Flower :
A flower, sometimes known as a bloom or blossom,
is the reproductive structure found in flowering
plants (plants of the division Magnoliophyta,
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, 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.
Morphology :
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 anther
where 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.
Christmas Lily (Lilium longiflorum). 1. Stigma, 2. Style, 3. Stamens, 4. Filament, 5. Petal
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, the double flowers of Peonies and Roses 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 are connate, 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 a radial 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 called pedicels. The apex of a flowering stem forms a terminal swelling which is called the torus or receptacle.
Floral formula
Inflorescence:
The familiar calla lily is not a single flower. It is actually an inflorescence of tiny flowers pressed together on a central stalk that is surrounded by a large petal-like bract.
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 biennial plants 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. These flowers are described by botanists as being perfect or bisexual. 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.
A flower, sometimes known as a bloom or blossom,
is the reproductive structure found in flowering plants (plants of the division Magnoliophyta,
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, 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.
Morphology :
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
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 anther
where 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.
Christmas Lily (Lilium longiflorum). 1. Stigma, 2. Style, 3. Stamens, 4. Filament, 5. Petal
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, the double flowers of Peonies and Roses 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 are connate, 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 a radial 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 called pedicels. The apex of a flowering stem forms a terminal swelling which is called the torus or receptacle.
Floral formula
Inflorescence:
The familiar calla lily is not a single flower. It is actually an inflorescence of tiny flowers pressed together on a central stalk that is surrounded by a large petal-like bract.
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 inflorescencesare 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 biennial plants 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
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. These flowers are described by botanists as being perfect or bisexual. 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.
