PLANTS are mainly multicellular , predominantly photosynthetic
eukaryotes of the kingdom PLANTAE.
The term is today generally limited to the GREEN PLANTS, which form
an unranked clade VIRIDIPLANTAE (Latin for "green plants"). This
includes the flowering plants , conifers and other gymnosperms , ferns
, clubmosses , hornworts , liverworts , mosses and the green algae ,
and excludes the red and brown algae . Historically, plants formed one
of two kingdoms covering all living things that were not animals , and
both algae and fungi were treated as plants; however all current
definitions of "plant" exclude the fungi and some algae, as well as
the prokaryotes (the archaea and bacteria ).
Green plants have cell walls containing cellulose and obtain most of
their energy from sunlight via photosynthesis by primary chloroplasts
, derived from endosymbiosis with cyanobacteria . Their chloroplasts
contain chlorophylls a and b, which gives them their green color. Some
plants are parasitic and have lost the ability to produce normal
amounts of chlorophyll or to photosynthesize. Plants are characterized
by sexual reproduction and alternation of generations , although
asexual reproduction is also common.
There are about 300–315 thousand species of plants, of which the
great majority, some 260–290 thousand, are seed plants (see the
table below ). Green plants provide most of the world's molecular
oxygen and are the basis of most of Earth's ecologies, especially on
land. Plants that produce grains , fruits and vegetables form
humankind's basic foodstuffs, and have been domesticated for
millennia. Plants play many roles in culture . They are used as
ornaments and, until recently and in great variety, they have served
as the source of most medicines and drugs . The scientific study of
plants is known as botany , a branch of biology .
* 1 Definition
* 1.1 Current definitions of Plantae
* 2 Diversity
* 2.1 Evolution
* 2.2 Embryophytes
* 2.3 Fossils
* 3 Structure, growth and development
* 3.1 Factors affecting growth
* 3.1.1 Effects of freezing
* 3.2 DNA damage and repair
* 4 Physiology
* 4.3 Internal distribution
* 5.1 Distribution
* 5.2 Ecological relationships
* 6 Importance
* 6.1 Foods and beverages
* 6.2 Nonfood products
* 6.3 Aesthetic uses
* 6.4 Scientific and cultural uses
* 6.5 Negative effects
* 7 See also
* 8 References
* 9 Further reading
* 10 External links
Plants are one of the two groups into which all living things were
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
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 four concepts. From
least to most inclusive, these four groupings are:
Land plants , also known as Embryophyta
Plantae _sensu strictissimo _
PLANTS IN THE STRICTEST SENSE include the liverworts , hornworts ,
mosses , and vascular plants , as well as fossil plants similar to
these surviving groups (e.g., Metaphyta Whittaker, 1969, Plantae
Margulis , 1971 ).
GREEN PLANTS, also known as VIRIDIPLANTAE , VIRIDIPHYTA or
Plantae _sensu stricto _
PLANTS IN A STRICT SENSE include the green algae , and land plants
that emerged within them, including stoneworts . The names given to
these groups vary considerably as of July 2011 . Viridiplantae
encompass a group of organisms that have cellulose in their cell walls
, possess chlorophylls _a_ and _b_ and have plastids that are bound by
only two membranes that are capable of storing starch. It is this
clade that is mainly the subject of this article (e.g., Plantae
Copeland , 1956 ).
Archaeplastida , also known as Plastida or Primoplantae
Plantae _sensu lato _
PLANTS IN A BROAD SENSE comprise the green plants listed above plus
Rhodophyta (red algae) and
Glaucophyta (glaucophyte algae). This clade
includes the organisms that eons ago acquired their chloroplasts
directly by engulfing cyanobacteria (e.g., Plantae Cavalier-Smith,
Old definitions of plant (obsolete)
Plantae _sensu amplo _
PLANTS IN AN AMPLE SENSE refers to older, obsolete classifications
that placed diverse algae, fungi or bacteria in Plantae (e.g., Plantae
or Vegetabilia Linnaeus, Plantae Haeckel 1866, Metaphyta Haeckel,
1894, Plantae Whittaker, 1969 ).
