fertilizes the ovules to produce seeds, which then develop within characteristic cones. Conifers grow throughout the world, and are dominant trees in many northern forests. Many conifers are used for lumber, paper, and other important products.

The Gnetophyta is a phylum of unusual gymnosperms, with about 70 species in three genera, Gnetum, Ephedra, and Welwitschia. These three genera differ significantly from one another in their vegetative and reproductive structures, although all are semi-desert plants. The mode of fertilization of species in the Ephedra genus resembles that of the Angiospermophyta (flowering plants), and many botanists consider them to be close relatives.

The Angiospermophyta is the largest and most important plant phylum, with at least 300,000 species. All species reproduce by making flowers, which develop into fruits with seeds upon fertilization. The flower originated about 130 million years ago, as a structure adapted to protect the ovules (immature seeds), which are born naked and unprotected in the more primitive gymnosperms. The highly specialized characteristics of many flowers evolved to facilitate pollination. There are two natural groups of angiosperms, the monocots, whose seeds have one cotyledon (or seed-leaf), and the dicots, whose seeds have two cotyledons. Nearly all of the plant foods of humans and many drugs and other economically important products come from angiosperms.

A recent scientific effort has created new theories about the classification of plants. Many genetic experiments were performed by plant biologists around the world in an effort to answer questions of the evolution of plants as a single large group of organisms. Some startling, and controversial results were attained just before the turn of the new century. In 1999, the group of scientists concluded that the kingdom Plantae should, in fact, be split into at least three separate kingdoms because the group is so highly diverse and the genetic evidence gathered indicated sufficient divergence among members. Also, the studies uncovered that the three proposed kingdoms each formed from a single plant-like ancestor that colonized land, not directly from the sea as was previously thought, but from fresh water. These ideas have yet to be accepted by the majority of biologists, and remain a matter of debate.

Plant structure

The seed plants (gymnosperms and angiosperms) are the dominant and most studied group of plants, so their anatomy and development are considered here. The leaves and other aerial portions are all covered with a cuticle, a waxy layer that inhibits water loss. The leaves have stomata, microscopic pores which open in response to certain environmental cues for uptake of carbon dioxide and release of oxygen during photosyn-thesis. Leaves have veins, which connect them to the stem through a vascular system which is used for transport of water and nutrients throughout the plant.

There are two special types of cells in the vascular system, xylem and phloem. Xylem is mainly responsible for the movement of water and minerals from the roots to the aerial portions, the stems and leaves. Phloem is mainly responsible for the transport of food, principally carbohydrates produced by photo-synthesis, from the leaves throughout the plant. The vascular system of plants differs from the circulatory system of animals in that water moves out of a plants leaves by transpiration, whereas an animals 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 uptake of water and minerals. Mycorrhizae are symbioses between fungi and most plant roots and are important for water and mineral uptake in most plants. The fungal partner benefits by receiving carbohydrates from the plant, which benefits by being better able to absorb minerals and water from the soil. Mycorrhizae form on the roots of nearly all land plants, and many biologists believe they played a vital role in the evolution of the terrestrial habit.

Plant development

As a plant grows, it undergoes developmental changes, known as morphogenesis, which include the formation of specialized tissues and organs. Most plants continually produce new sets of organs, such as leaves, flowers, and fruits, as they grow. In contrast, animals typically develop their organs only once, and these organs merely increase in size as the animal grows. The meristematic tissues of plants (see below) have the capacity for cell division and development of new and complex tissues and organs, even in older plants. Most of the developmental changes of plants are mediated by hormonal and other chemical changes, which selectively alter the levels of expression of specific genes.

A plant begins its life as a seed, a quiescent stage in which the metabolic rate is greatly reduced. Various environmental cues such as light, temperature changes, or nutrient availability, signal a seed to germinate. During early germination, the young seedling depends upon nutrients stored within the seed itself for growth.

As the seedling grows, it begins to synthesize chlorophyll and turn green. Most plants become green only when exposed to sunlight, because chlorophyll synthesis is light-induced. As plants grow larger, new organs develop according to certain environmental cues and genetic programs of the individual.

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 containing undifferentiated, 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 parallel to the elongation axis of the shoots and roots, and are responsible for thickening of the plant. Differences in apical meristems give different species their unique leaf arrangements; differences in lateral meristems give different species their unique stems and bark.

