Plants, also called green plants (Viridiplantae in Latin),
are living organisms of the kingdom Plantae including such multicellular groups
as flowering plants, conifers, ferns and mosses, as well as, depending on
definition, the green algae, but not red or brown seaweeds like kelp, nor fungi
or bacteria.
Green plants have cell walls with cellulose and
characteristically obtain most of their energy from sunlight via photosynthesis
using chlorophyll contained in chloroplasts, which gives them their green
color. Some plants are parasitic and may not produce normal amounts of
chlorophyll or photosynthesize. Plants are also characterized by sexual
reproduction, modular and indeterminate growth, and an alternation of
generations, although asexual reproduction is common, and some plants bloom
only once while others bear only one bloom.
Precise numbers are difficult to determine, but as of 2010,
there are thought to be 300–315 thousand species of plants, of which the great
majority, some 260–290 thousand, are seed plants (see the table below).[2]
Green plants provide most of the world's molecular oxygen and are the basis of
most of the earth's ecologies, especially on land. Plants described as grains,
fruits and vegetables form mankind's basic foodstuffs, and have been
domesticated for millennia. Plants enrich our lives as flowers and ornaments.
Until recently and in great variety they have served as the source of most of
our medicines and drugs. Their scientific study is known as botany.
Definition
Plants are one of the two groups into which all living
things have been traditionally divided; the other is animals. The division goes
back at least as far as Aristotle (384 BC – 322 BC) who distinguished between
plants which generally do not move, and animals which often are mobile to catch
their food. Much later, when Linnaeus (1707–1778) created the basis of the
modern system of scientific classification, these two groups became the
kingdoms Vegetabilia (later Metaphyta or Plantae) and Animalia (also called
Metazoa). Since then, it has become clear that the plant kingdom as originally
defined included several unrelated groups, and the fungi and several groups of
algae were removed to new kingdoms. However, these organisms are still often
considered plants, particularly in popular contexts.
Outside of formal scientific contexts, the term "plant"
implies an association with certain traits, such as being multicellular,
possessing cellulose, and having the ability to carry out photosynthesis.[3][4]
Current definitions of Plantae
When the name Plantae or plant is applied to a specific
group of organisms or taxon, it usually refers to one of three concepts. From
least to most inclusive, these three groupings are:
Name(s) Scope Description
Land plants, also known as Embryophyta or Metaphyta. Plantae sensu strictissimo This group includes the liverworts,
hornworts, mosses, and vascular plants, as well as fossil plants similar to
these surviving groups.
Green plants - also known as Viridiplantae, Viridiphyta or
Chlorobionta Plantae sensu stricto This group includes the land plants plus
various groups of green algae, including stoneworts. The names given to these
groups vary considerably as of July 2011. Viridiplantae encompass a group of
organisms that possess chlorophyll a and b, have plastids that are bound by
only two membranes, are capable of storing starch, and have cellulose in their
cell walls. It is this clade which is mainly the subject of this article.
Archaeplastida, Plastida or Primoplantae Plantae sensu lato This group comprises the green plants
above plus Rhodophyta (red algae) and Glaucophyta (glaucophyte algae). This
clade includes the organisms that eons ago acquired their chloroplasts directly
by engulfing cyanobacteria.
Another way of looking at the relationships between the
different groups which have been called "plants" is through a
cladogram, which shows their evolutionary relationships. The evolutionary
history of plants is not yet completely settled, but one accepted relationship
between the three groups described above is shown below.[5] Those which have
been called "plants" are in bold.
Archaeplastida
Glaucophyta (glaucophyte algae)
Rhodophyta (red algae)
Viridiplantae
Chlorophyta (part of green algae)
Streptophyta
streptophyte algae (part of green algae)
Charales (stoneworts, often included
in green algae)
land plants or embryophytes
groups traditionally called "algae"
The way in which the groups of green algae are combined and
named varies considerably between authors.
Algae
Green algae from Ernst Haeckel's Kunstformen der Natur,
1904.