Another way of looking at the relationships between the different
groups that 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. Those
which have been called "plants" are in bold.
Glaucophyta (glaucophyte algae)
Rhodophyta (red algae)
Chlorophyta (part of green algae)
streptophyte algae (part of green algae)
Charales (stoneworts, often included
in green algae)
LAND PLANTS or embryophytes
included in the "algae"
The way in which the groups of green algae are combined and named
varies considerably between authors.
Green algae from
Ernst Haeckel 's
Kunstformen der Natur
Kunstformen der Natur _,
1904. Main article:
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 the brown algae , for example, are no
longer classified within the plant kingdom as it is defined here.
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 green algae, the green
plants have the following features in common; cell walls containing
cellulose , chloroplasts containing chlorophylls _a_ and _b_, and food
stores in the form of starch contained within the plastids. 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 that appear to be derived
directly from endosymbiotic cyanobacteria , although they differ in
the pigments which are used in photosynthesis from those of the
Viridiplantae and so are different in colour. In these groups, the
storage polysaccharide is floridean starch and is stored in the
cytoplasm rather than in the plastids. These groups appear to have had
a common origin with
Viridiplantae and the three groups form the clade
Archaeplastida , whose name implies that their chloroplasts were
derived from a single ancient endosymbiotic event. This is the
broadest modern definition of the term 'plant'.
In contrast, most other algae (e.g. brown algae/diatoms , 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 that excludes some of the descendants of a common
Paraphyletic groups are generally avoided in modern
classifications, so that in recent treatments the
been divided into two clades, the
Chlorophyta and the Streptophyta
(including the land plants and Charophyta).
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 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, which consists of the land plants together
with the Charophyta, itself consisting of several groups of green
algae such as the desmids and stoneworts . Streptophyte algae are
either unicellular or form multicellular filaments, branched or
unbranched. 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" that pond-owners so
dislike. The freshwater stoneworts strongly resemble land plants and
are believed to be their closest relatives. Growing in fresh water,
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.
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
Robert Whittaker proposed the creation of the kingdom
Fungi. Molecular evidence has since shown that the most recent common
ancestor (concestor), of the
Fungi was probably more similar to that
Animalia than to that of Plantae or any other kingdom.
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 autotrophs , 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 the most familiar example, are
the reproductive structures of fungi, and are unlike any structures
produced by plants.
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
NO. OF LIVING SPECIES
APPROXIMATE NO. IN INFORMAL GROUP
green algae (chlorophytes)
green algae (e.g. desmids vertical-align:top;">19,000
ferns, whisk ferns vertical-align:top;">260,000
The naming of plants is governed by the International Code of
Nomenclature for algae, fungi, and plants and International Code of
Nomenclature for Cultivated Plants (see cultivated plant taxonomy ).
Life timeline view • discuss • edit -4500 — – -4000 —
– -3500 — – -3000 — – -2500 — – -2000 — – -1500 —
– -1000 — – -500 — – 0 — _WATER _ Single-celled
life _PHOTOSYNTHESIS _ EUKARYOTES Multicellular
life LAND LIFE DINOSAURS MAMMALS FLOWERS ←
Earth (−4540 ) ← Earliest water ← Earliest
life ← LHB meteorites ← Earliest oxygen ←
Atmospheric oxygen ←
Oxygen crisis ← Earliest sexual
reproduction ← Ediacara biota ←
← Earliest humans P
n Pongola Huronian
Cryogenian Andean Karoo Quaternary
Axis scale : millions of years .
Orange labels: known _ICE AGES_.
Also see: _
Human timeline _ and _
Nature timeline _ Further
Evolutionary history of plants
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.
Plants in all of these groups continue to thrive, especially in the
environments in which they evolved.