Many of the morphogenetic changes of developing plants are mediated by hormoneschemical messengers that are active in very small concentrations. The major plant hormones are auxins, gibberellins, cytokinins, abscissic acid, and ethylene. Auxins control cell expansion, apical dominance, and fruit growth. Gibberellins control cell expansion, seed germination, and fruit development. Cytokinins promote cell division and organ development, but impede senescence. Abscissic acid can induce dormancy of seeds and buds, and accelerate plant senescence. Ethylene accelerates senescence and fruit ripening, and inhibits stem growth.

Characteristics of plant cells

Like all other organisms, plants are made up of cells, which are semi-autonomous units consisting of protoplasts surrounded by a special layer of lipids and proteins, termed the plasma membrane. Plant cells are all eukaryotic, in that their genetic material (DNA) is sequestered within a nucleus inside the cell, although some DNA also occurs inside plastids and mitochondria (see below). Plant cells have rigid cell walls external to their plasma membrane.

In addition to nuclei, plant cells contain many other small structures, which are specialized for specific functions. Many of these structures are membrane-enclosed, and are referred to as organelles (small organs).

Cell structures and their functions

The cells of plants, fungi, and bacteria are surrounded by rigid cell walls. Plant cell walls are typically one to five micrometers thick, and their primary constituent is cellulose, a molecule consisting of many glucose units connected end-to-end. In plant cell walls, many cellulose molecules are bundled together into microfibrils (small fibers), like the fibers of a string. These microfibrils have great tensile strength, because the component strands of cellulose are interconnected by hydrogen bonds. The cellulose microfibrils are embedded in a dense, cell-wall matrix consisting of other complex molecules such as hemicellulose, pectic substances, and enzymes and other proteins. Some plant cells become specialized for transport of water or physical support, and these cells develop a secondary wall that is thick and impregnated with lignin, another complex carbohydrate.

All living cells are surrounded by a plasma membrane, a viscous lipid-and-protein matrix which is about 10 nm thick. The plasma membrane of plant cells lies just inside the cell wall, and encloses the rest of the cell, the cytoplasm and nucleus. The plasma membrane regulates transport of various molecules into and out of the cell, and also serves as a sort of two-dimensional scaffolding, upon which many biochemical reactions occur.

The nucleus is often considered to be the control center of a cell. It is typically about 10 micrometers in diameter, and is surrounded by a special double-membrane with numerous pores. The most important molecules in the nucleus are DNA (deoxyribonucleic acid), RNA (ribonucleic acid), and proteins. DNA is a very long molecule, and is physically associated with numerous proteins in plants and other eukaryotes. Specific segments of DNA make up genes, the functional units of heredity which encode specific characteristics of an organism. Genes are connected together into chromosomes, thread-like structures that occur in a characteristic number in each species. Special enzymes within the nucleus use DNA as a template to synthesize RNA. Then, the RNA moves out of the nucleus where it is used as a template for the synthesis of enzymes and other proteins.

Plastids are organelles only present in plants and algae. They have a double membrane on their outside, and are specilized for the storage of starch (amyloplasts), storage of lipids (elaioplasts), photosynthesis (chloroplasts), or other functions. Chloroplasts are the most important type of plastid, and are typically about 10 micrometers in diameter. Chloroplasts are specialized for photosynthesis, the biological conversion of light energy absorbed by chlorophylls, the green leaf pigments, into potential chemical energy such as carbohydrates. Some of the component reactions of photosynthesis occur on special, inner membranes of the chloroplasts, referred to as thylakoids; other reactions occur in the aqueous interior of the chloroplast, referred to as the stroma. Interestingly, plastids are about the size of bacteria and, like bacteria, they also contain a circular loop of DNA. These and many other similarities suggest that cells with chloroplasts originated several billion years ago by symbiogenesis, the union of formerly separate, prokaryotic cells.

Mitochondria are organelles which are present in nearly all living, eukaryotic cells. A mitochondrion has a double membrane on its outside, is typically ovoid or oblong in shape, and is about 0.5 micrometers wide and several micrometers long. Mitochondria are mainly responsible for the controlled oxidation (metabolic breakdown) of high-energy food molecules, such as fats and carbohydrates, and the consequent synthesis of ATP (adenosine triphosphate), the energy source for cells. Many of the mitochondrial enzymes that oxidize food molecules are embedded in special internal membranes of the mitochondria. Like plastids, mitochondria contain a circular loop of DNA, and are believed to have originated by symbiogenesis.

