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Flower
A flower, (< Old French flo(u)r < Latin florem < flos), also known as a
bloom or blossom, is the reproductive structure found in flowering plants
(plants of the division Magnoliophyta, also called angiosperms). The
flower's structure contains the plant's reproductive organs, and its
function is to produce seeds. After fertilization, portions of the flower
develop into a fruit containing the seeds. For the higher plants, seeds are
the next generation, and serve as the primary means by which individuals of
a species are dispersed across the landscape. The grouping of flowers on a
plant is called the inflorescence.
In addition to serving as the reproductive organs of flowering plants,
flowers have long been admired and used by humans, mainly to beautify their
environment but also as a source of food.
Function
The biological function of a flower is to mediate the union of male and
female gametes in order to produce seeds. The process begins with
pollination, is followed by fertilization, and continues with the formation
and dispersal of the seed.
Morphology
Flowering plants are heterosporangiate, producing two types of reproductive
spores). The pollen (male spores) and ovules (female spores) are produced in
different organs, but the typical flower is a bisporangiate strobilus in
that it contains both organs.
A flower is regarded as a modified stem with shortened internodes and
bearing, at its nodes, structures that may be highly modified leaves.[1] In
essence, a flower structure forms on a modified shoot or axis with an apical
meristem that does not grow continuously (growth is determinate). The stem
is called a pedicel, the end of which is the torus or receptacle. The parts
of a flower are arranged in whorls on the torus. The four main parts or
whorls (starting from the base of the flower or lowest node and working
upwards) are as follows:
* Calyx – the outer whorl of sepals; typically these are green, but are
petal-like in some species.
* Corolla – the whorl of petals, which are usually thin, soft and colored to
attract insects that help the process of pollination.
* Androecium (from Greek andros oikia: man's house) – one or two whorls of
stamens, each a filament topped by an anther where pollen is produced.
Pollen contains the male gametes.
* Gynoecium (from Greek gynaikos oikia: woman's house) – one or more
pistils. The female reproductive organ is the carpel: this contains an ovary
with ovules (which contain female gametes). A pistil may consist of a number
of carpels merged together, in which case there is only one pistil to each
flower, or of a single individual carpel (the flower is then called
apocarpous). The sticky tip of the pistil, the stigma, is the receptor of
pollen. The supportive stalk, the style becomes the pathway for pollen tubes
to grow from pollen grains adhering to the stigma, to the ovules, carrying
the reproductive material.
Although the floral structure described above is considered the "typical"
structural plan, plant species show a wide variety of modifications from
this plan. These modifications have significance in the evolution of
flowering plants and are used extensively by botanists to establish
relationships among plant species. For example, the two subclasses of
flowering plants may be distinguished by the number of floral organs in each
whorl: dicotyledons typically having 4 or 5 organs (or a multiple of 4 or 5)
in each whorl and monocotyledons having three or some multiple of three. The
number of carpels in a compound pistil may be only two, or otherwise not
related to the above generalization for monocots and dicots.
In the majority of species individual flowers have both pistils and stamens
as described above. These flowers are described by botanists as being
perfect, bisexual, or hermaphrodite. However, in some species of plants the
flowers are imperfect or unisexual: having only either male (stamens) or
female (pistil) parts. In the latter case, if an individual plant is either
male or female the species is regarded as dioecious. However, where
unisexual male and female flowers appear on the same plant, the species is
considered monoecious.
Additional discussions on floral modifications from the basic plan are
presented in the articles on each of the basic parts of the flower. In those
species that have more than one flower on an axis—so-called composite
flowers— the collection of flowers is termed an inflorescence; this term can
also refer to the specific arrangements of flowers on a stem. In this
regard, care must be exercised in considering what a ‘‘flower’’ is. In
botanical terminology, a single daisy or sunflower for example, is not a
flower but a flower head—an inflorescence composed of numerous tiny flowers
(sometimes called florets). Each of these flowers may be anatomically as
described above. Many flowers have a symmetry, if the perianth is bisected
through the central axis from any point, symmetrical halves are produced -
the flower is called regular or actinomorphic e.g. rose or trillium. When
flowers are bisected and produce only one line that produces symmetrical
halves the flower is said to be irregular or zygomorphic. e.g. snapdragon or
most orchids.