Main article: Algae
Algae comprise several different groups of organisms which
produce energy through photosynthesis and for that reason have been included in
the plant kingdom in the past. Most conspicuous among the algae are the
seaweeds, multicellular algae that may roughly resemble land plants, but are
classified among the brown, red and green algae. Each of these algal groups
also includes various microscopic and single-celled organisms. There is good
evidence that some of these algal groups arose independently from separate
non-photosynthetic ancestors, with the result that many groups of algae are no
longer classified within the plant kingdom as it is defined here.[6][7]
The Viridiplantae, the green plants – green algae and land
plants – form a clade, a group consisting of all the descendants of a common
ancestor. With a few exceptions among the green algae, all green plants have
many features in common, including cell walls containing cellulose,
chloroplasts containing chlorophylls a and b, and food stores in the form of
starch. They undergo closed mitosis without centrioles, and typically have
mitochondria with flat cristae. The chloroplasts of green plants are surrounded
by two membranes, suggesting they originated directly from endosymbiotic
cyanobacteria.
Two additional groups, the Rhodophyta (red algae) and
Glaucophyta (glaucophyte algae), also have chloroplasts which appear to be
derived directly from endosymbiotic cyanobacteria, although they differ in the
pigments which are used in photosynthesis and so are different in colour. All
three groups together are generally believed to have a single common origin,
and so are classified together in the taxon Archaeplastida, whose name implies
that the chloroplasts or plastids of all the members of the taxon were derived
from a single ancient endosymbiotic event. This is the broadest modern
definition of the plants.
In contrast, most other algae (e.g. heterokonts,
haptophytes, dinoflagellates, and euglenids) not only have different pigments
but also have chloroplasts with three or four surrounding membranes. They are
not close relatives of the Archaeplastida, presumably having acquired
chloroplasts separately from ingested or symbiotic green and red algae. They
are thus not included in even the broadest modern definition of the plant
kingdom, although they were in the past.
The green plants or Viridiplantae were traditionally divided
into the green algae (including the stoneworts) and the land plants. However,
it is now known that the land plants evolved from within a group of green
algae, so that the green algae by themselves are a paraphyletic group, i.e. a
group which excludes some of the descendants of a common ancestor. Paraphyletic
groups are generally avoided in modern classifications, so that in recent
treatments the Viridiplantae have been divided into two clades, the Chlorophyta
and the Streptophyta (or Charophyta).[8][9]
The Chlorophyta (a name that has also been used for all
green algae) are the sister group to the group from which the land plants
evolved. There are about 4,300 species[10] of mainly marine organisms, both
unicellular and multicellular. The latter include the sea lettuce, Ulva.
The other group within the Viridiplantae are the mainly
freshwater or terrestrial Streptophyta (or Charophyta), which consist of
several groups of green algae plus the stoneworts and land plants. (The names
have been used differently, e.g. Streptophyta to mean the group which excludes
the land plants and Charophyta for the stoneworts alone or the stoneworts plus
the land plants.) Streptophyte algae are either unicellular or form multicellular
filaments, branched or unbranched.[9] The genus Spirogyra is a filamentous
streptophyte alga familiar to many, as it is often used in teaching and is one
of the organisms responsible for the algal "scum" which pond-owners
so dislike. The freshwater stoneworts strongly resemble land plants and are
believed to be their closest relatives. Growing underwater, they consist of a
central stalk with whorls of branchlets, giving them a superficial resemblance
to horsetails, species of the genus Equisetum, which are true land plants.
Fungi
Main article: Fungi
The classification of fungi has been controversial until
quite recently in the history of biology. Linnaeus' original classification
placed the fungi within the Plantae, since they were unquestionably not animals
or minerals and these were the only other alternatives. With later developments
in microbiology, in the 19th century Ernst Haeckel felt that another kingdom
was required to classify newly discovered micro-organisms. The introduction of
the new kingdom Protista in addition to Plantae and Animalia, led to
uncertainty as to whether fungi truly were best placed in the Plantae or
whether they ought to be reclassified as protists. Haeckel himself found it
difficult to decide and it was not until 1969 that a solution was found whereby
Robert Whittaker proposed the creation of the kingdom Fungi. Molecular evidence
has since shown that the last common ancestor (concestor) of the Fungi was
probably more similar to that of the Animalia than of any other kingdom, including
the Plantae.