Golgi bodies are organelles present in most eukaryotic cells, and function as biochemical processing centers for many cellular molecules. They appear as a cluster of flattened vesicles, termed cisternae, and associated spherical vesicles. The Golgi bodies process carbohydrates, which are used to synthesize the cell wall, and lipids, which are used to make up the plasma membrane. They also modify many proteins by adding sugar molecules to them, a process referred to as glycosylation.

Vacuoles are fluid-filled vesicles which are separated from the cytoplasm by a special membrane, referred to as a tonoplast. Vacuoles are present in many eukaryotic cells. The vacuoles of many plant cells are very large, and can constitute 90% or more of the total cell volume. The main constituent of vacuoles is water. Depending on the type of cell, vacuoles are specialized for storage of foods, ions, or water-soluble plant pigments.

The endoplasmic reticulum is a complex system of interconnected double membranes, which is distributed throughout most eukaryotic cells. The membranes of the endoplasmic reticulum are often continuous with the plasma membrane, the outer nuclear membrane, the tonoplast, and Golgi bodies. Thus, the endoplasmic reticulum functions as a conduit for chemical communication between different parts of the cell. The endoplasmic reticulum is also a region where many proteins, lipids, and carbohydrates are biochemically modified. Many regions of the endoplasmic reticulum have ribosomes associated with them. Ribosomes are subcellular particles made up of proteins and RNA, and are responsible for synthesis of proteins from information encoded in RNA.

Importance to humans

Plants provide food to humans and all other nonphotosynthetic organisms, either directly or indirectly. Agriculture began about 10,000 years ago in the fertile crescent of the Near East, where people first cultivated wheat and barley. Scientists believe that as people of the fertile crescent gathered wild seeds, they selected for certain genetically determined traits, which made the plants produced from those seeds more suited for cultivation and as foods. For example, most strains of wild wheat bear their seeds on stalks that break off to disperse the mature seeds. As people selected wild wheat plants for food, they unknowingly selected genetic variants in the wild population whose seed stalks did not break off. This trait made it easier to harvest and cultivate wheat, and is a feature of all of our modern varieties of wheat.

The development of agriculture led to enormous development of human cultures, as well as growth in the human population. This, in turn, spurred new technologies in agriculture. One of the more recent agricultural innovations is the Green Revolution, the development of new genetic varieties of crop plants. In the past 20-30 years, many new plant varieties have been developed that are capable of very high yields, surely an advantage to an ever-growing human population.

Nevertheless, the Green Revolution has been criticized by some people. One criticism is that these new crop varieties often require large quantities of fertilizers and other chemicals to attain their high yields, making them unaffordable to the relatively poor farmers of the developing world. Another criticism is that the rush to use these new genetic varieties may hasten the extinction of native varieties of crop plants, which themselves have many valuable, genetically-determined characteristics.

Regardless of ones view of the Green Revolution, it is clear that high-tech agriculture cannot provide a

KEY TERMS

Diploid Nucleus or cell containing two copies of each chromosome, generated by fusion of two haploid nuclei.

Eukaryote A cell whose genetic material is carried on chromosomes inside a nucleus encased in a membrane. Eukaryotic cells also have organelles that perform specific metabolic tasks and are supported by a cytoskeleton which runs through the cytoplasm, giving the cell form and shape.

Gametophyte The haploid, gamete-producing generation in a plants life cycle.

Haploid Nucleus or cell containing one copy of each chromosome.

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

Morphogenesis Developmental changes that occur during growth of an organism, such as formation of specialized tissues and organs.

Prokaryote A cell without a nucleus, considered more primitive than a eukaryote.

Sporophyte The diploid spore-producing generation in a plants life cycle.

Symbiosis A biological relationship between two or more organisms that is mutually beneficial. The relationship is obligate, meaning that the partners cannot successfully live apart in nature.

simple solution to poverty and starvation. Improvements in our crop plants must surely be coupled to advances in politics and diplomacy to ensure that people of the developing nations are fed in the future.

See also Angiosperm; Bryophyte; Mycorrhiza; Plant pigment; Root system.

Resources

BOOKS

Graham, Linda, Lee Wilcox, and Jim Graham. Plant Biology, 2nd ed. Upper Saddle River, NJ: Prentice Hall, 2005.

