Plant

From New World Encyclopedia
Plants
Fern.jpg
Fern frond
Scientific classification
Domain: Eukaryota
Kingdom: Plantae
Haeckel,1866
Divisions
Adiantum pedatum (a fern)

Plants are eukaryotic, multicellular, nonmotile photosynthetic organisms, and comprise one of the five Kingdoms of living things, Plantae. They are classified as plants by their ability to create food through photosynthesis, have cell walls composed of cellulose, do not have a central nervous system, and reproduce by alternation of generations. This kingdom includes familiar organisms such as trees, shrubs, herbs, and ferns. Over 350,000 species of plants have been estimated to exist. As of 2004, some 287,655 species had been identified, of which 258,650 are flowering plants.

Aristotle divided all living things between plants, which generally do not move or have sensory organs, and animals. In Carolus Linnaeus' system, these became the Kingdoms Vegetabilia (later Plantae) and Animalia. Since then, it has become clear that the Plantae as originally defined included several unrelated groups, and the fungi and several groups of algae were removed to new kingdoms. However, these are still often considered plants in many contexts. Indeed, any attempt to match "plant" with a single taxon is doomed to fail, because plant is a vaguely defined concept unrelated to the presumed phylogenic concepts on which modern taxonomy is based.

Embryophytes

The most familiar plants are the multicellular land plants with specialized reproductive organs, called embryophytes. They include the vascular plants—plants with full systems of leaves, stems, and roots. They also include a few of their close relatives, often called bryophytes, of which mosses and liverworts are the most common.

All of these plants have eukaryotic cells with cell walls composed of cellulose, and most obtain their energy through photosynthesis, using light and carbon dioxide to synthesize food. Plants are distinguished from green algae, from which they are considered to have evolved, by having specialized reproductive organs protected by non-reproductive tissues.

Some plants live symbiotically off of other organisms (myco-heterotrophs) rather than producing food through photosynthesis. Most of these are actually fungi, but some plants, such as Monotropastrum humile and Monotropa hypopitys,in the Ericaceae family, contain no chlorophyll and obtain nutrients from non-living organic matter. Mycotrophs engage in mutualistic mycorrhizal relationships, where a fungus attaches to a host plant's roots. The fungus obtains sugars produced by the plant, and the fungus aids the plant in absorbing essential nutrients from the soil. These mycorrhizal plants are mostly photosynthetic. About three hundred known plant species do not photosynthesize, but are parasites on other species of photosynthetic plants.

Vascular Plants

Vascular plants comprise those embryophytic plants that have specialized tissues for conducting water. Vascular plants include the seed plantsflowering plants (angiosperms), and gymnosperms—as well as non-seed (vascular) plants, such as ferns, clubmosses, and horsetails. Water transport happens in either xylem or phloem: the xylem carries water and inorganic solutes upward toward the leaves from the roots, while phloem carries organic solutes throughout the plant.

Seed plants

The spermatophytes (also known as phanerogams) comprise those plants that produce seeds. They are a subset of the embryophytes or land plants: living spermatophytes include cycads, Ginkgo, conifers, gnetae, and angiosperms

Seed-bearing plants were traditionally divided into angiosperms, or flowering plants, and gymnosperms, which includes the gnetae, cycads, ginkgo, and conifers. Angiosperms are now thought to have evolved from a gymnosperm ancestor, which would make gymnosperms a paraphyletic group if it includes extinct taxa. Modern cladistics attempts to define taxa that are monophyletic, traceable to a common ancestor and inclusive therefore of all descendants of that common ancestor. Although not a monophyletic taxonomic unit, "gymnosperm" is still widely used to distinguish the four taxa of non-flowering, seed-bearing plants from the angiosperms.

Molecular phylogenies have conflicted with morphologically-based evidence as to whether extant gymnosperms comprise a monophyletic group. Some morphological data suggests that the Gnetophytes are the sister-group to angiosperms, but molecular phylogenies have generally shown a monophyletic gymnosperm clade that includes the Gnetophytes as sister-group to the conifers.

