A flower, sometimes
known as a bloom or blossom, is the reproductive structure found in flowering plants (plants of the divisionMagnoliophyta, also called angiosperms). The biological
function of a flower is to effect reproduction, usually by providing a
mechanism for the union of sperm with eggs. Flowers may facilitate outcrossing
(fusion of sperm and eggs from different individuals in a population) or allow
selfing (fusion of sperm and egg from the same flower). Some flowers
produce diaspores without fertilization (parthenocarpy). Flowers contain sporangia and are the site
where gametophytes develop. Flowers give rise to fruit and seeds. Many flowers have evolved to be attractive
to animals (Verbal Sentence) , so as to cause them to be vectors for the
transfer of pollen.
In addition to facilitating
the reproduction of flowering plants, flowers have long been admired and used
by humans to beautify their environment, and also as objects of romance,
ritual, religion, medicine and as a source of food.
A stereotypical
flower consists of four kinds of structures attached to the tip of a short
stalk. Each of these kinds of parts is arranged in a whorl on the receptacle. The four main whorls
(starting from the base of the flower or lowest node and working upwards) are
as follows:
Calyx: the outermost whorl consisting of units
called sepals; these are typically green and enclose the
rest of the flower in the bud stage, however, they can be absent or prominent
and petal-like in some species.
Corolla: the next whorl toward the apex, composed of units
called petals, which are typically thin, soft and colored to
attract animals that help the process of pollination.
Androecium (from Greek andros oikia:
man's house): the next whorl (sometimes multiplied into several whorls),
consisting of units called stamens. Stamens consist of two parts: a stalk called a filament, topped by an antherwhere pollen is produced by meiosis and eventually dispersed.
Gynoecium (from Greek gynaikos oikia:
woman's house): the innermost whorl of a flower, consisting of one or more
units called carpels. The carpel or multiple fused carpels form a hollow structure called an ovary,
which produces ovules internally. Ovules are megasporangia and they in turn
produce megaspores by meiosis which develop into female gametophytes. These
give rise to egg cells. The gynoecium of a flower is also described using an
alternative terminology wherein the structure one sees in the innermost whorl
(consisting of an ovary, style and stigma) is called a pistil. A pistil may
consist of a single carpel or a number of carpels fused together. 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.
Although the
arrangement described above is considered "typical", plant species
show a wide variation in floral structure. These modifications have
significance in the evolution of flowering plants and are used extensively by
botanists to establish relationships among plant species.
The four main parts
of a flower are generally defined by their positions on the receptacle and not
by their function. Many flowers lack some parts or parts may be modified into
other functions and/or look like what is typically another part. In some
families, like Ranunculaceae, the petals are greatly reduced and in many
species the sepals are colorful and petal-like. Other flowers have modified stamens that are petal-like (Verbal
Sentence), the double flowers of Peonies andRoses are mostly petaloid stamens.[1] Flowers show great variation and plant
scientists describe this variation in a systematic way to identify and
distinguish species.
Specific
terminology is used to describe flowers and their parts. Many flower parts are
fused together; fused parts originating from the same whorl areconnate, while fused parts originating from different
whorls are adnate, parts that are not fused are free.
When petals are fused into a tube or ring that falls away as a single unit,
they are sympetalous (also called gamopetalous.) Connate petals may
have distinctive regions: the cylindrical base is the tube, the expanding
region is the throat and the flaring outer region is the limb. A sympetalous
flower, with bilateral symmetry with an upper and lower lip, is bilabiate.
Flowers with connate petals or sepals may have various shaped corolla or calyx
including: campanulate, funnelform, tubular, urceolate, salverform or rotate.
Many flowers have a
symmetry. When the perianth is bisected through the central axis from
any point, symmetrical halves are produced, forming aradial symmetry. These flowers are also known to be
actinomorphic or regular, 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.
Flowers may be
directly attached to the plant at their base (sessile—the supporting stalk or
stem is highly reduced or absent). The stem or stalk subtending a flower is
called a peduncle. If a peduncle supports more than one flower,
the stems connecting each flower to the main axis are calledpedicels. The apex of a flowering stem forms a terminal
swelling which is called the torus or receptacle.
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"
∞: to represent "many"
A floral formula
would appear something like this:
Ca5Co5A10
- ∞G1
Several additional
symbols are sometimes used (see Key to Floral Formulas).