Floral formula
A floral formula is a way to represent the structure of a flower using
specific letters, numbers, and symbols. Typically, a general formula will be
used to represent the flower structure of a plant family rather than a
particular species. The following representations are used:
Ca = calyx (sepal whorl; e.g. Ca5 = 5 sepals)
Co = corolla (petal whorl; e.g., Co3(x) = petals some multiple of three )
Z = add if zygomorphic (e.g., CoZ6 = zygomorphic with 6 petals)
A = androecium (whorl of stamens; e.g., A∞ = many stamens)
G = gynoecium (carpel or carpels; e.g., G1 = monocarpous)
x - to represent a "variable number"
∞ - to represent "many"
A floral formula would appear something like this:
Ca5Co5A10 - ∞G1
Several additional symbols are sometimes used (see Key to Floral Formulas).
Pollination
The primary purpose of a flower is reproduction by the joining of pollen of
one plant with the ovules of another (or in some cases its own ovules) in
order to form seed which grows into the next generation of plants. Sexual
reproduction produces genetically unique offspring, allowing for adaptation
to occur. As such, each flower has a specific design which best encourages
the transfer of this pollen. Many flowers are dependent upon the wind to
move pollen between flowers of the same species. Others rely on animals
(especially insects) to accomplish this feat. Even large animals such as
birds, bats, and pygmy possums can be employed. The period of time during
which this process can take place (the flower is fully expanded and
functional) is called anthesis.
Attraction methods
Many flowers in nature have evolved to attract animals to pollinate the
flower, the movements of the pollinating agent contributing to the
opportunity for genetic recombination within a dispersed plant population.
Flowers that are insect-pollinated are called entomophilous (literally
"insect-loving"). Flowers commonly have glands called nectaries on their
various parts that attract these animals. Birds and bees are common
pollinators: both having color vision, thus opting for "colorful" flowers.
Some flowers have patterns, called nectar guides, that show pollinators
where to look for nectar; they may be visible to us or only under
ultraviolet light, which is visible to bees and some other insects. Flowers
also attract pollinators by scent. Many of their scents are pleasant to our
sense of smell, but not all. Some plants, such as Rafflesia, the titan arum,
and the North American pawpaw (Asimina triloba), are pollinated by flies, so
they produce a scent imitating rotting meat. Flowers pollinated by night
visitors such as bats or moths are especially likely to concentrate on scent
- which can attract pollinators in the dark - rather than color: most such
flowers are white.
Still other flowers use mimicry to attract pollinators. Some species of
orchids, for example, produce flowers resembling female bees in color,
shape, and scent. Male bees move from one such flower to another in search
of a mate.
Pollination mechanism
The pollination mechanism employed by a plant depends on what method of
pollination is utilized.
Most flowers can be divided between two broad groups of pollination methods:
Entomophilous - flowers attract and use insects, bats, birds or other
animals to transfer pollen from one flower to the next. often they are
specialized in shape and have an arrangement of the stamens that ensures
that pollen grains are transferred to the bodies of the pollinator when it
lands in search of its attractant (such as nectar, pollen, or a mate). In
pursuing this attractant from many flowers of the same species, the
pollinator transfers pollen to the stigmas - arranged with equally pointed
precision - of all of the flowers it visits. Many flower rely on simple
proximity between flower parts to ensure pollination. Others, such as the
Sarracenia or lady-slipper orchids, have elaborate designs to ensure
pollination while preventing self-pollination.
Anemophilous - flowers use the wind to move pollen from one flower to the
next, examples include the grasses, Birch trees, Ragweed and Maples. They
have no need to attract pollinators and therefore tend not to be "showy"
flowers. Whereas the pollen of entomophilous flowers tends to be
large-grained, sticky, and rich in protein (another "reward" for
pollinators), anemophilous flower pollen is usually small-grained, very
light, and of little nutritional value to insects, though it may still be
gathered in times of dearth. Honeybees and bumblebees actively gather
anemophilous corn (maize) pollen, though it is of little value to them.
Some flowers are self pollinated and use flowers that never open or are self
pollinated before the flowers open, these flowers are called clestigomous.