Whittaker's original reclassification was based on the
fundamental difference in nutrition between the Fungi and the Plantae. Unlike
plants, which generally gain carbon through photosynthesis, and so are called
autotrophic phototrophs, fungi generally obtain carbon by breaking down and
absorbing surrounding materials, and so are called heterotrophic saprotrophs.
In addition, the substructure of multicellular fungi is different from that of
plants, taking the form of many chitinous microscopic strands called hyphae,
which may be further subdivided into cells or may form a syncytium containing
many eukaryotic nuclei. Fruiting bodies, of which mushrooms are most familiar
example, are the reproductive structures of fungi, and are unlike any structures
produced by plants.
Diversity
The table below shows some species count estimates of
different green plant (Viridiplantae) divisions. It suggests there are about
300,000 species of living Viridiplantae, of which 85-90% are flowering plants.
(Note: as these are from different sources and different dates, they are not
necessarily comparable, and like all species counts, are subject to a degree of
uncertainty in some cases.)
Diversity of living green plant (Viridiplantae) divisions
Informal group Division
name Common name No. of living species Approximate
No. in informal group
Green algae Chlorophyta green algae (chlorophytes) 3,800 [11] – 4,300 [12] 8,500
(6,600 - 10,300)
Charophyta green
algae (e.g. desmids & stoneworts) 2,800;[13]
4,000-6,000 [14]
Bryophytes Marchantiophyta liverworts 6,000-8,000 [15] 19,000
(18,100 - 20,200)
Anthocerotophyta hornworts 100-200 [16]
Bryophyta mosses 12,000 [17]
Pteridophytes Lycopodiophyta club mosses 1,200 [7] 12,000
(12,200)
Pteridophyta ferns,
whisk ferns & horsetails 11,000 [7]
Seed plants Cycadophyta cycads 160
[18] 260,000
(259,511)
Ginkgophyta ginkgo 1 [19]
Pinophyta conifers 630 [7]
Gnetophyta gnetophytes 70 [7]
Magnoliophyta flowering
plants 258,650 [20]
The naming of plants is governed by the International Code
of Botanical Nomenclature and International Code of Nomenclature for Cultivated
Plants (see cultivated plant taxonomy).
Evolution
Further information: Evolutionary history of plants
The evolution of plants has resulted in increasing levels of
complexity, from the earliest algal mats, through bryophytes, lycopods, ferns
to the complex gymnosperms and angiosperms of today. The groups which appeared
earlier continue to thrive, especially in the environments in which they
evolved.
Evidence suggests that an algal scum formed on the land
1,200 million years ago, but it was not until the Ordovician Period, around 450
million years ago, that land plants appeared.[21] However, new evidence from
the study of carbon isotope ratios in Precambrian rocks has suggested that
complex photosynthetic plants developed on the earth over 1000 m.y.a.[22] These
began to diversify in the late Silurian Period, around 420 million years ago,
and the fruits of their diversification are displayed in remarkable detail in
an early Devonian fossil assemblage from the Rhynie chert. This chert preserved
early plants in cellular detail, petrified in volcanic springs. By the middle
of the Devonian Period most of the features recognised in plants today are
present, including roots, leaves and secondary wood, and by late Devonian times
seeds had evolved.[23] Late Devonian plants had thereby reached a degree of
sophistication that allowed them to form forests of tall trees. Evolutionary
innovation continued after the Devonian period. Most plant groups were
relatively unscathed by the Permo-Triassic extinction event, although the
structures of communities changed. This may have set the scene for the
evolution of flowering plants in the Triassic (~200 million years ago), which
exploded in the Cretaceous and Tertiary. The latest major group of plants to
evolve were the grasses, which became important in the mid Tertiary, from
around 40 million years ago. The grasses, as well as many other groups, evolved
new mechanisms of metabolism to survive the low CO2 and warm, dry conditions of
the tropics over the last 10 million years.
This article may be confusing or unclear to readers. Please
help us clarify the article; suggestions may be found on the talk page. (March
2009)
A proposed phylogenetic tree of Plantae, after Kenrick and
Crane,[24] is as follows, with modification to the Pteridophyta from Smith et
al.[25] The Prasinophyceae may be a paraphyletic basal group to all green
plants.