Inflorescence
Main article: Inflorescence
In those species
that have more than one flower on an axis, the collective cluster of flowers is
termed an inflorescence. Some inflorescences are composed of many
small flowers arranged in a formation that resembles a single flower. The
common example of this is most members of the very large composite (Asteraceae)
group. A single daisy or sunflower, for example, is not a flower but a
flower head—an inflorescence composed of numerous flowers
(or florets).
An inflorescence
may include specialized stems and modified leaves known as bracts.
Development
A flower is a
modified stem tip with compressed internodes, bearing
structures that are highly modified leaves.[2] In essence, a flower develops on a
modified shoot or axis from a determinate apical meristem (determinate meaning the axis
grows to a set size).
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 is favorable for fertilization and the formation of seeds, hence ensuring maximal reproductive success. To meet these needs a plant is
able to interpret important endogenous and environmental cues such as changes
in levels of plant
hormones and
seasonable temperature and photoperiod changes.[3] Many perennial and most biennialplants require vernalization to flower. The molecular interpretation
of these signals is through the transmission of a complex signal known as florigen, which involves a variety of genes, including
CONSTANS, FLOWERING LOCUS C and FLOWERING LOCUS T. Florigen is produced in the
leaves in reproductively favorable conditions and acts in buds and
growing tips to induce a number of different physiological and morphological
changes.[4] 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.[5] Once the process begins, even if that cue
is removed the stem will continue to develop a flower.
Organ 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, when there is a loss of B-gene
function, mutant flowers are produced with sepals in the first whorl as usual,
but also in the second whorl instead of the normal petal formation. 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.
Floral function
The principal
purpose of a flower is the reproduction of the individual and the species. All
flowering plants are heterosporous, producing two types of spores. Microspores are produced by meiosis inside anthers while megaspores are
produced inside ovules, inside an ovary. In fact, anthers typically consist of
four microsporangia and an ovule is an integumented megasporangium. Both types
of spores develop into gametophytes inside sporangia. As with all heterosporous
plants, the gametophytes also develop inside the spores (are endosporic).
In the majority of
species, individual flowers have both
functional carpels and stamens (verbal sentence). These flowers are
described by botanists as being perfect orbisexual.
Some flowers lack one or the other reproductive organ and called imperfect or unisexual If
unisex flowers are found on the same individual plant but in different
locations, the species is said to be monoecious. If each type of unisex flower is found only
on separate individuals, the plant is dioecious.
Flower specialization and pollination
Further
information: Pollination syndrome
Flowering plants
usually face selective pressure to optimize the transfer of their pollen, and this is typically reflected in the morphology of the flowers and the
behaviour of the plants. Pollen may be transferred between plants via a number
of 'vectors'. Some plants make use of abiotic vectors — namely wind (anemophily) or, much less commonly, water (hydrophily). Others use biotic vectors including insects
(entomophily), birds (ornithophily), bats (chiropterophily) or other animals. Some plants make use of
multiple vectors, but many are highly specialised.
Cleistogamous
flowers are self
pollinated, after which they may or may not open. Many Viola and some Salvia species are known to have these types of
flowers (Verbal Sentence) .
The flowers of plants that make use of biotic
pollen vectors commonly have glands called nectaries that act as an incentive
for animals to visit the flower (Verbal Sentence). Some flowers have patterns,
called nectar guides,
that show pollinators where to look for nectar (Verbal Sentence). Flowers also attract pollinators by scent and
color. Still other flowers use mimicry to attract pollinators. Some species of
orchids, for example, produce flowers resembling female bees in color, shape,
and scent. Flowers are also 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.
Anemophilous
flowers use the wind to
move pollen from one flower to the next. Examples include grasses, birch trees,
ragweed and maples. They have no need to attract pollinators and therefore tend
not to be "showy" flowers. Male and female reproductive organs are
generally found in separate flowers, the male flowers having a number of long
filaments terminating in exposed stamens, and the female flowers having long,
feather-like stigmas. Whereas the pollen of animal-pollinated 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 animals.
