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a sam creates 63 the abc model of flower development can be used for many plants to describe how groups of genes come together to induce each organ being produced 65 in general all aspects of floral development are controlled by a gene regulatory network of specialised mads box genes which includes the abc genes and associated proteins 66 67 for plants the transition into flowering is a major change and must occur at the right time so as to ensure reproductive success plants determine this time by interpreting both internal and environmental cues such as day length 64 the abc model was the first unifying principle in the development of flowers and its major tenets have been found to hold in most flowering plants 68 it describes how three groups of genes a b and c are responsible for the development of flowers these three gene groups activities interact together to determine the developmental identities of the primordia organ within the floral apical meristem alone a genes produce sepals in the first whorl together a and b produce the petals in the second whorl c genes alone produce carpels in the centre of the flower c and b together produce the stamens in the third whorl 65 this can also be extended to the more complex abcde model which adds an additional two gene groups to explain the development of structures like ovules 69 the transition to flowering is one of the major phase changes that a plant makes during its life cycle 14 the transition must take place at a time that is favourable for fertilisation and the formation of seeds hence ensuring maximal reproductive success to meet these needs a plant can interpret important internal and environmental cues such as changes in levels of plant hormones such as gibberellins 70 seasonable temperature and day length changes 64 many plants including many of those that have more than two year lifespans and just two year lifespans require cold exposure to flower 70 71 72 these cues are interpreted molecularly through a complex signal called florigen which involves a variety of genes florigen is produced in the leaves in reproductively favourable conditions and acts in stem tips to force switching from developing leaves to flowers 73 once developed flowers may selectively open and close their flowers at different times of day usually around dusk and dawn 74 they may also track the path of the sun to remain warm potentially both for their own benefit and to attract pollinators both of these mechanisms are controlled by a plant s circadian rhythm and in response to environmental changes 75 diagram showing the floral organs within a developing rose flower or rose bud diagram of the abc model of development pollination main article pollination a hoverfly pollinating oxalis pes caprae a grey headed flying fox feeding on nectar ophrys apifera which has evolved to mimic a female bee 76 mexican violetear colibri thalassinus pollinating a flower various examples of biotic pollination since the flowers are the reproductive organs of the plant they mediate the joining of the sperm contained within pollen to the eggs in the ovules contained in the ovary 77 pollination is this movement of pollen from the male parts to the female parts 78 it occurs either between flowers or from one part of a flower to another of the same plant as in self pollination or between flowers of different plants as in cross pollination cross pollination is more common in flowering plants as it increases genetic variation 11 79 pollination typically only takes place when the flower is fully expanded and functional 80 flowering plants usually face evolutionary pressure to optimise the transfer of their pollen and this is typically reflected in the morphology of their flowers and their reproductive strategies 81 82 agents that transport pollen between plants are called vectors around 80 of flowering plants make use of biotic or living vectors others use abiotic or non living vectors or some combination of the two 83 84 biotic pollination further information pollination syndrome flowers that use biotic vectors attract and use animals to transfer pollen from one flower to the next often they are shaped and designed to both attract pollinators and ensure pollen is transferred effectively 12 85 flowers most commonly employ insects 86 87 but also birds bats lizards 88 other mammals 89 snails and slugs 86 and in rare cases crustaceans and worms 89 rewards given to pollinators by flowers to encourage pollination include food such as pollen starch or nectar mates shelter a place to raise their young and pseudocopulation sexual deception 85 in the latter the flower is scented or shaped so as to encourage sexual arousal and pollination from the subsequent intercourse 90 they may also be attracted by various stimuli such as size and scent as in carrion flowers colour is also a factor and includes nectar guides which show pollinators where to look for nectar they may be visible only under ultraviolet light 12 91 92 many flowers have close relationships with just one or a few specific pollinators they may be structured to allow or encourage pollination from these organisms this increases efficiency because there is a higher chance pollination comes from pollen of the same species of plant 93 this close relationship is an example of coevolution as the plant and pollinator have developed together over a long period to match each other s needs 58 abiotic pollination main articles anemophily and hydrophily a male catkin or inflorescence of small wind pollinated flowers 44 of populus tremula the female flower of enhalus acoroides which is pollinated by the flow of water flowers that use abiotic or non living vectors use the wind or much less commonly water to move pollen from one flower to the next 84 wind dispersed species do not need to attract pollinators and therefore tend not to grow large showy or colourful flowers and do not have nectaries nor a noticeable scent 93 whereas the pollen of insect pollinated flowers is usually large sticky and rich in protein to act as a reward wind pollinated flowers pollen is typically small very light smooth and of little nutritional value 94 95 fertilisation and seed development a flower s ovules are fertilised by sperm cells from pollen grains fertilisation is the fusion of the male and female sex cells to produce a zygote from which a new organism develops 10 in humans sexual intercourse results in the depositing of sperm cells into the vagina although not all survive they travel until one reaches the egg in the fallopian tube where the male and female sex cells fuse in the process of fertilisation 96 in flowering plants fertilisation is preceded by pollination which is the movement of pollen from the stamen to the carpel it encompasses both plasmogamy the fusion of the protoplasts cell without cell wall and karyogamy the fusion of the nuclei when pollen lands on the stigma of the flower it begins creating a pollen tube which runs down through the style and into the ovary after penetrating the centre most part of the ovary it enters the egg apparatus and is guided by a specialised cell 97 next the end of the pollen tube bursts and releases the two sperm cells one of which makes its way to an egg while also losing its cell membrane and much of the jelly like substance that fills its cells the sperm s nucleus then fuses with the egg s nucleus resulting in the formation of a zygote a diploid cell containing