Gurevitch, Jessica, Samuel M. Scheiner, and Gordon A. Fox. The Ecology of Plants, 3rd ed. Sunderland, MA: Sinauer, 2006.

Lambers, Hans, F. Stuart Chapin III, and Thijs L. Pons. Plant Physiological Ecology. New York: Springer, 2000.

Margulis, Lynn, and Karlene V. Schwartz. Five Kingdoms, 3rd ed. San Francisco: W. H. Freeman and Company, 1998.

PERIODICALS

Palmer, J. D., et al. Dynamic Evolution of Plant Mitochondrial Genomes: Mobile Genes and Introns and Highly Variable Mutation Rates. Proceedings of the National Academy of Sciences of the United States of America 97 (2000): 6960-6966.

Peter A. Ensminger

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Plant

Plant

A plant is an organism in the kingdom Plantae. According to the five-kingdom classification system used by most biologists, plants have the following characteristics: they are multicellular during part of their life; they are eukaryotic, in that their cells have nuclei; they reproduce sexually; they have chloroplasts with chlorophyll-a, chlorophyll-b and carotenoids as photosynthetic pigments; they have cell walls with cellulose , a complex carbohydrate ; they have life cycles with an alternation of a sporophyte phase and a gametophyte phase; they develop organs which become specialized for photosynthesis , reproduction, or mineral uptake; and most live on land during their life cycle.

Biologists have identified about 500,000 species of plants, although there are many undiscovered species in the tropics.


Plant evolution and classification

From the time of Aristotle until the 1950s, most people classified all organisms into the animal kingdom or the plant kingdom. Fungi and plant-like, single-celled organisms were placed into the plant kingdom, in view of certain highly derived, but superficial characteristics of these organisms.

In 1959, Robert Whittaker advocated a five-kingdom classification system. According to a recent modification of that system, the five kingdoms are: Monera (single-celled, prokaryotic organisms, such as bacteria ), Protoctista (various eukaryotic groups, such as algae and water molds), Fungi (spore-forming eukaryotes which lack flagella , such as mushrooms and various molds), Animalia (various multicellular eukaryotic groups, such as jellyfish and vertebrates ), and Plantae, or plants.

Biologists now recognize an additional kingdom of prokaryotes, the Archaebacteria or ancient bacteria, which have unique characteristics that distinguish them from Eubacteria , or true bacteria in the kingdom Monera. The evolutionary relationships of Eukaryotes, Archaebacteria, and Eubacteria are uncertain at the present time. Undoubtedly, as our knowledge of evolution and biological diversity increases, Whittaker's five kingdom classification system will require further modification.


Evolution of plants

There was little life on land 500 million years ago, although the oceans abounded with diverse photosynthetic organisms, as well as species in the Monera, Protoctista, and Animalia kingdoms. Land plants appear to have evolved from photosynthetic, aquatic ancestors about 500 million years ago, probably from the Chlorophyta, or green algae. Both groups use chlorophyll-a and chlorophyll-b as photosynthetic pigments, store their energy reserves as starch, and have cellulose in their cell walls.

The evolution of the terrestrial habit required special adaptations of reproductive and vegetative tissues for protection against desiccation. The most significant adaptation of the reproductive tissues is enclosure of the sex cells (egg and sperm) within specialized tissues, and retention of the fertilized egg as it develops into a multicellular embryo. The most significant adaptation of the vegetative tissue is development of a parenchymatous cell organization, in which unspecialized cells (parenchyma) are embedded in a dense matrix of cells. This reduces water loss by reducing the overall surface area of the plant per cell, and also provides the plant with a body matrix for differentiation of specialized tissues.

The life cycle of all plants consists of an alternation of generations, in which a haploid gametophyte (tissue in which each cell has one copy of each chromosome ) alternates with a diploid sporophyte (tissue in which each cell has two copies of each chromosome). A major trend in plant evolution has been the increasing dominance of the sporophyte. Chlorophyta (green algae), the ancestors of land plants, have a dominant gametophyte and greatly reduced sporophyte. Bryophyta, the most primitive land plants, have a more elaborate sporophyte than Chlorophyta, although their gametophyte is still dominant. Free-sporing vascular plants (Filicinophyta, Lycopodophyta, and Sphenophyta) have a somewhat more dominant sporophyte phase than gametophyte phase. However, seed plants, the most advanced of the land plants, have a greatly reduced gametophyte, and a dominant sporophyte.