The fossil record contains evidence of many extinct taxa of seed plants. The so-called "seed ferns" (Pteridospermae) were one of the earliest successful groups of land plants, and forests dominated by seed ferns were prevalent in the late Paleozoic. Glossopteris was the most prominent tree genus in the ancient southern supercontinent of Gondwana during the Permian period. By the Triassic period, seed ferns had declined in ecological importance, and representatives of modern gymnosperm groups were abundant and dominant through the end of the Cretaceous, when angiosperms radiated.

Modern classification classifies the seed plants as follows:

  • Cycadophyta, the cycads
  • Ginkgophyta, the ginkgo
  • Pinophyta, the conifers
  • Gnetophyta, including Gnetum, Welwitschia, Ephedra
  • Magnoliophyta, the flowering plants

Nonseed Plants

The nonseed plants are comprised of Filicophyta or Pteridophyta (ferns), Psilotophyta (whisk ferns), Lycopodiophyta (clubmosses, spikemosses, and quillworts), Sphenophyta or Equisetophyta (horsetails), and Ophioglossophyta (adderstongues, which have also been grouped with the Pteridophyta).

Pteridophyta

Pteridophyta (previously known as Filicophyta) is a vast group of 20,000 species of plants found globally, and know as ferns. Ferns can vary in complexity and size, from 2cm aquatic ferns to several-meter tree ferns of the tropics. Ferns can be either terrestrial and grow in the soil, or they are epiphytes, growing on another plant. The fern life cycle differs from that of the angiosperms and gymnosperms in that its gametophyte is a free-living organism. Each frond (leaf) is capable of bearing spores (sporophyll) when conditions are right. (See fern article for a complete description.)

Psilotophyta

Psilotophyta, orPsilotales (the "whisk ferns") is an order of the Class Ophioglossopsida. This order contains only two living genera, Psilotum, a small shrubby plant of the dry tropics, and Tmesipteris, anepiphyte found in Australia, New Zealand, and New Caledonia. There has long been controversy about the relationships of the Psilotophyta, with some claiming that they are ferns (Pteridophyta), and others maintaining that they are descendants of the first vascular plants (the Psilophyta of the Devonian period). Recent evidence from DNA demonstrates a much closer relationship to the ferns, and that they are closely related to the Ophioglossales, in particular. Psilotales lack leaves, instead having small outgrowths called enations. The enations are not considered true leaves because there is only a vascular bundle just underneath them, but not inside, as in leaves. Psilotales also do not have true roots. They are anchored by rhizoids. Absorption is aided by symbiotic fungi called mycorrhizae.

Three sporangia are united into a synangium, which is considered to be a very reduced series of branches. There is a thick tapetum to nourish the developing spores, as is typical of eusporangiate plants. The gametophyte looks like a small piece of subterranean stem, but produces antheridia and archegonia.

Lycopodiophyta

The Division Lycopodiophyta (sometimes called Lycophyta) is the oldest extant (living) vascular plant division and includes some of the most "primitive" extant species. These species reproduce by shedding spores and have macroscopic alternation of generations, although some are homosporous while others are heterosporous. They differ from all other vascular plants in having microphylls, leaves that have only a single vascular trace (vein) rather than the much more complex megaphylls found in ferns and seed plants.

There are three main groups within the Lycopodiophyta, sometimes separated at the level of order and sometimes at the level of class. These are subdivided at the class level here:

  • Class Lycopodiopsida – clubmosses and firmosses
  • Class Selaginellopsida – spikemosses
  • Class Isoetopsida – quillworts

The members of this division have a long evolutionary history, and fossils are abundant worldwide, especially in coal deposits. In fact, most known genera are extinct. The Silurian species Baragwanathia longifolia represents the earliest identifable Lycopodiophyta, while some Cooksonia seem to be related.

The Lycopodiophyta are one of several classes of plants that expanded onto land during the Silurian and Devonian periods. They developed and specialized roots to extract nutrients from the soil and developed leaves for photosynthesis and gas exchange, using a stem for transport. A waxy cuticle helped retain moisture, and stoma allowed respiration. The vulnerable meiotic gametophyte is protected from radiation by its reduced size and often by the use of subterranean mycorrhiza for its energy source instead of photosynthesis. Club-mosses are homosporous, but spike-mosses and quillworts are heterosporous. In heterospores the female spores are larger than the male because they store food for the new generation.