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. 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. For
more than a century it has been assumed that the ancestors of land
plants evolved in aquatic environments and then adapted to a life on
land, an idea usually credited to botanist
Frederick Orpen Bower in
his 1908 book "The Origin of a Land Flora". A more recent alternative
view, supported by genetic evidence, is that they evolved from
single-celled algae that were already terrestrial. Primitive land
plants 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
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
seeds had evolved. 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
A 1997 proposed phylogenetic tree of Plantae, after Kenrick and
Crane, is as follows, with modification to the Pteridophyta from
Smith _et al._ The
Prasinophyceae are a paraphyletic assemblage of
early diverging green algal lineages, but are treated as a group
outside the Chlorophyta: later authors have not followed this
SPERMATOPHYTES (seed plants)
Pteridopsida (true ferns)
Psilotopsida (whisk ferns border-left:1px
Aglaophyton _ †
A newer proposed classification follows Leliaert et al. 2011 and
modified with Silar 2016 for the green algae clades and Novíkov
Palmophyllales Zechman et al. 2010
Prasinophyceae Christensen 1962 s.s.
Nephroselmidophyceae Cavalier-Smith 1993
Pseudoscourfieldiales Melkonian 1990
Mamiellophyceae Marin border-top: 0; border-right: 0; border-bottom:
0; vertical-align: top;">
Picocystis _ Lewin 2001
Scourfieldiales Moestrup 1991
Pedinophyceae Moestrup 1991
Chlorodendrophyceae Massjuk 2006
Trebouxiophyceae Friedl 1995
Ulvophyceae Mattox border-top: 0; border-right: 0; border-bottom: 0;
Chlorophyceae Christensen 1994
Charophyta Rabenhorst 1863 emend. Lewis border-top: 0; border-right:
0; border-bottom: 0; vertical-align: top;">
Chaetosphaeridiales Marin border-top: 0; border-right: 0;
border-bottom: 0; vertical-align: top;">
Bryophyta (True mosses)
Anthocerotophyta (Non-flowering hornworts)
Tracheophyta (Vascular Plants)
Dicksonia antarctica _, a species of
The plants that are likely most familiar to us are the multicellular
land plants, called embryophytes . Embryophytes include the vascular
plants , such as ferns, conifers and flowering plants. They also
include the _bryophytes _, of which mosses and liverworts are the most
All of these plants have eukaryotic cells with cell walls composed of
cellulose , and most obtain their energy through photosynthesis ,
using light , water and carbon dioxide to synthesize food. About three
hundred plant species do not photosynthesize but are parasites on
other species of photosynthetic plants. Embryophytes 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
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 bryophytes
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 . In bryophytes, the sporophyte
is always unbranched and remains nutritionally dependent on its parent
gametophyte. The bryophytes have the ability to secrete a cuticle on
their outer surface, a waxy layer that confers resistant to
desiccation. In the mosses and hornworts a cuticle is usually only
produced on the sporophyte.
Stomata are absent from liverworts, but
occur on the sporangia of mosses and hornworts, allowing gas exchange
while controlling water loss.
Vascular plants first appeared during the
Silurian period, and by the
Devonian had diversified and spread into many different terrestrial
environments. They developed a number of adaptations that allowed them
to spread into increasingly more arid places, notably the vascular
tissues xylem and phloem , that transport water and food throughout
Root systems capable of obtaining soil water and
nutrients also evolved during the Devonian. In modern vascular plants,
the sporophyte is typically large, branched, nutritionally independent
and long-lived, but there is increasing evidence that Paleozoic
gametophytes were just as complex as the sporophytes. The gametophytes
of all vascular plant groups evolved to become reduced in size and
prominence in the life cycle.
The first seed plants, Pteridosperms (seed ferns), now extinct,
appeared in the
Devonian and diversified through the Carboniferous. In
these the microgametophyte is reduced to pollen and the
megagametophyte remains inside the megasporangium, attached to the
parent plant. A megasporangium invested in protective layer called an
integument is known as an ovule . After fertilisation by means of
sperm deposited by pollen grains, an embryo develops inside the ovule.