Classification of plants

All species are classified hierarchically. Related species are grouped into a genus; related genera into a family; related families into an order; related orders into a class; related classes into a phylum; and related phyla into a kingdom. Below, the most significant characteristics of the nine phyla of the kingdom Plantae are briefly considered.

Bryophyta is a phylum with three classes, the largest of which is the mosses, with about 15,000 species. The gametophyte phase is dominant, and in mosses this is the familiar, small, green, leafy plant. Bryophytes do not have true leaves, stems, or roots, and they lack a vascular system for transporting food and water. They reproduce by making spores, and are mostly found in bogs or moist woodlands, so their sperm can swim through water to reach the eggs. Mosses are particularly prominent in the northern boreal forest and arctic and alpine tundra .

The Lycopodophyta is a phylum with about 1,000 species. The sporophyte phase is dominant, and is the familiar, low-growing, green plant in many species which superficially resembles the branch of a pine. Their leaves are tiny structures, termed microphylls, and are arranged in whorls on the stem. The stems of lycopods and all subsequent phyla have vascular tissues for efficient transport of food and water. Like bryophytes, they reproduce by making spores, and are mostly found in wet areas so their sperm can swim to reach the eggs. Lycopods are most abundant in the tropics, although numerous species of Lycopodium (ground pine) grow in woodlands in the temperate zone.

The Sphenophyta has a single genus, Equisetum, with about 10 species. Equisetum is commonly called horsetail, because the dominant sporophyte phase of these plants superficially resembles a horse's tail. It is an erect stem, with whorls of microphylls, and a spore-producing, cone-like structure, termed a strobilus, on top. Horsetails are mostly found in moist woodlands of the temperate zone, since their sperm must swim to reach the eggs.

The Filicinophyta has about 11,000 species, which are known commonly as ferns. The sporophyte phase is dominant, and is the more familiar form of ferns that is commonly seen in temperate-zone woodlands. Like the leaves of all subsequent phyla, those of ferns have a complex system of branched veins , and are referred to as megaphylls. Ferns reproduce by making spores, and they are mostly restricted to moist environments so their sperm can swim to reach the eggs. Most species occur in tropical and subtropical ecosystems.

The Cycadophyta has about 200 species, which are known commonly as cycads . Like all subsequent phyla, cycads are seed-producing plants. They are considered gymnosperms, because they bear their seeds naked on specialized leaves called sporophylls. The sporophyte phase is dominant, and appears rather like a shrublike palm in many species, although cycads are only distantly related to palms . Cycads have flagellated sperm which swim to fertilize the eggs, a characteristic of evolutionarily primitive, free-sporing plants (all phyla above), but not of other seed plants (except for Ginkgo, see below). Cycads grow in tropical and subtropical regions of the world.

The Ginkgophyta consists of a single species, Ginkgo biloba, a gymnosperm which bears its seeds in green, fruit-like structures. The sporophyte phase of Ginkgo is dominant, and is a tree with fan-shaped leaves that arise from spurs on the branches. Like the cycads, Ginkgo has flagellated sperm that swim to fertilize the eggs. Ginkgo only exists in cultivation, and is widely planted as an ornamental tree throughout the United States and other temperate countries.

The Coniferophyta has about 600 species, and includes familiar evergreen trees such as pines , spruces, and firs . The conifers are the best known and most abundant of the gymnosperms. The sporophyte phase is dominant, and is the familiar cone-bearing tree. Male reproductive structures produce pollen grains, or male gametophytes, which travel by wind to the female reproductive structures. The pollen fertilizes the ovules to produce seeds, which then develop within characteristic cones. Conifers grow throughout the world, and are dominant trees in many northern forests . Many conifers are used for lumber, paper , and other important products.

The Gnetophyta is a phylum of unusual gymnosperms, with about 70 species in three genera, Gnetum, Ephedra, and Welwitschia. These three genera differ significantly from one another in their vegetative and reproductive structures, although all are semi-desert plants. The mode of fertilization of species in the Ephedra genus resembles that of the Angiospermophyta (flowering plants), and many botanists consider them to be close relatives.