Sphenophyta

The horsetails comprise 15 species of plants in the genus Equisetum. This genus is the only one in the family Equisetaceae, which in turn is the only family in the order Equisetales and the class Equisetopsida. This class is often placed as the sole member of the Division Equisetophyta (also called Arthrophyta in older works), though some recent molecular analyses place the genus within Pteridophyta, related to Marattiales. The molecular data, however, remain somewhat. Other classes and orders of Equisetophyta are known from the fossil record, where they were important members of the world flora during the Carboniferous period.

The name horsetail arose because it was thought that the stalk resembled a horse's tail; the name Equisetum is from the Latin equus, "horse", and seta, "bristle". Other names, rarely used, include candock (applied to branching species only), and scouring-rush (applied to the unbranched or sparsely branched species). The name scouring-rush refers to its rush-like appearance and because the stems are coated with abrasive silica that led them to be used for scouring cooking pots in the past.

The genus is near-cosmopolitan, being absent only from Australasia and Antarctica. They are perennial plants, either herbaceous, dying back in winter (most temperate species) or evergreen (some tropical species, and the temperate Equisetum hyemale). They mostly grow 0.2-1.5 m tall, though E. telmateia can exceptionally reach 2.5 m, and the tropical American species E. giganteum 5 m, and E. myriochaetum 8 m.

In these plants the leaves are greatly reduced, being represented only by whorls of small, translucent scales. The stems are green and photosynthetic, also distinctive in being hollow, jointed, and ridged (with (3-) 6-40 ridges). There may or may not be whorls of branches at the nodes; when present, these branches are identical to the main stem except smaller.

The spores are borne in cone-like structures (strobilus, pl. strobili) at the tips of some of the stems. In many species they are unbranched, and in some (e.g. E. arvense) they are non-photosynthetic, produced early in spring separately from photosynthetic sterile stems. In some other species (e.g. E. palustre) they are very similar to sterile stems, photosynthetic and with whorls of branches.

Horsetails are mostly homosporous, though in E. arvense, smaller spores give rise to male prothalli. The spores have four elaters that act as moisture-sensitive springs, ejecting the spores through a weak spot of the sporangia.

The horsetails were a much larger and more diverse group in the distant past before seed plants became dominant across the Earth. Some species were large trees reaching to 30 m tall. The genus Calamites (Family Calamitaceae) is abundant in coal deposits from the Carboniferous period.

Nonvascular Plants

Non-vascular plants include those land plants without a vascular system. Bryophytes - the Bryophyta (mosses), the Hepaticophyta (liverworts), and the Anthocerotophyta (hornworts), are the only nonvascular plants grouped within the Kingdom Plantae. In these groups, the primary plants are haploid, with the only diploid portion being the attached sporophyte, consisting of a stalk and sporangium. Because these plants lack the water-conducting tissues, they fail to achieve the structural complexity and size of most vascular plants. Some algae - especially the green algae- are also nonvascular, but these are no longer grouped in the plant kingdom. Recent studies have demonstrated that the algae actually consist of several unrelated groups. It turns out that common features of living in water and photosynthesis were misleading as indicators of close relationship. Only the green algae are still considered relatives of the plants.

Plant History and Evolution

Bryophytes first appeared during the early Palaeozoic (~450 million years ago). They can only survive where moisture is available for significant periods, although some species are desiccation tolerant. Most species of bryophyte remain small throughout their life cycle. This involves an alternation between two generations: a haploid stage, called the gametophyte, and a diploid stage, called the sporophyte. The sporophyte is short-lived and remains dependent on its parent gametophyte.

Vascular plants first appeared during the Silurian (450-415 million years ago) period, and by the Devonian (415 million years ago) had diversified and spread into many different land environments. They have a number of adaptations that allowed them to overcome the limitations of the bryophytes. These include a cuticle resistant to desiccation, and vascular tissues which transport water throughout the organism. In most the sporophyte acts as a separate individual, while the gametophyte remains small.