Prasinophyceae (micromonads)
Streptobionta
Embryophytes
Stomatophytes
Polysporangiates
Tracheophytes
Eutracheophytes
Euphyllophytina
Lignophytia
Spermatophytes (seed plants)
Progymnospermophyta †
Pteridophyta
Pteridopsida (true ferns)
Marattiopsida
Equisetopsida (horsetails)
Psilotopsida (whisk ferns & adders'-tongues)
Cladoxylopsida †
Lycophytina
Lycopodiophyta
Zosterophyllophyta †
Rhyniophyta †
Aglaophyton †
Horneophytopsida †
Bryophyta (mosses)
Anthocerotophyta (hornworts)
Marchantiophyta (liverworts)
Charophyta
Chlorophyta
Trebouxiophyceae (Pleurastrophyceae)
Chlorophyceae
Ulvophyceae
Embryophytes
Main article: Embryophyte
Dicksonia antarctica, a species of tree fern
The plants that are likely most familiar to us are the
multicellular land plants, 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. About three hundred plant
species do not photosynthesize but are parasites on other species of
photosynthetic plants. Plants are distinguished from green algae, which
represent a mode of photosynthetic life similar to the kind modern plants are
believed to have evolved from, by having specialized reproductive organs
protected by non-reproductive tissues.
Bryophytes first appeared during the early Paleozoic. 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 period,
and by the Devonian 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.
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 and diversifying rapidly during the Cretaceous. These
differ in that the seed embryo (angiosperm) 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
Main articles: Paleobotany and Evolutionary history of
plants
A petrified log in Petrified Forest National Park.
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.
The earliest fossils clearly assignable to Kingdom Plantae
are fossil green algae from the Cambrian. These fossils resemble calcified
multicellular members of the Dasycladales. Earlier Precambrian fossils are
known which resemble single-cell green algae, but definitive identity with that
group of algae is uncertain.
The oldest known fossils of embryophytes date from the Ordovician,
though such fossils are fragmentary. By the Silurian, fossils of whole plants
are preserved, including the lycophyte Baragwanathia longifolia. From the
Devonian, detailed fossils of rhyniophytes have been found. 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.
The Coal measures are a major source of Paleozoic 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.
Structure, growth, and development
Further information: Plant morphology
Most of the solid material in a plant is taken from the
atmosphere. Through a process known as photosynthesis, most plants use the
energy in sunlight to convert carbon dioxide from the atmosphere, plus water,
into simple sugars. (Parasitic plants, on the other hand, use the resources of
its host to grow.) These sugars are then used as building blocks and form the
main structural component of the plant. Chlorophyll, a green-colored,
magnesium-containing pigment is essential to this process; it is generally
present in plant leaves, and often in other plant parts as well.
Plants usually rely on soil primarily for support and water
(in quantitative terms), but also obtain compounds of nitrogen, phosphorus,
potassium, magnesium and other elemental nutrients. Epiphytic and lithophytic
plants depend on air and nearby debris for nutrients, and carnivorous plants
supplement their nutrient requirements with insect prey that they capture. For
the majority of plants to grow successfully they also require oxygen in the
atmosphere and around their roots (soil gas) for respiration. However, some
plants grow as submerged aquatics, using oxygen dissolved in the surrounding
water, and a few specialized vascular plants, such as mangroves, can grow with
their roots in anoxic conditions.
The leaf is usually the primary site of photosynthesis in
plants.
There is no photosynthesis in deciduous leaves in autumn.
Factors affecting growth
The genotype of a plant affects its growth. For example,
selected varieties of wheat grow rapidly, maturing within 110 days, whereas
others, in the same environmental conditions, grow more slowly and mature
within 155 days.[26]
Growth is also determined by environmental factors, such as
temperature, available water, available light, and available nutrients in the
soil. Any change in the availability of these external conditions will be
reflected in the plants growth.
Biotic factors are also capable of affecting plant growth.