The Angiospermophyta is the largest and most important plant phylum, with at least 300,000 species. All species reproduce by making flowers, which develop into fruits with seeds upon fertilization. The flower originated about 130 million years ago, as a structure adapted to protect the ovules (immature seeds), which are born naked and unprotected in the more primitive gymnosperms. The highly specialized characteristics of many flowers evolved to facilitate pollination . There are two natural groups of angiosperms, the monocots, whose seeds have one cotyledon (or seed-leaf), and the dicots, whose seeds have two cotyledons. Nearly all of the plant

foods of humans and many drugs and other economically important products come from angiosperms.

A recent scientific effort has created new theories about the classification of plants. Many genetic experiments were performed by plant biologists around the world in an effort to answer questions of the evolution of plants as a single large group of organisms. Some startling, and controversial results were attained just before the turn of the new century. In 1999, the group of scientists concluded that the kingdom Plantae should, in fact, be split into at least three separate kingdoms because the group is so highly diverse and the genetic evidence gathered indicated sufficient divergence among members. Also, the studies uncovered that the three proposed kingdoms each formed from a single plant-like ancestor that colonized land, not directly from the sea as was previously thought, but from fresh water. These ideas have yet to be accepted by the majority of biologists, and remain a matter of debate.


Plant structure

The seed plants (gymnosperms and angiosperms) are the dominant and most studied group of plants, so their anatomy and development are considered here. The leaves and other aerial portions are all covered with a cuticle, a waxy layer that inhibits water loss. The leaves have stomata, microscopic pores which open in response to certain environmental cues for uptake of carbon dioxide and release of oxygen during photosynthesis. Leaves have veins, which connect them to the stem through a vascular system which is used for transport of water and nutrients throughout the plant.

There are two special types of cells in the vascular system, xylem and phloem. Xylem is mainly responsible for the movement of water and minerals from the roots to the aerial portions, the stems and leaves. Phloem is 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 moves out of a plant's leaves by 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 uptake of water and minerals. Mycorrhizae are symbioses between fungi and most plant roots and are important for water and mineral uptake in most plants. The fungal partner benefits by receiving carbohydrates from the plant, which benefits by being better able to absorb minerals and water from the soil. Mycorrhizae form on the roots of nearly all land plants, and many biologists believe they played a vital role in the evolution of the terrestrial habit.


Plant development

As a plant grows, it undergoes developmental changes, known as morphogenesis, which include the formation of specialized tissues and organs. Most plants continually produce new sets of organs, such as leaves, flowers, and fruits, as they grow. In contrast, animals typically develop their organs only once, and these organs merely increase in size as the animal grows. The meristematic tissues of plants (see below) have the capacity for cell division and development of new and complex tissues and organs, even in older plants. Most of the developmental changes of plants are mediated by hormonal and other chemical changes, which selectively alter the levels of expression of specific genes.

A plant begins its life as a seed, a quiescent stage in which the metabolic rate is greatly reduced. Various environmental cues such as light , temperature changes, or nutrient availability, signal a seed to germinate. During early germination , the young seedling depends upon nutrients stored within the seed itself for growth.

As the seedling grows, it begins to synthesize chlorophyll and turn green. Most plants become green only when exposed to sunlight, because chlorophyll synthesis is light-induced. As plants grow larger, new organs develop according to certain environmental cues and genetic programs of the individual .

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 containing undifferentiated, 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 parallel to the elongation axis of the shoots and roots, and are responsible for thickening of the plant. Differences in apical meristems give different species their unique leaf arrangements; differences in lateral meristems give different species their unique stems and bark .

Many of the morphogenetic changes of developing plants are mediated by hormones—chemical messengers that are active in very small concentrations. The major plant hormones are auxins, gibberellins, cytokinins, abscissic acid, and ethylene. Auxins control cell expansion, apical dominance, and fruit growth. Gibberellins control cell expansion, seed germination, and fruit development. Cytokinins promote cell division and organ development, but impede senescence. Abscissic acid can induce dormancy of seeds and buds, and accelerate plant senescence. Ethylene accelerates senescence and fruit ripening, and inhibits stem growth.

Characteristics of plant cells

Like all other organisms, plants are made up of cells, which are semi-autonomous units consisting of protoplasts surrounded by a special layer of lipids and proteins , termed the plasma membrane . Plant cells are all eukaryotic, in that their genetic material (DNA) is sequestered within a nucleus inside the cell, although some DNA also occurs inside plastids and mitochondria (see below). Plant cells have rigid cell walls external to their plasma membrane.