Many Viola species and some Salvia have these types of flowers.
Flower-pollinator relationships
Many flowers have close relationships with one or a few specific pollinating
organisms. Many flowers, for example, attract only one specific species of
insect, and therefore rely on that insect for successful reproduction. This
close relationship is often given as an example of coevolution, as the
flower and pollinator are thought to have developed together over a long
period of time to match each other's needs.
This close relationship compounds the negative effects of extinction. The
extinction of either member in such a relationship would mean almost certain
extinction of the other member as well. Some endangered plant species are so
because of shrinking pollinator populations.
Fertilization and dispersal
Some flowers with both stamens and a pistil are capable of
self-fertilization, which does increase the chance of producing seeds but
limits genetic variation. The extreme case of self-fertilization occurs in
flowers that always self-fertilize, such as many dandelions. Conversely,
many species of plants have ways of preventing self-fertilization. Unisexual
male and female flowers on the same plant may not appear or mature at the
same time, or pollen from the same plant may be incapable of fertilizing its
ovules. The latter flower types, which have chemical barriers to their own
pollen, are referred to as self-sterile or self-incompatible (see also:
Plant sexuality).
Evolution
While land plants have existed for about 425 million years, the first ones
reproduced by a simple adaptation of their aquatic counterparts: spores. In
the sea, plants -- and some animals -- can simply scatter out little living
copies of themselves to float away and grow elsewhere. This is how early
plants, such as the modern fern, are thought to have reproduced. But plants
soon began protecting these copies to deal with drying out and other abuse
which is even more likely on land than in the sea. The protection became the
seed...but not, yet, flowers. Early seed-bearing plants include the ginkgo,
conifers (like pines), and fir trees. The earliest fossil of a flowering
plant, Archaefructus liaoningensis, is dated about 125 million years old.[2]
Several groups of extinct gymnosperms, particularly seed ferns, have been
proposed as the ancestors of flowering plants but there is no continuous
fossil evidence showing exactly how flowers evolved. The apparently sudden
appearance of relatively modern flowers in the fossil record posed such a
problem for the theory of evolution that it was called an "abominable
mystery" by Charles Darwin. Recently discovered angiosperm fossils such as
Archaefructus, along with further discoveries of fossil gymnosperms, suggest
how angiosperm characteristics may have been acquired in a series of steps.
Recent DNA analysis (molecular systematics)[3][4] show that Amborella
trichopoda, found on the Pacific island of New Caledonia, is the sister
group to the rest of the flowering plants, and morphological studies[5]
suggest that it has features which may have been characteristic of the
earliest flowering plants.
The general assumption is that the function of flowers, from the start, was
to involve other animals in the reproduction process. Pollen can be
scattered without bright colors and obvious shapes, which would therefore be
a liability, using the plant's resources, unless they provide some other
benefit. One proposed reason for the sudden, fully developed appearance of
flowers is that they evolved in an isolated setting like an island, or chain
of islands, where the plants bearing them were able to develop a highly
specialized relationship with some specific animal (a wasp, for example),
the way many island species develop today. This symbiotic relationship, with
a hypothetical wasp bearing pollen from one plant to another much the way
fig wasps do today, could have eventually resulted in both the plant(s) and
their partners developing a high degree of specialization. Island genetics
is believed to be a common source of speciation, especially when it comes to
radical adaptations which seem to have required inferior transitional forms.
Note that the wasp example is not incidental; bees, apparently evolved
specifically for symbiotic plant relationships, are descended from wasps.
Likewise, most fruit used in plant reproduction comes from the enlargement
of parts of the flower. This fruit is frequently a tool which depends upon
animals wishing to eat it, and thus scattering the seeds it contains.
While many such symbiotic relationships remain too fragile to survive
competition with mainland animals and spread, flowers proved to be an
unusually effective means of production, spreading (whatever their actual
origin) to become the dominant form of land plant life.
While there is only hard proof of such flowers existing about 130 million
years ago, there is some circumstantial evidence that they did exist up to
250 million years ago. A chemical used by plants to defend their flowers,
oleanane, has been detected in fossil plants that old, including
gigantopterids[6], which evolved at that time and bear many of the traits of
modern, flowering plants, though they are not known to be flowering plants
themselves, because only their stems and prickles have been found preserved
in detail; one of the earliest examples of petrification.