Phylogeny of the modern Spermatophyta (seed plants) and some allied vascular plant groups. Note that the spore-bearing vascular plants are paraphyletic with respect to the seed plants, with ferns (Pteridophyta) more closely allied to seed plants than they are to clubmosses (Lycopodiophyta)

The first primitive seed plants, Pteridosperms (seed ferns) and Cordaites, both groups now extinct, appeared in the late Devonian and diversified through the Carboniferous, with further evolution through the Permian and Triassic periods. In these the gametophyte stage is completely reduced, and the sporophyte begins life inside an enclosure called a seed, which develops while on the parent plant, and with fertilisation by means of pollen grains. Whereas other vascular plants, such as ferns, reproduce by means of spores and so need moisture to develop, some seed plants can survive and reproduce in extremely arid conditions.

Early seed plants are referred to as gymnosperms (naked seeds), as the seed embryo is not enclosed in a protective structure at pollination, with the pollen landing directly on the embryo. Four surviving groups remain widespread now, particularly the conifers, which are dominant trees in several biomes. The angiosperms, comprising the flowering plants, were the last major group of plants to appear, emerging from within the gymnosperms during the Jurassic (200-160 million years ago) and diversifying rapidly during the Cretaceous (100-150 million years ago). These differ in that the seed embryo is enclosed, so the pollen has to grow a tube to penetrate the protective seed coat; they are the predominant group of flora in most biomes today.

Fossils

Plant fossils include roots, wood, leaves, seeds, fruit, pollen, spores, phytoliths, and amber (the fossilized resin produced by some plants). Fossil land plants are recorded in terrestrial, lacustrine, fluvial and nearshore marine sediments. Pollen, spores and algae (dinoflagellates and acritarchs) are used for dating sedimentary rock sequences. The remains of fossil plants are not as common as fossil animals, although plant fossils are locally abundant in many regions worldwide.

Early fossils of these ancient plants show the individual cells within the plant tissue. The Devonian period also saw the evolution of what many believe to be the first modern tree, Archaeopteris. This fern-like tree combined a woody trunk with the fronds of a fern, but produced no seeds.

Fossil Ginkgo leaves from the Jurassic of England

The Coal Measures are a major source of Palaeozoic plant fossils, with many groups of plants in existence at this time. The spoil heaps of coal mines are the best places to collect; coal itself is the remains of fossilised plants, though structural detail of the plant fossils is rarely visible in coal. In the Fossil Forest at Victoria Park in Glasgow, Scotland, the stumps of Lepidodendron trees are found in their original growth positions.

The fossilized remains of conifer and angiosperm roots, stems and branches may be locally abundant in lake and inshore sedimentary rocks from the Mesozoic and Cenozoic eras. Sequoia and its allies, magnolia, oak, and palms are often found.

Petrified wood is common in some parts of the world, and is most frequently found in arid or desert areas where it is more readily exposed by erosion. Petrified wood is often heavily silicified (the organic material replaced by silicon dioxide), and the impregnated tissue is often preserved in fine detail. Such specimens may be cut and polished using lapidary equipment. Fossil forests of petrified wood have been found in all continents.

Fossils of seed ferns such as Glossopteris are widely distributed throughout several continents of the southern hemisphere, a fact that gave support to Alfred Wegener's early ideas regarding Continental drift theory.

Algae and Fungi

The algae comprise several different groups of organisms that produce energy through photosynthesis. However, they are not classified within the Kingdom Plantae but mostly in the Kingdom Protista. Most conspicuous are the seaweeds, multicellular algae that may roughly resemble terrestrial plants, but are classified among the green, red, and brown algae. These and other algal groups also include various single-celled organisms.

The embryophytes developed from green algae; the two groups are collectively referred to as the green plants or Viridiplantae. The Kingdom Plantae is often taken to mean this monophyletic grouping. With a few exceptions among the green algae, all such forms have cell walls containing cellulose and chloroplasts containing chlorophylls a and b, and store food in the form of starch. They undergo closed mitosis without centrioles (barrel-shaped microtubules that aids in the cell division process), and typically have mitochondria with flat cristae.

The chloroplasts of green plants are surrounded by two membranes, suggesting they originated directly from endosymbiotic cyanobacteria. The same is true of the red algae, and the two groups are generally believed to have a common origin. In contrast, most other algae have chloroplasts with three or four membranes. They are not close relatives of the green plants, presumably in origin acquiring chloroplasts separately from ingested or symbiotic green and red algae.