Plants compete with other plants for space, water, light and nutrients. Plants
can be so crowded that no single individual produces normal growth, causing
etiolation and chlorosis. Optimal plant growth can be hampered by grazing
animals, suboptimal soil composition, lack of mycorrhizal fungi, and attacks by
insects or plant diseases, including those caused by bacteria, fungi, viruses,
and nematodes.[26]
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 plants live and
reproduce within one growing season, biennial plants live for two growing
seasons and usually reproduce in second year, and perennial plants live for
many growing seasons and continue to reproduce once they are mature. These
designations often depend on climate and other environmental factors; plants
that are annual in alpine or temperate regions can be biennial or perennial in
warmer climates. 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 of it. 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 millimeters per hour (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.
Dried dead plants
Plants protect themselves from frost and dehydration stress
with antifreeze proteins, heat-shock proteins and sugars (sucrose is common).
LEA (Late Embryogenesis Abundant) protein expression is induced by stresses and
protects other proteins from aggregation as a result of desiccation and
freezing.[27]
Plant cell
Plant cell structure
Main article: Plant cell
Plant cells are typically distinguished by their large
water-filled central vacuole, chloroplasts, and rigid cell walls that are made
up of cellulose, hemicellulose, and pectin. Cell division is also characterized
by the development of a phragmoplast for the construction of a cell plate in
the late stages of cytokinesis. Just as in animals, plant cells differentiate
and develop into multiple cell types. Totipotent meristematic cells can
differentiate into vascular, storage, protective (e.g. epidermal layer), or
reproductive tissues, with more primitive plants lacking some tissue types.[28]
Physiology
Main article: Plant physiology
Photosynthesis
Main articles: Photosynthesis and Biological pigment
Plants are photosynthetic, which means that they manufacture
their own food molecules using energy obtained from light. The primary
mechanism plants have for capturing light energy is the pigment chlorophyll.
All green plants contain two forms of chlorophyll, chlorophyll a and
chlorophyll b. The latter of these pigments is not found in red or brown algae.
Immune system
See also: Immune system and Plant disease resistance
By means of cells that behave like nerves, plants receive
and distribute within their systems information about incident light intensity
and quality. Incident light which stimulates a chemical reaction in one leaf,
will cause a chain reaction of signals to the entire plant via a type of cell
termed a bundle sheath cell. Researchers from the Warsaw University of Life
Sciences in Poland, found that plants have a specific memory for varying light
conditions which prepares their immune systems against seasonal pathogens.[29]
Plants use pattern-recognition receptors to recognize conserved microbial
signatures. This recognition triggers an immune response. The first plant
receptors of conserved microbial signatures were identified in rice (XA21,
1995)[30] and in Arabidopsis thaliana (FLS2, 2000).[31] Plants also carry
immune receptors that recognize highly variable pathogen effectors. These
include the NBS-LRR class of proteins.
Internal distribution
Main article: Vascular tissue
Vascular plants differ from other plants in that they
transport nutrients between different parts through specialized structures,
called xylem and phloem. They also have roots for taking up water and minerals.
The xylem moves water and minerals from the root to the rest of the plant, and
the phloem provides the roots with sugars and other nutrient produced by the
leaves.[28]
Ecology
Main article: Plant ecology
The photosynthesis conducted by land plants and algae is the
ultimate source of energy and organic material in nearly all ecosystems.
Photosynthesis radically changed the composition of the early Earth's
atmosphere, which as a result is now 21% oxygen. Animals and most other
organisms are aerobic, relying on oxygen; those that do not are confined to
relatively rare anaerobic environments. Plants are the primary producers in
most terrestrial ecosystems and form the basis of the food web in those
ecosystems. Many animals rely on plants for shelter as well as oxygen and food.
Land plants are key components of the water cycle and
several other biogeochemical cycles. Some plants have coevolved with nitrogen
fixing bacteria, making plants an important part of the nitrogen cycle. Plant
roots play an essential role in soil development and prevention of soil
erosion.
Distribution
This
section requires expansion. (June 2008)
Plants are distributed worldwide in varying numbers. While
they inhabit a multitude of biomes and ecoregions, few can be found beyond the
tundras at the northernmost regions of continental shelves. At the southern
extremes, plants have adapted tenaciously to the prevailing conditions. (See
Antarctic flora.)