The similarity in leaf and stem structure can be very important, because
flowers are genetically just an adaptation of normal leaf and stem
components on plants, a combination of genes normally responsible for
forming new shoots.[7] The most primitive flowers are thought to have had a
variable number of flower parts, often separate from (but in contact with)
each other. The flowers would have tended to grow in a spiral pattern, to be
bisexual (in plants, this means both male and female parts on the same
flower), and to be dominated by the ovary (female part). As flowers grew
more advanced, some variations developed parts fused together, with a much
more specific number and design, and with either specific sexes per flower
or plant, or at least "ovary inferior".
Flower evolution continues to the present day; modern flowers have been so
profoundly influenced by humans that many of them cannot be pollinated in
nature. Many modern, domesticated flowers used to be simple weeds, which
only sprouted when the ground was disturbed. Some of them tended to grow
with human crops, and the prettiest did not get plucked because of their
beauty, developing a dependence upon and special adaptation to human
affection.[8]
Development
The molecular control of floral organ identity determination is fairly well
understood. In a simple model, three gene activities interact in a
combinatorial manner to determine the developmental identities of the organ
primordia within the floral meristem. These gene functions are called A, B
and C-gene functions. In the first floral whorl only A-genes are expressed,
leading to the formation of sepals. In the second whorl both A- and B-genes
are expressed, leading to the formation of petals. In the third whorl, B and
C genes interact to form stamens and in the center of the flower C-genes
alone give rise to carpels. The model is based upon studies of homeotic
mutants in Arabidopsis thaliana and snapdragon, Antirrhinum majus. For
example, in a loss of B-gene function mutant flower we get sepals in the
first whorl as usual, but also in the second whorl (the B-function lost that
is needed for petal development). In the third whorl the lack of B function
but presence of C-function mimics the fourth whorl, leading to the formation
of carpels also in the third whorl. See also The ABC Model of Flower
Development.
Most genes central in this model belong to the MADS-box genes and are
transcription factors that regulate the expression of the genes specific for
each floral organ.
Flowering transition
The transition to flowering is one of the major phase changes that a plant
makes during its life cycle. The transition must take place at a time that
will ensure maximal reproductive success. To meet these needs a plant is
able to interpret important endogenous and environmental cues such as
changes in plant hormones levels and seasonable temperature and
photoperiodchanges. Many perennial and most biennial plants require
vernalization to flower. The molecular interpretation of these signals
through genes such as CONSTANS and FLC ensures that flowering occurs at a
time that is favorable for fertilization and the formation of seeds[9].
Flower formation is initiated at the ends of stems, and involves a number of
different physiological and morphological changes. The first step is the
transformation of the vegetative stem primordia into floral primordia. This
occurs as biochemical changes take place to change cellular differentiation
of leaf, bud and stem tissues into tissue that will grow into the
reproductive organs. Growth of the central part of the stem tip stops or
flattens out and the sides develop protuberances in a whorled or spiral
fashion around the outside of the stem end. These protuberances develop into
the sepals, petals, stamens, and carpels. Once this process begins, in most
plants, it cannot be reversed and the stems develop flowers, even if the
initial start of the flower formation event was dependent of some
environmental cue. Once the process begins, even if that cue is removed the
stem will continue to develop a flower.
Uses by humans
In everyday life
In modern times, people have sought ways to cultivate, buy, wear, or just be
around flowers and blooming plants, partly because of their agreeable smell.
Around the world, people use flowers for a wide range of events and
functions that, cumulatively, encompass one's lifetime:
* For new births or Christenings
* As a corsage or boutonniere to be worn at social functions or for holidays
* For wedding flowers for the bridal party, and decorations for the hall
* As brightening decorations within the home
* As a gift of remembrance for bon voyage parties, welcome home parties, and
"thinking of you" gifts
* For funeral flowers and expressions of sympathy for the grieving
People therefore grow flowers around their homes, dedicate entire parts of
their living space to flower gardens, pick wildflowers, or buy flowers from
florists who depend on an entire network of commercial growers and shippers
to support their trade.