The integument becomes a seed coat, and the ovule develops into a
Seed plants can survive and reproduce in extremely arid
conditions, because they are not dependent on free water for the
movement of sperm, or the development of free living gametophytes.
Early seed plants are gymnosperms , as the ovules and subsequent
seeds are not enclosed in a protective structure (carpels or fruit),
but are found naked, typically on cone scales.
Pollen typically lands
directly on the ovule. Four surviving groups remain widespread now,
particularly the conifers , which are dominant trees in several biomes
Evolutionary history of plants
Evolutionary history of plants A
petrified log in
Petrified Forest National Park
Petrified Forest National Park , Arizona
Plant fossils include roots, wood, leaves, seeds, fruit, pollen ,
spores , phytoliths , and amber (the fossilized resin produced by some
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
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 that resemble single-cell green algae, but
definitive identity with that group of algae is uncertain.
The oldest known fossils of embryophytes date from the
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.
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 Grove at Victoria
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
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
The earliest fossils attributed to green algae date from the
Precambrian (ca. 1200 mya). The resistant outer walls of
prasinophyte cysts (known as phycomata) are well preserved in fossil
deposits of the
Paleozoic (ca. 250-540 mya). A filamentous fossil
(Proterocladus) from middle Neoproterozoic deposits (ca. 750 mya) has
been attributed to the
Cladophorales , while the oldest reliable
records of the
Dasycladales ) and stoneworts are from
STRUCTURE, GROWTH AND DEVELOPMENT
Plant morphology The leaf is usually the
primary site of photosynthesis in plants.
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 their 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 . Plants use oxygen
and glucose (which may be produced from stored starch ) to provide
energy. 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
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.
Growth is also determined by environmental factors, such as
temperature , available water , available light , carbon dioxide and
available nutrients in the soil. Any change in the availability of
these external conditions will be reflected in the plant's 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.
There is no photosynthesis in deciduous leaves in autumn.
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 that 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.
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 .
Effects Of Freezing
When water freezes in plants, the consequences for the plant depend
very much on whether the freezing occurs within cells
(intracellularly) or outside cells in intercellular spaces (Glerum
1985). Intracellular freezing, which usually kills the cell (Lyons et
al. 1979) regardless of the hardiness of the plant and its tissues,
seldom occurs in nature because rates of cooling are rarely high
enough to support it. Rates of cooling of several degrees Celsius per
minute are typically needed to cause intracellular formation of ice
At rates of cooling of a few degrees Celsius per hour, segregation of
ice occurs in intercellular spaces, the “extraorgan ice” of Sakai
and Larcher (1987) and their coworkers. This may or may not be
lethal, depending on the hardiness of the tissue.
The process of intercellular ice formation was described by Glerum
(1985). At freezing temperatures, water in the intercellular spaces
of plant tissue freezes first, though the water may remain unfrozen
until temperatures drop below −7 °C (19 °F). After the initial
formation of ice intercellularly, the cells shrink as water is lost to
the segregated ice, and the cells undergo freeze-drying. This
dehydration is now considered the fundamental cause of freezing
DNA DAMAGE AND REPAIR
Plants are continuously exposed to a range of biotic and abiotic
stresses. These stresses often cause DNA damage directly, or
indirectly via the generation of reactive oxygen species . Plants are
capable of a DNA damage response that is a critical mechanism for
maintaining genome stability. The DNA damage response is particularly
important during seed germination , since seed quality tends to
deteriorate with age in association with DNA damage accumulation.
During germination repair processes are activated to deal with this
accumulated DNA damage. In particular, single- and double-strand
breaks in DNA can be repaired . The DNA checkpoint kinase ATM has a
key role in integrating progression through germination with repair
responses to the DNA damages accumulated by the aged seed.