Plants are often the dominant physical and structural
component of habitats where they occur. Many of the Earth's biomes are named
for the type of vegetation because plants are the dominant organisms in those
biomes, such as grasslands and forests.
Ecological relationships
The Venus flytrap, a species of carnivorous plant.
Numerous animals have coevolved with plants. Many animals
pollinate flowers in exchange for food in the form of pollen or nectar. Many animals
disperse seeds, often by eating fruit and passing the seeds in their feces.
Myrmecophytes are plants that have coevolved with ants. The plant provides a
home, and sometimes food, for the ants. In exchange, the ants defend the plant
from herbivores and sometimes competing plants. Ant wastes provide organic
fertilizer.
The majority of plant species have various kinds of fungi
associated with their root systems in a kind of mutualistic symbiosis known as
mycorrhiza. The fungi help the plants gain water and mineral nutrients from the
soil, while the plant gives the fungi carbohydrates manufactured in
photosynthesis. Some plants serve as homes for endophytic fungi that protect
the plant from herbivores by producing toxins. The fungal endophyte, Neotyphodium
coenophialum, in tall fescue (Festuca arundinacea) does tremendous economic
damage to the cattle industry in the U.S.
Various forms of parasitism are also fairly common among
plants, from the semi-parasitic mistletoe that merely takes some nutrients from
its host, but still has photosynthetic leaves, to the fully parasitic broomrape
and toothwort that acquire all their nutrients through connections to the roots
of other plants, and so have no chlorophyll. Some plants, known as
myco-heterotrophs, parasitize mycorrhizal fungi, and hence act as epiparasites
on other plants.
Many plants are epiphytes, meaning they grow on other
plants, usually trees, without parasitizing them. Epiphytes may indirectly harm
their host plant by intercepting mineral nutrients and light that the host
would otherwise receive. The weight of large numbers of epiphytes may break
tree limbs. Hemiepiphytes like the strangler fig begin as epiphytes but
eventually set their own roots and overpower and kill their host. Many orchids,
bromeliads, ferns and mosses often grow as epiphytes. Bromeliad epiphytes
accumulate water in leaf axils to form phytotelmata, complex aquatic food
webs.[32]
Approximately 630 plants are carnivorous, such as the Venus
Flytrap (Dionaea muscipula) and sundew (Drosera species). They trap small
animals and digest them to obtain mineral nutrients, especially nitrogen and
phosphorus.[33]
Importance
Potato plant. Potatoes spread to the rest of the world after
European contact with the Americas in the late 15th and early 16th centuries
and have since become an important field crop.
Timber in storage for later processing at a sawmill.
A section of a Yew branch showing 27 annual growth rings,
pale sapwood and dark heartwood, and pith (centre dark spot). The dark radial
lines are longitudinal sections of small branches which became included by
growth of the tree.
The study of plant uses by people is termed economic botany
or ethnobotany; some consider economic botany to focus on modern cultivated
plants, while ethnobotany focuses on indigenous plants cultivated and used by
native peoples. Human cultivation of plants is part of agriculture, which is
the basis of human civilization. Plant agriculture is subdivided into agronomy,
horticulture and forestry.
Food
Much of human nutrition depends on land plants, either
directly or indirectly.
Human nutrition depends to a large extent on cereals,
especially maize (or corn), wheat and rice. Other staple crops include potato,
cassava, and legumes. Human food also includes vegetables, spices, and certain
fruits, nuts, herbs, and edible flowers.
Beverages produced from plants include coffee, tea, wine,
beer and alcohol.
Sugar is obtained mainly from sugar cane and sugar beet.
Cooking oils and margarine come from maize, soybean,
rapeseed, safflower, sunflower, olive and others.
Food additives include gum arabic, guar gum, locust bean
gum, starch and pectin.
Livestock animals including cows, pigs, sheep, and goats are
all herbivores; and feed primarily or entirely on cereal plants, particularly
grasses.
Nonfood products
Wood is used for buildings, furniture, paper, cardboard,
musical instruments and sports equipment. Cloth is often made from cotton, flax
or synthetic fibers derived from cellulose, such as rayon and acetate.