Symbolism
Many flowers have important symbolic meanings in Western culture. The
practice of assigning meanings to flowers is known as floriography. Some of
the more common examples include:
* Red roses are given as a symbol of love, beauty, and passion.
* Poppies are a symbol of consolation in time of death. In the UK, Australia
and Canada, red poppies are worn to commemorate soldiers who have died in
times of war.
* Irises/Lily are used in burials as a symbol referring to
"resurrection/life". It is also associated with stars (sun) and its petals
blooming/shining.
* Daisies are a symbol of innocence.
Flowers within art are also representative of the female genitalia, as seen
in the works of artists such as Georgia O'Keefe, Imogen Cunningham, Veronica
Ruiz de Velasco, and Judy Chicago, and in fact in Asian and western
classical art.
Edible flowers
Flowers provide less food than other major plants parts (seeds, fruits,
roots, stems and leaves) but they provide several important foods and
spices. Flower vegetables include broccoli, cauliflower and artichoke. The
most expensive spice, saffron, consists of dried stigmas of a crocus. Other
flower spices are cloves and capers. Hops flowers are used to flavor beer.
Marigold flowers are fed to chickens to give their egg yolks a golden yellow
color, which consumers find more desirable. Dandelion flowers are often made
into wine. Bee Pollen, pollen collected from bees, is considered a health
food by some people. Honey consists of bee-processed flower nectar and is
often named for the type of flower, e.g. orange blossom honey, clover honey
and tupelo honey.
Hundreds of fresh flowers are edible but few are widely marketed as food.
They are often used to add color and flavor to salads. Squash flowers are
dipped in breadcrumbs and fried. Edible flowers include nasturtium,
chrysanthemum, carnation, cattail, honeysuckle, chicory, cornflower, Canna,
and sunflower. Some edible flowers are sometimes candied such as daisy and
rose (you may also come across a candied pansy).
Floristry
Under Construction
Main and related articles at: Floristry, Flower garden, Gardening, and List
of flowers Flowers can also be made into tea. Dried flowers such as
chrysanthemum, rose, jasmine, camomile are infused into tea both for their
fragrance and medical properties. Sometimes, they are also mixed with tea
leaves for the added fragrance.
In the arts
The great variety of delicate and beautiful flowers has inspired the works
of numerous poets, especially from the 18th-19th century Romantic era.
Famous examples include William Wordsworth's I Wandered Lonely as a Cloud
and William Blake's Ah! Sun-Flower.
Because of their varied and colorful appearance, flowers have long been a
favorite subject of visual artists as well. Some of the most celebrated
paintings from well-known painters are of flowers, such as Van Gogh's
sunflowers series or Monet's water lilies.
Flowers are also dried, freeze dried and pressed in order to create
permanent, three-dimensional pieces of flower art.
Mythology
The Roman goddess of flowers, gardens, and the season of Spring is Flora.
The Greek goddess of spring, flowers and nature is Chloris.
In Hindu mythology, flowers have a significant status. Vishnu, one of the
three major gods in the Hindu system, is often depicted standing straight on
a lotus flower.[10] Apart from the association with Vishnu, the Hindu
tradition also considers the lotus to have spiritual significance.[11] For
example, it figures in the Hindu stories of creation.[12]
References
1. ^ Eames, A. J. (1961) Morphology of the Angiosperms McGraw-Hill Book Co.,
New York.
2. ^ Flowers Modern & Ancient
3. ^ First Flower
4. ^ Amborella not a "basal angiosperm"? Not so fast
5. ^ South Pacific plant may be missing link in evolution of flowering
plants
6. ^ Oily Fossils Provide Clues To The Evolution Of Flowers
7. ^ Age-Old Question On Evolution Of Flowers Answered
8. ^ Human Affection Altered Evolution of Flowers
9. ^ Ausin et al (2005), Environmental regulation of flowering. Int J Dev
Biol. 2005;49(5-6):689-705
10. ^ Vishnu
11. ^ Flowers & Religion
12. ^ The Lotus
* Eames, A. J. (1961) Morphology of the Angiosperms McGraw-Hill Book Co.,
New York.
* Esau, Katherine (1965) Plant Anatomy (2nd ed.) John Wiley & Sons, New
York.
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