Plant cell structure Main article:
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
Totipotent meristematic cells can differentiate into
vascular , storage, protective (e.g. epidermal layer ), or
reproductive tissues, with more primitive plants lacking some tissue
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. The simple equation of photosynthesis
is as follows:-
6CO2 + 6H2O → (in the presence of light and chlorophyll) C6H12O6 +
Immune system and
Plant disease resistance
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 that 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.
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) and in _
Arabidopsis thaliana _ (FLS2,
2000). Plants also carry immune receptors that recognize highly
variable pathogen effectors. These include the NBS-LRR class of
Vascular plants differ from other plants in that nutrients are
transported between their 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.
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 .
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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 .
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
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 that may contain complex aquatic food webs.
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 .
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Plants in culture
Plants in culture
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 .
FOODS AND BEVERAGES
Mechanical harvest of oats. Main article:
Much of human nutrition depends on plants, either directly through
foods and beverages consumed by people, or indirectly as feed for
animals or the flavoring of foods. The science of agriculture deals
with the planting, raising, nutrition, and harvest of food crops, and
has played a key role in the history of world civilizations.
Human nutrition depends to a large extent on cereals , especially
maize (or corn), wheat , rice , oats , and millet . Large areas of
many countries are given over to the cultivation of cereals for local
consumption or export to other countries.
Livestock animals including
cows , pigs , sheep , goats and camels are all herbivores ; and most
feed primarily or entirely on cereal plants. Cereals are staple crops
, meaning that they provide calories (in the form of complex
carbohydrates such as starch ) that are needed to fuel daily
activities, and thus form the foundation of a daily diet. Other staple
crops include potatoes , cassava , yams , and legumes .
Human food also includes vegetables , which consist principally of
leaves and stems eaten as food. Vegetables are important for the
vitamins , minerals , and dietary fiber they supply. Fruits provide a
higher quantity of sugars and have a sweeter taste than vegetables.
However, whether a particular food is considered a "vegetable" or a
"fruit" will depend on context, since the word _fruit_ has a more
precise definition in botany than it does in general use. Nuts and
seeds , including foods such as peanuts , walnuts , almonds , and
pistachios , contain unsaturated fats that are also necessary for a
healthy diet. As with fruits, the terms _nut_ and _seed_ have stricter
definitions in plant science.
Many plants are used to flavor foods. Such plants include herbs (e.g.
rosemary and mint ), which come from the green leafy parts of plants,
and spices (e.g. cumin and cinnamon ), which come from other plant
parts. Some plants produce edible flowers , which may be added to
salads or used to decorate foods. Sweeteners such as sugar and stevia
are derived from plants.
Sugar is obtained mainly from sugar cane and
sugar beet , and honey is created when bees regurgitate the nectar
from flowers. Cooking oils and margarine come from maize, soybean ,
rapeseed , safflower , sunflower , olive and others.
include gum arabic , guar gum , locust bean gum , starch and pectin .
Plants are also the source of beverages produced either by infusion ,
such as coffee and tea ; by fermentation , such as beer and wine ; or
by distillation , such as whisky , vodka , rum , and other alcoholic
Timber in storage for later processing at a sawmill .
Plants are the source of many natural products such as essential oils
, natural dyes , pigments, waxes, resins , tannins , alkaloids, amber
and cork . Products derived from plants include soaps, shampoos,
perfumes, cosmetics, paint, varnish, turpentine, rubber, latex ,
lubricants, linoleum, plastics, inks, and gums . Renewable fuels from
plants include firewood , peat and many other biofuels .
petroleum are fossil fuels derived from the remains of plants. Olive
oil has been used in lamps for centuries to provide illumination.
Structural resources and fibers from plants are used in both the
construction of dwellings and the manufacture of clothing.
used not only for buildings, boats, and furniture, but also for
smaller items such as musical instruments and sports equipment. Wood
also may be pulped for the manufacture of paper and cardboard. Cloth
is often made from cotton , flax , ramie or synthetic fibers derived
from cellulose , such as rayon and acetate . The thread that is used
to sew cloth likewise comes from plant fibers. Hemp and jute are grown
for their fibers, which may be woven into rope or rough sacking.