In addition to nuclei, plant cells contain many other small structures, which are specialized for specific functions. Many of these structures are membrane-enclosed, and are referred to as organelles (small organs).


Cell structures and their functions

The cells of plants, fungi, and bacteria are surrounded by rigid cell walls. Plant cell walls are typically one to five micrometers thick, and their primary constituent is cellulose, a molecule consisting of many glucose units connected end-to-end. In plant cell walls, many cellulose molecules are bundled together into microfibrils (small fibers), like the fibers of a string. These microfibrils have great tensile strength, because the component strands of cellulose are interconnected by hydrogen bonds. The cellulose microfibrils are embedded in a dense, cell-wall matrix consisting of other complex molecules such as hemicellulose, pectic substances, and enzymes and other proteins. Some plant cells become specialized for transport of water or physical support, and these cells develop a secondary wall that is thick and impregnated with lignin, another complex carbohydrate.

All living cells are surrounded by a plasma membrane, a viscous lipid-and-protein matrix which is about 10 nm thick. The plasma membrane of plant cells lies just inside the cell wall, and encloses the rest of the cell, the cytoplasm and nucleus. The plasma membrane regulates transport of various molecules into and out of the cell, and also serves as a sort of two-dimensional scaffolding, upon which many biochemical reactions occur.

The nucleus is often considered to be the control center of a cell. It is typically about 10 micrometers in diameter, and is surrounded by a special double-membrane with numerous pores. The most important molecules in the nucleus are DNA (deoxyribonucleic acid), RNA (ribonucleic acid), and proteins. DNA is a very long molecule, and is physically associated with numerous proteins in plants and other eukaryotes. Specific segments of DNA make up genes, the functional units of heredity which encode specific characteristics of an organism. Genes are connected together into chromosomes, thread-like structures that occur in a characteristic number in each species. Special enzymes within the nucleus use DNA as a template to synthesize RNA. Then, the RNA moves out of the nucleus where it is used as a template for the synthesis of enzymes and other proteins.

Plastids are organelles only present in plants and algae. They have a double membrane on their outside, and are specilized for the storage of starch (amyloplasts), storage of lipids (elaioplasts), photosynthesis (chloroplasts), or other functions. Chloroplasts are the most important type of plastid, and are typically about 10 micrometers in diameter. Chloroplasts are specialized for photosynthesis, the biological conversion of light energy absorbed by chlorophylls, the green leaf pigments, into potential chemical energy such as carbohydrates. Some of the component reactions of photosynthesis occur on special, inner membranes of the chloroplasts, referred to as thylakoids; other reactions occur in the aqueous interior of the chloroplast , referred to as the stroma. Interestingly, plastids are about the size of bacteria and, like bacteria, they also contain a circular loop of DNA. These and many other similarities suggest that cells with chloroplasts originated several billion years ago by symbiogenesis, the union of formerly separate, prokaryotic cells.

Mitochondria are organelles which are present in nearly all living, eukaryotic cells. A mitochondrion has a double membrane on its outside, is typically ovoid or oblong in shape, and is about 0.5 micrometers wide and several micrometers long. Mitochondria are mainly responsible for the controlled oxidation (metabolic breakdown) of high-energy food molecules, such as fats and carbohydrates, and the consequent synthesis of ATP (adenosine triphosphate ), the energy source for cells. Many of the mitochondrial enzymes that oxidize food molecules are embedded in special internal membranes of the mitochondria. Like plastids, mitochondria contain a circular loop of DNA, and are believed to have originated by symbiogenesis.

Golgi bodies are organelles present in most eukaryotic cells, and function as biochemical processing centers for many cellular molecules. They appear as a cluster of flattened vesicles, termed cisternae, and associated spherical vesicles. The Golgi bodies process carbohydrates, which are used to synthesize the cell wall, and lipids, which are used to make up the plasma membrane. They also modify many proteins by adding sugar molecules to them, a process referred to as glycosylation.

Vacuoles are fluid-filled vesicles which are separated from the cytoplasm by a special membrane, referred to as a tonoplast. Vacuoles are present in many eukaryotic cells. The vacuoles of many plant cells are very large, and can constitute 90% or more of the total cell volume . The main constituent of vacuoles is water. Depending on the type of cell, vacuoles are specialized for storage of foods, ions, or water-soluble plant pigments.