two copies of each chromosome 10 97 flowering plants undergo double fertilisation which involves both karyogamy and plasmogamy in double fertilisation the second sperm cell subsequently also fuses with the two polar nuclei of the central cell since all three nuclei are haploid they result in a large nutrient tissue nucleus which is triploid 97 seed and fruit development main articles seed development and fruit development development of seeds peas and fruit pod from pea flower following its formation the zygote begins to grow through nuclear and cellular divisions called mitosis eventually becoming a small group of cells one section of it becomes the embryo 98 while the other becomes the suspensor a structure which forces the embryo into the endosperm and is later undetectable two small groups of cells also form at this time which later become the cotyledon or initial leaf which is used as an energy store the next stage involves the growth of several key structures including the embryotic root the embryotic stem and the root or shoot junction itself in the final step vascular tissue develops around the seed 99 the ovary inside which the seed is forming from the ovule grows into a fruit all the other main floral parts wither and die during this development including the style stigma stamens petals and sepals this process is called floral senescence it is often accelerated or initiated by the completion of pollination death is preferred because flowers are costly to the plant nevertheless flowers can last for between a few hours and several months 100 101 the fruit contains three main structures the outer layer of peel the fleshy part and the stone or innermost layer the pericarp which may include one or more of these structures represents collectively the fruit wall everything but the seed the size shape toughness and thickness of the pericarp varies among different dry and fleshy fruits these traits are directly connected to the plant s method of seed dispersal since the purpose of fruit is to encourage or enable the seed s dispersal and protect the seed while doing so 99 102 seed dispersal main articles biological dispersal and seed dispersal following the pollination of a flower fertilisation and finally the development of a seed and fruit a mechanism or vector is typically used to disperse the fruit away from the plant 103 in flowering plants seeds are dispersed away from the plant so as to not force competition between the mother and daughter plants 104 as well as to enable the colonisation of new areas vectors can generally be divided into two categories external vectors and internal vectors 105 106 external vectors include living things like birds or bats or non living things such as water and wind 105 107 internal vectors which are derived from the plant itself 105 106 include for example the fruit exploding to release the seeds as in dwarf mistletoes 108 evolution further information evolutionary history of plants flowers and floral biology angiosperms now account for most of land plant diversity thanks in large part to flowers 109 110 flowers originated between 150 and 190 million years ago during the jurassic 111 112 although molecular analyses indicate this early appearance of angiosperms flowering plants the earliest definitive evidence from the fossil record comes from between 125 and 130 million years ago during the early cretaceous 113 114 115 note 5 the exact time at which angiosperms diverged from other seed plants is a classic open question in evolutionary biology 112 116 117 prior to the advent of flowers plants reproduced using cones as in gymnosperms 118 and spores as in pteridophytes 119 the transformation of spore producing leaves into structures like stamens and carpels is the most clear milestone in the complex evolution of flowers 115 there is debate both over whether these and other changes happened gradually or as sudden shifts like homeotic mutations and which aspect of flower morphology came first 120 121 the flower was the angiosperms most significant evolutionary innovation 109 granting the ability to effectively take advantage of animal pollinators 115 other evolutionary advantages included being able to have both male and female parts on the same axis and on this axis have carpels to protect the ovules stamens to present the pollen and the perianth to provide protection in addition they pioneered double fertilisation which allows energy investment into endosperm to be prolonged until after pollination the gametophytes which lead to sex cells were very reduced which allowed for greater protection of this critical process 122 the net effect of these features was greater reproductive security and efficiency 112 this allowed the angiosperms to replace many other seed plants such as pinales cycads gnetophyta and ginkgoales in the majority of ecosystems 112 a key driving force in the evolution of flowers is coevolution where pollinator and flower evolve with one another 123 often to their mutual benefit this is particularly prominent in insect species such as bees but is also found in flower pollinator relationships with birds and bats many flowers have evolved in such a way so as to make pollination by specific species easier thus providing greater efficiency and also ensuring higher rates of pollination this is because they receive less pollen from other plant species 58 93 however this close interdependence increases the risk of extinction since the extinction of either member almost certainly means the extinction of the other member as well 124 modern day flowers exhibit a variety of features derived through coevolution including shape size symmetry timing of flower opening colour scent and pollinator rewards including pollen nectar and oils 12 92 for example japanese honeysuckle flowers strategically open during the night to attract nocturnal moths which are more efficient pollinators than diurnal bees 125 with the innovation of the flower and other adaptations angiosperms rapidly diversified note 6 approximately 90 of all living land plant species are angiosperms 110 this is attributed in part to coevolution which caused specialisation and so speciation where populations diverge into separate species 126 both the strength of close pollinator flower relationships and the survival of either species are affected by climate change reducing numbers of pollinators have led to the extinction of many flowering plants 127 taxonomy further information plant taxonomy and linnaean taxonomy linnaeus s diagram of 24 classes of sexual systems from systema naturae maximum likelihood diagram of a phylogenetic tree of orchids in the genus vanilla classical and modern approaches to angiosperm taxonomy in plant taxonomy which is the study of plant classification and identification the morphology of plants flowers is used extensively and has been since at least classical greece 128 129 despite earlier works carl linnaeus s 1753 book species plantarum in which he laid out his system of classification is regarded as the first taxonomic work to recognise the significance of flowers 129 130 he identified 24 classes of flowering plants based mainly on the number length and union of the stamens 130 131 132 subsequent systems in the 18th and 19th centuries focused more on natural characteristics this included taking into account the rest of the plant so that diverse plants weren t put into the same groups as often happened in linnaeus s system 132 133 134 in 1963 the biologist...