Plants are also a primary source of basic chemicals , both for their
medicinal and physiological effects, as well as for the industrial
synthesis of a vast array of organic chemicals. 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 . Certain plants contain psychotropic
chemicals that are extracted and ingested, including tobacco ,
cannabis (marijuana), opium , and cocaine . Poisons from plants
include ricin , hemlock and curare .
A rose espalier at Niedernhall in Germany. Capitals of
ancient Egyptian columns decorated to resemble papyrus plants. (at
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. Plants are the basis
of a multibillion-dollar per year tourism industry, which includes
travel to historic gardens , national parks , rainforests , forests
with colorful autumn leaves, and the National Cherry Blossom Festival
While some gardens are planted with food crops, many are planted for
aesthetic, ornamental, or conservation purposes. Arboretums and
botanical gardens are public collections of living plants. In private
outdoor gardens, lawn grasses, shade trees, ornamental trees, shrubs,
vines, herbaceous perennials and bedding plants are used. Gardens may
cultivate the plants in a naturalistic state, or may sculpture their
growth, as with topiary or espalier .
Gardening is the most popular
leisure activity in the U.S., and working with plants or horticulture
therapy is beneficial for rehabilitating people with disabilities.
Plants may also be grown or kept indoors as houseplants , or in
specialized buildings such as greenhouses that are designed for the
care and cultivation of living plants.
Venus Flytrap , sensitive plant
and resurrection plant are examples of plants sold as novelties. There
are also art forms specializing in the arrangement of cut or living
plant, such as bonsai , ikebana , and the arrangement of cut or dried
flowers. Ornamental plants have sometimes changed the course of
history, as in tulipomania .
Architectural designs resembling plants appear in the capitals of
ancient Egyptian columns, which were carved to resemble either the
Egyptian white lotus or the papyrus . Images of plants are often used
in painting and photography, as well as on textiles, money, stamps,
flags and coats of arms.
SCIENTIFIC AND CULTURAL USES
Barbara McClintock (1902–1992) was a pioneering cytogeneticist
who used maize (or corn) to study the mechanism of inheritance of
Basic biological research has often been done with plants. In
genetics , the breeding of pea plants allowed
Gregor Mendel to derive
the basic laws governing inheritance, and examination of chromosomes
in maize allowed
Barbara McClintock to demonstrate their connection to
inherited traits. The plant _
Arabidopsis thaliana _ is used in
laboratories as a model organism to understand how genes control the
growth and development of plant structures.
Space stations or space
colonies may one day rely on plants for life support .
Ancient trees are revered and many are famous .
Tree rings themselves
are an important method of dating in archeology, and serve as a record
of past climates.
Plants figure prominently in mythology , religion and literature .
They are used as national and state emblems, including state trees and
state flowers . Plants are often used as memorials, gifts and to mark
special occasions such as births, deaths, weddings and holidays. The
arrangement of flowers may be used to send hidden messages .
The field of ethnobotany studies plant use by indigenous cultures,
which helps to conserve endangered species as well as discover new
medicinal plants .
Weeds are uncultivated and usually unwanted plants growing in managed
environments such as farms , urban areas , gardens , lawns , and parks
. 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 costly damage in
crop losses annually by displacing crop plants, they further increase
the cost of production and the use of chemicals to control them, which
in turn 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. Both illegal and legal drugs derived
from plants may have negative effects on the economy, affecting worker
productivity and law enforcement costs. Some plants cause allergic
reactions when ingested, while other plants cause food intolerances
that negatively affect health.
Evolutionary history of plants
Evolutionary history of plants
Plant defense against herbivory
Plant defense against herbivory
Plants in space
The Plant List
The Plant List
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