Renewable fuels from plants include firewood, peat and many other biofuels.
Coal and petroleum are fossil fuels derived from plants. Medicines derived from
plants include aspirin, taxol, morphine, quinine, reserpine, colchicine,
digitalis and vincristine. There are hundreds of herbal supplements such as
ginkgo, Echinacea, feverfew, and Saint John's wort. Pesticides derived from
plants include nicotine, rotenone, strychnine and pyrethrins. Drugs obtained
from plants include opium, cocaine and marijuana. Poisons from plants include
ricin, hemlock and curare. Plants are the source of many natural products such
as fibers, essential oils, natural dyes, pigments, waxes, tannins, latex, gums,
resins, alkaloids, amber and cork. Products derived from plants include soaps,
paints, shampoos, perfumes, cosmetics, turpentine, rubber, varnish, lubricants,
linoleum, plastics, inks, chewing gum and hemp rope. Plants are also a primary source
of basic chemicals for the industrial synthesis of a vast array of organic
chemicals. These chemicals are used in a vast variety of studies and
experiments.
Aesthetic uses
Thousands of plant species are cultivated for aesthetic
purposes as well as to provide shade, modify temperatures, reduce wind, abate
noise, provide privacy, and prevent soil erosion. People use cut flowers, dried
flowers and houseplants indoors or in greenhouses. In outdoor gardens, lawn
grasses, shade trees, ornamental trees, shrubs, vines, herbaceous perennials
and bedding plants are used. Images of plants are often used in art,
architecture, humor, language, and photography and on textiles, money, stamps,
flags and coats of arms. Living plant art forms include topiary, bonsai, ikebana
and espalier. Ornamental plants have sometimes changed the course of history,
as in tulipomania. Plants are the basis of a multi-billion dollar per year
tourism industry which includes travel to arboretums, botanical gardens,
historic gardens, national parks, tulip festivals, rainforests, forests with
colorful autumn leaves and the National Cherry Blossom Festival. Venus Flytrap,
sensitive plant and resurrection plant are examples of plants sold as
novelties.
Scientific and cultural uses
Tree rings are an important method of dating in archeology
and serve as a record of past climates. Basic biological research has often
been done with plants, such as the pea plants used to derive Gregor Mendel's
laws of genetics. Space stations or space colonies may one day rely on plants
for life support. Plants are used as national and state emblems, including
state trees and state flowers. Ancient trees are revered and many are famous.
Numerous world records are held by plants. Plants are often used as memorials, gifts
and to mark special occasions such as births, deaths, weddings and holidays.
Plants figure prominently in mythology, religion and literature. The field of
ethnobotany studies plant use by indigenous cultures which helps to conserve
endangered species as well as discover new medicinal plants. Gardening is the
most popular leisure activity in the U.S. Working with plants or horticulture
therapy is beneficial for rehabilitating people with disabilities. Certain
plants contain psychotropic chemicals which are extracted and ingested,
including tobacco, cannabis (marijuana), and opium.
Negative effects
Weeds are plants that grow where people do not want them.
People have spread plants beyond their native ranges and some of these
introduced plants become invasive, damaging existing ecosystems by displacing
native species. Invasive plants cause billions of dollars in crop losses
annually by displacing crop plants, they increase the cost of production and
the use of chemical means to control them affects the environment.
Plants may cause harm to animals, including people. Plants
that produce windblown pollen invoke allergic reactions in people who suffer
from hay fever. A wide variety of plants are poisonous. Toxalbumins are plant
poisons fatal to most mammals and act as a serious deterrent to consumption.
Several plants cause skin irritations when touched, such as poison ivy. Certain
plants contain psychotropic chemicals, which are extracted and ingested or
smoked, including tobacco, cannabis (marijuana), cocaine and opium. Smoking
causes damage to health or even death, while some drugs may also be harmful or
fatal to people.[34][35] Both illegal and legal drugs derived from plants may
have negative effects on the economy, affecting worker productivity and law
enforcement costs.[36][37] Some plants cause allergic reactions when ingested,
while other plants cause food intolerances that negatively affect health.
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