Pollination
The primary purpose
of a flower is reproduction. Since the flowers are the reproductive organs
of plant, they mediate the joining of the sperm, contained within pollen, to
the ovules — contained in the ovary. Pollination is the movement of pollen
from the anthers to the stigma. The joining of the sperm to the ovules is
called fertilization. Normally pollen is moved from one plant to another, but
many plants are able to self pollinate. The fertilized ovules produce seeds
that are the next generation. Sexual reproduction produces genetically unique
offspring, allowing for adaptation. Flowers have specific designs which
encourages the transfer of pollen from one plant to another of the same
species. Many plants are dependent upon external factors for pollination,
including: wind and animals, and especially insects. 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
Plants cannot move
from one location to another, thus many
flowers have evolved to attract animals to transfer pollen (Verbal Sentence)between
individuals in dispersed populations. Flowers that are insect-pollinated are
called entomophilous; literally "insect-loving" in Greek.
They can be highly modified along with the pollinating insects by co-evolution. Flowers
commonly have glands called nectaries on various parts that
attract animals looking for nutritious nectar (Verbal
Sentence). Birds andbees have color vision, enabling them to seek
out "colorful" flowers. Some flowers have patterns, called nectar guides, that show pollinators where to look for
nectar; they may be visible only under ultraviolet light, which is visible to bees and some
other insects. Flowers also attract pollinators by scent and some of those scents are pleasant to our sense of smell. Not all
flower scents are appealing to humans; a number of flowers are pollinated by
insects that are attracted to rotten flesh and have flowers that smell like
dead animals, often called Carrion flowers, including Rafflesia, the titan arum, and the North American pawpaw (Asimina triloba). Flowers
pollinated by night visitors, including bats and moths, are likely to
concentrate on scent to attract pollinators and 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 flowers 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 toinsects, 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 cleistogamous. 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 (Verbal Sentence).
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
Main
article: biological 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
Evolution of flowers
While land plants have existed for about 425
million years, the first ones reproduced by a simple adaptation of their aquatic
counterparts: spores
(Verbal Sentence). In the sea, plants—and some animals—can simply scatter out genetic clones of themselves to float away and grow elsewhere. This is how early
plants reproduced. But plants soon evolved methods of 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,
though it had not yet evolved the flower. Early seed-bearing plants include
the ginkgo and conifers. The earliest fossil of a flowering
plant, Archaefructus
liaoningensis, is dated
about 125 million years old.[6] 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 (Non verbal Sentence). 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 (Non Verbal Sentence).
Recent DNA analysis
(molecular systematics)[7][8] shows 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[9] suggest
that it has features which may have been characteristic of the earliest
flowering plants (Non verbal Sentence).
The general
assumption is that the function of flowers, from the start, was to involve
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 (Verbal Sentence). 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 organisms, 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,(Non Verbal Sentence) including gigantopterids,[10] 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 (Non
Verbal Sentence).
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.[11] 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 (Non Verbal Sentence). 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.]
Symbolism
Many flowers have important symbolic meanings
in Western culture (Non Verbal Sentence). 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 United Kingdom, New Zealand, Australia and Canada, red poppies
are worn to commemorate soldiers who
have died in times of war (Verbal Sentence).
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'Keeffe, Imogen Cunningham,Veronica Ruiz de Velasco,
and Judy
Chicago, and in fact in Asian
and western classical art. Many cultures around the world have a marked
tendency to associate flowers with femininity.
The great variety
of delicate and beautiful flowers have
been inspired the works of numerous poets (Non Verbal Sentence), 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.
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 alotus flower.[13] Apart from the association with Vishnu,
the Hindu tradition also considers the lotus to have spiritual significance.[14] For example, it figures in the Hindu
stories of creation.[15]
Usage
In modern times, people have been sought ways to cultivate(Non
Verbal Sentence), buy, wear, or
otherwise be around flowers and blooming plants, partly because of their
agreeable appearance and 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
As tokens of love
or esteem
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
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.
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 have
been given their egg yolks a golden yellow color, which consumers find more
desirable (Non Verbal Sentence). 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).
Flowers can also be
made into herbal
teas. 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.
Flowers have been used since as far back as
50,000 years (Non Verbal Sentence) in funeral rituals. Many cultures do draw a connection between flowers and
life and death, and because of their seasonal return flowers also suggest
rebirth, which is the why many people place flowers upon graves. In ancient
times the Greeks would place a crown of flowers on the
head of the deceased as well as cover the tomb with wreaths and flower petals,
rich and powerful women in ancient Egypt would wear floral headdresses and
necklaces upon their death as representations of renewal and a joyful
afterlife, and the Mexicans to this day use flowers prominently in
their Day
of the Dead celebrations
in the same way that their Aztec ancestors did.