Unlike embryophytes and algae, fungi are not photosynthetic, but are saprophytic, obtaining food by breaking down and absorbing surrounding materials. Most fungi are formed by microscopic structures called hyphae, which may or may not be divided into cells but contain eukaryotic nuclei. Fruiting bodies, of which mushrooms are most familiar, are the reproductive structures of fungi. They are not related to any of the photosynthetic groups, but are close relatives of animals. Therefore, the fungi are in a kingdom of their own.

Reproduction

See articles on life cycle, gymnosperm, angiosperm, bryophyte, and fern for more complete discussion of plant reproduction.

In flowering plants, a stamen produces gametes called pollen grains, which attach to a pistil, in which the female gametes (ovules) are located. Here, the female gamete is fertilized and develops into a seed. The ovary, which produced the gamete then grows into a fruit, which surrounds the seed(s). Plants may either self-pollinate or cross-pollinate.

Vegetative reproduction is a type of asexual reproduction found in plants also called vegetative propagation or vegetative multiplication. It is a process by which new plant "individuals" arise or are obtained without production of seeds or spores. It is both a natural process in many plant species (including organisms outside of the plant kingdom, such as bacteria and fungi) and one utilized or encouraged by horticulturists to obtain quantities of economically valuable plants.

Natural vegetative reproduction is mostly a process found in herbaceous and woody perennials, and typically involves structural modifications of the stem, although any horizontal, underground part of a plant (whether stem or a root) can contribute to vegetative reproduction of a plant. And, in a few species (such as Kalanchoë shown at right), leaves are involved in vegetative reproduction. Most plant species that survive and significantly expand by vegetative reproduction would be perennial almost by definition, since specialized organs of vegetative reproduction, like seeds of annuals, serve to survive seasonally harsh conditions. A plant that persists in a location through vegetative reproduction of individuals over a long period of time constitutes a clonal colony.

In a sense, this process is not one of "reproduction" but one of survival and expansion of biomass of the individual. When an individual organism increases in size via cell multiplication and remains intact, the process is called "vegetative growth". However, in vegetative reproduction, the new plants that result are new individuals in almost every respect except genetic. And of considerable interest is how this process appears to reset the aging clock.


A rhizome is a modified stem serving as an organ of vegetative reproduction. Prostrate aerial stems, called runners or stolons are important vegetative reproduction organs in some species, such as the strawberry, numerous grasses, and some ferns. Adventitious buds develop into above ground stems and leaves, forming on roots near the ground surface and on damaged stems (as on the stumps of cut trees). Adventitious roots form on stems where the latter touch the soil surface.

A form of budding called suckering is the reproduction or regeneration of a plant by shoots that arise from an existing root system. Species that characteristically produce suckers include Elm (Ulmus), Dandelion (Taraxacum), and members of the Rose Family (Rosa).

Another type of vegetative reproduction is the production of bulbs. Plants like onion (Allium cepa), hyacinth (Hyacinth), narcissus (Narcissus) and tulips (Tulipa) reproduce by forming bulbs. Other plants like potatoes (Solanum tuberosum) and dahlia (Dahlia) reproduce by a similar method of producing tubers. Gladioluses and crocuses (Crocus) reproduce by forming a bulb-like structure called a corm.


Vegetative propagation is usually considered a cloning method. However, there are several cases where vegetatively propagated plants are not genetically identical. Rooted stem cuttings of thornless blackberries will revert to thorny type because the adventitious shoot develops from a cell that is genetically thorny. Thornless blackberry is a chimera, with the epidermal layers genetically thornless but the tissue beneath it genetically thorny. Leaf cutting propagation of certain chimeral variegated plants, such as snake plant, will produce mainly nonvariegated plants.

Growth

It is a common misconception that most of the solid material in a plant is taken from the soil, when in fact almost all of it is actually taken from the atmosphere. Through a process known as photosynthesis, plants use the energy in sunlight to convert carbon dioxide from the atmosphere into simple sugars. These sugars are then used as building blocks and form the main structural component of the plant. Plants rely on soil primarily for water (in quantitative terms), but also obtain nitrogen, phosphorus and other crucial elemental nutrients.

Importance

The photosynthesis and carbon fixation conducted by land plants and algae are the ultimate source of energy and organic material in nearly all ecosystems. These processes radically changed the composition of the early Earth's atmosphere, which as a result is now 20% oxygen. Animals and most other organisms are aerobic, relying on oxygen; those that do not are confined to relatively rare anaerobic, oxygen-depleted, environments.

