Flower

A Phalaenopsis flower

Flowers are unique structures housing reproductive parts of plants belonging to the angiosperm branch of the plant family. All flowers share similar underlying features allowing them to produce seed, but there are a huge variety of shapes, colors, sizes and fragrances. Cultivation has led to additional varieties and diversity thanks to selective breeding.

Structure and function

Flower parts

Ornamental attractions

Life cycle

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Bud
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Flower
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Fruit

Different structures

There are countless flower forms, evolutionarily meant to aid pollination by either by insects, other animals, wind or even water. Flowers pollinated by insects and other animals tend to be brightly colored with a sweet scent, and usually having sugar-rich nectar. Some have evolved specialized forms to encourage particular pollinators - for example, the flowers of certain orchids can resemble female insects in order to attract the males. Flowers which are wind pollinated tend to be less easily visible and smaller, though in plants like grasses, they often are crowded in large inflorescences.

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Flowerhead
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Iris
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Bracts
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Spathe
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Spurred flower

Inflorescences

Flowers on some plants come as one flower on its own stem. Many other flowers come grouped into inflorescences. The types of inflorescences can be identified by the arrangement of flowers on the stem. Sometimes compound flowerheads resemble a single flower, such as in lantana.

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Solitary
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Cluster
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Flowerhead
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Umbel
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Cyme
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Spike
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Raceme
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Corymb
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Panicle

Shape

There are two types of flower shapes; either regular or radially symmetrical and rounded in outline; or long and irregular or symmetrical along one axis only. Petals can be separate (free) or else partly fused, forming a flower which is tubular or funnel-shaped. Composite flowers may have elongated florets, but usually the flowerhead is rounded.

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Cross-shaped (cruciform)
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Star-shaped (stellate)
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Saucer-shaped
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Cup-shaped
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Bell-shaped (campanulate)
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Tubular
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Funnel-shaped
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Salverform
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Trumpet-shaped
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Rosette
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Pompon
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Pea-like
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Pitcher-shaped
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Slipper-shaped

Petal arrangement

Almost all flowers in the wild have a single whorl or fused group of 2-6 petals. A few do have more, but this is more a tendency far more often seen in cultivated plants, which occurred as a mutation and was perpetuated through selective breeding. Semi-double flowers tend to have 2 or 3 whorls of petals, while double flowers have 3 or more whorls and no stamens (or just a few), sometimes they don't even have carpels. Many times stamens have been modified into structures like petals called staminodes. Doubling occurs in a good number of members of the Asteraceae, like Dahlia and Chrysanthemums, but when this happens the number of ray florets goes up, and partially or fully replaces the disk florets.

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Single
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Semi-double
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Double

Habits

The habit of a flower or an inflorescence is a description of its orientation on its stalk at maturity. This can change on some plants during the development of the flower.

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Erect
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Horizontal
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Nodding
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Pendant

Colors

Both colors and markings on a flower evolved originally as a means of attracting pollinators. For cultivated plants, they are modified to enhance their decorative value.

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Self-colored
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Bicolored
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Picotee
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Striped

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 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:

Anatomy of a Sarracenia flower. The umbrella shaped style is unique to this genus, and will look different in most flowers

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 pla nts 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.

This Crateva religiosa flower is perfect: it has both stamens (outer ring) and a pistil (center)

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., Co^3(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:

Ca⁵Co⁵A^{10 - ∞}G¹

Several additional symbols are sometimes used (see [1]).

Pollination

Grains of pollen sticking to this bee will be transfered to the next flower it visits

Main article: pollination

The primary purpose of a flower is to join the pollen of one plant with the ovules of another (or in some cases its own ovules) in order to form seed which is genetically unique, 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 pr ocess can take place (the flower is fully expanded and functional) is called anthesis.

Attraction methods

Bee orchid mimics a female bee in order to attract a male bee pollinator

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 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 desired.

Entomophilous flowers (those which employ insects to transfer pollen) 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.

The flowers of other species are pollinated by the wind (for example, grasses); they have no need to attract pollinators and therefore tend not to be "showy". Wind-pollinated flowers are referred to as anemophilous. 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.

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 polli nator 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

Main article: biological dispersal

In this picture you can clearly see the stamens of the flower

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).

Various flower colors and shapes

A Syrphid fly on a Virginia Bluebell

Lilies are often used to denote life or resurrection

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

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 are infused into tea by the oriental people both for their fragrance and medical properties. Sometimes, they are also mixed with tea leaves for the added fragrance.

References

↑ Eames, A. J. (1961) Morphology of the Angiosperms McGraw-Hill Book Co., New York.

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.

External links

[1] Eames, A. J. (1961) Morphology of the Angiosperms McGraw-Hill Book Co., New York.

[1]