The endoplasmic reticulum is a complex system of interconnected double membranes, which is distributed throughout most eukaryotic cells. The membranes of the endoplasmic reticulum are often continuous with the plasma membrane, the outer nuclear membrane, the tonoplast, and Golgi bodies. Thus, the endoplasmic reticulum functions as a conduit for chemical communication between different parts of the cell. The endoplasmic reticulum is also a region where many proteins, lipids, and carbohydrates are biochemically modified. Many regions of the endoplasmic reticulum have ribosomes associated with them. Ribosomes are subcellular particles made up of proteins and RNA, and are responsible for synthesis of proteins from information encoded in RNA.


Importance to humans

Plants provide food to humans and all other nonphotosynthetic organisms, either directly or indirectly. Agriculture began about 10,000 years ago in the fertile crescent of the Near East, where people first cultivated wheat and barley . Scientists believe that as people of the fertile crescent gathered wild seeds, they selected for certain genetically determined traits, which made the plants produced from those seeds more suited for cultivation and as foods. For example, most strains of wild wheat bear their seeds on stalks that break off to disperse the mature seeds. As people selected wild wheat plants for food, they unknowingly selected genetic variants in the wild population whose seed stalks did not break off. This trait made it easier to harvest and cultivate wheat, and is a feature of all of our modern varieties of wheat.

The development of agriculture led to enormous development of human cultures, as well as growth in the human population. This, in turn, spurred new technologies in agriculture. One of the most recent agricultural innovations is the "Green Revolution," the development of new genetic varieties of crop plants. In the past 20-30 years, many new plant varieties have been developed that are capable of very high yields, surely an advantage to an ever-growing human population.

Nevertheless, the Green Revolution has been criticized by some people. One criticism is that these new crop varieties often require large quantities of fertilizers and other chemicals to attain their high yields, making them unaffordable to the relatively poor farmers of the developing world. Another criticism is that the rush to use these new genetic varieties may hasten the extinction of native varieties of crop plants, which themselves have many valuable, genetically-determined characteristics.

Regardless of one's view of the Green Revolution, it is clear that high-tech agriculture cannot provide a simple solution to poverty and starvation. Improvements in our crop plants must surely be coupled to advances in politics and diplomacy to ensure that people of the developing nations are fed in the future.

See also Angiosperm; Bryophyte; Mycorrhiza; Plant pigment; Root system.


Resources

books

Attenborough, D. The Private Life of Plants. Princeton: Princeton University Press, 1995.

Galston, A.W. Life Processes of Plants: Mechanisms for Survival. San Francisco: W. H. Freeman Press, 1993.

Kaufman, P.B., et al. Plants: Their Biology and Importance. New York: HarperCollins, 1990.

Margulis, L., and K. V. Schwartz. Five Kingdoms. San Francisco: W. H. Freeman and Company, 1988.

Wilkins, M. Plant Watching. New York: Facts on File, 1988.


periodicals

Palmer, J. D., et al. "Dynamic Evolution of Plant Mitochondrial Genomes: Mobile Genes and Introns and Highly Variable Mutation Rates." Proceedings of the National Academy of Sciences of the United States of America 97 (2000): 6960-6966.


Peter A. Ensminger

KEY TERMS

. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Diploid

—Nucleus or cell containing two copies of each chromosome, generated by fusion of two haploid nuclei.

Eukaryote

—A cell whose genetic material is carried on chromosomes inside a nucleus encased in a membrane. Eukaryotic cells also have organelles that perform specific metabolic tasks and are supported by a cytoskeleton which runs through the cytoplasm, giving the cell form and shape.

Gametophyte

—The haploid, gamete-producing generation in a plant's life cycle.

Haploid

—Nucleus or cell containing one copy of each chromosome.

Meristem

—Special plant tissues that contain undifferentiated, actively growing and dividing cells.

Morphogenesis

—Developmental changes that occur during growth of an organism, such as formation of specialized tissues and organs.

Prokaryote

—A cell without a nucleus, considered more primitive than a eukaryote.

Sporophyte

—The diploid spore-producing generation in a plant's life cycle.

Symbiosis

—A biological relationship between two or more organisms that is mutually beneficial. The relationship is obligate, meaning that the partners cannot successfully live apart in nature.

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