Much of human nutrition depends on cereals. Other plants that are eaten include fruits, vegetables, legumes herbs, and spices. Strict vegetarians rely entirely on plants (as well as some algae and fungi) for their nutrition. Some vascular plants, referred to as trees and shrubs, produce woody stems and are an important source of building material. A number of plants are used decoratively, including a variety of flowers, and in landscaping.

Some plants grow special defence measures such as the spines on a blackberry

Simple plants like algae may have short life spans as individuals, but their populations are commonly seasonal. Other plants may be organized according to their seasonal growth pattern:

  • Annual: live and reproduce within one growing season.
  • Biennial: live for two growing seasons; usually reproduce in second year.
  • Perennial: live for many growing seasons; continue to reproduce once mature.

Among the vascular plants, perennials include both evergreens that keep their leaves the entire year, and deciduous plants which lose their leaves for some part. In temperate and boreal climates, they generally lose their leaves during the winter; many tropical plants lose their leaves during the dry season.

The growth rate of plants is extremely variable. Some mosses grow less than 0.001 mm/h, while most trees grow 0.025-0.250 mm/h. Some climbing species, such as kudzu, which do not need to produce thick supportive tissue, may grow up to 12.5 mm/h.

Distribution

Plants are found throughout the world, both on land and in water bodies. Plants are most abundant where resources (water, sunlight, adequate growth temperatures) are most abundant, and accordingly, the tropics overwhelmingly contain the greatest biomass and species diversity. The mostly dry, subtropical regions contain highly specialized, dessication-tolerant species, and the landscape is often sparse. The temperate midlatitudes once again increase in biodiversity and biomass, but for the most part do not surpass the tropics in either. Poleward of the midlatitudes biodiversity decreases, and tundra dominates. Poleward of the arctic circle, vegetation growth is highly seasonal, as it remains dark for a significant portion of the year, preventing photosynthesis from occurring. (Please see biome article for more detailed discussion on global distribution of plants and their biomes.)

References and further reading

  • Bowe, L. Michelle, Gwénaële Coat, and Claude W. dePamphilis. 2000. Phylogeny of seed plants based on all three genomic compartments: Extant gymnosperms are monophyletic and Gnetales' closest relatives are conifers. Proceedings of the National Academy of Sciences 97: 4092-4097.
  • Soltis, Douglas E., Pamela S. Soltis and Michael J. Zanis. 2002. Phylogeny of seed plants based on evidence from eight genes. American Journal of Botany 89: 1670-1681 (abstract here).
  • Chaw, Shu-Miaw, Christopher L. Parkinson, Yuchang Cheng, Thomas M. Vincent, and Jeffrey D. Palmer. 2000. Seed plant phylogeny inferred from all three plant genomes: Monophyly of extant gymnosperms and origin of Gnetales from conifers. Proceedings of the National Academy of Sciences 97: 4086-4091 (abstract here).
  • Species estimate and counts:
    • Prance, G. T. (2001). Discovering the Plant World. Taxon 50: 345-359.
    • International Union for Conservation of Nature and Natural Resources (IUCN) Species Survival Commission (2004). IUCN Red List of Threatened Species [1].
    • Both the above are cited in Nature Conservancy, Spring 2006, p. 14.
  • Kenrick, Paul & Crane, Peter R. (1997). The Origin and Early Diversification of Land Plants: A Cladistic Study. Washington, D. C.: Smithsonian Institution Press. ISBN 1-56098-730-8.
  • Raven, Peter H., Evert, Ray F., & Eichhorn, Susan E. (2005). Biology of Plants (7th ed.). New York: W. H. Freeman and Company. ISBN 0-7167-1007-2.
  • Taylor, Thomas N. & Taylor, Edith L. (1993). The Biology and Evolution of Fossil Plants. Englewood Cliffs, NJ: Prentice Hall. ISBN 0-13-651589-4.
  • Evans, L.T. (1998). Feeding the Ten Billion - Plants and Population Growth. Cambridge University Press. Paperback, 247 pages ISBN. 0-521-64685-5.

See also

External links

Botanical and vegetation databases

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