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Flower specialization and pollination Callistemon citrinus flowers Further information Pollination syndrome Flowering plants usually face selective pressure to optimise 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 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 Some flowers have patterns called nectar guides that show pollinators where to look for nectar 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 stigmasrranged with equally pointed precisionf 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 Morphology Raspberry flower 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 In essence a flower structure forms on a modified shoot or axis with an apical meristem that does not grow continuously growth is determinate Flowers may be attached to the plant in a few ways If the flower has no stem but forms in the axil of a leaf it is called sessile When one flower is produced the stem holding the flower is called a peduncle If the peduncle ends with groups of flowers each stem that holds a flower is called a pedicel The flowering stem forms a terminal end which is called 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 Diagram showing the main parts of a mature flower An example of a perfect flower this Crateva religiosa flower has both stamens outer ring and a pistil center 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 animals that help the process of pollination The coloration may extend into the ultraviolet which is visible to the compound eyes of insects but not to the eyes of birds 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 female or male 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 axiso called composite flowershe 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 lower is In botanical terminology a single daisy or sunflower for example is not a flower but a flower headn 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 producedhe 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 Christmas Lillium Lilium longiflorum 1 Stigma 2 Style 3 Stamens 4 Filament 5 Petal 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 1 Several additional symbols are sometimes used see Key to Floral Formulas 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 Flowers show great variation and plant scientists describe this variation in a systematic way to identify and distinguish species Specific terminology is used to descried 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 Petals that are connate 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 Development 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 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 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 Organ Development The ABC model of flower 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 Pollination Grains of pollen sticking to this bee will be transferred to the next flower it visits Tip of a tulip stamen Note the grains of pollen Main article 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 A Bee orchid has evolved over many generations to better mimic a female bee to attract male bees as pollinators Plants can not move from one location to another thus many flowers have evolved to attract animals to transfer pollen between individuals in dispersed populations Flowers that are insect pollinated are called entomophilous literally insect loving in Latin 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 Birds and bees have color vision enabling them 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