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Botany 12th (Presented by Wing DIET KUD H.O.D Sushma Gupta H.O.D Renuka Nagpal and Rashpal Singh Under the Guidance of Sh. Devinder Handoo H.O.D I/c DIET Kud)

Unit-01 in Flowering Plants

Unit-01Marks- 07

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Asexual reproduction is a mode of reproduction by which offspring arise from a single parent, and inherit the of that parent only, it is reproduction which does not involve , or fertilization. A more stringent definition is agamogenesis which is reproduction without the fusion of . Asexual reproduction is the primary form of reproduction for single-celled organisms such as the , , and . Many plants and fungi reproduce asexually as well. While all reproduce asexually (without the formation and fusion of gametes), mechanisms for lateral transfer such as conjugation, transformation and transduction are sometimes likened to . A lack of sexual reproduction is relatively rare among multicellular organisms, particularly , for reasons that are not completely understood. Current hypotheses suggest that asexual reproduction may have short term benefits when rapid population growth is important or in stable environments, while sexual reproduction offers a net advantage by allowing more rapid generation of genetic diversity, allowing to changing environments. Developmental constraints may underlie why few animals have relinquished sexual reproduction completely in their -cycles.

Contents

• 1 Types o 1.1 Binary fission o 1.2 Budding o 1.3 Vegetative reproduction o 1.4 formation o 1.5 Fragmentation o 1.6 o 1.7 Agamogenesis o 1.8 and nucellar embryony

• 2 Alternation between sexual and asexual reproduction 2 | Page ​ ​

• 3 Examples in animals

Types of Asexual Reproduction

1. Binary fission

In binary fission the parent organism is replaced by two daughter organisms, because it literally divides in two. Many single-celled organisms, both prokaryotes (the archaea and the bacteria), and (such as protists and unicellular fungi), reproduce asexually through binary fission;

Most of these are also capable of sexual reproduction. Some single-celled organisms rely on one or more organisms in order to reproduce.

2. Budding

Some cells split via budding (for example baker's ), resulting in a 'mother' and 'daughter' cell. The offspring organism is smaller than the parent. Budding is also known on a multicellular level; an example is the hydra, which reproduces by budding. The buds grow into fully matured individuals which eventually break away from the parent organism.

3. Vegetative reproduction

Vegetative reproduction is a type of asexual reproduction found in plants where new individuals are formed without the production of seeds or by meiosis or syngamy. Examples of vegetative reproduction include the formation of miniaturized plants called plantlets on specialized leaves (for example in kalanchoe) and some produce new plants out of rhizomes or stolon (for example in strawberry). Other plants reproduce by forming bulbs or tubers (for example tulip bulbs and dahlia tubers). Some plants produce adventitious shoots and suckers that form along their lateral roots. Plants that reproduce vegetatively may form a clonal colony, where all the individuals are clones, and the clones may cover a large area.

4. Spore formation ​ Sporogenesis

Many multicellular organisms form spores during their biological life cycle in a process called Sporogenesis. Exceptions are animals and some protists, which undergo gametic meiosis immediately followed by fertilization. Plants and many on the other hand undergo sporic meiosis where meiosis leads to the formation of haploid spores rather than gametes. These spores grow into multicellular individuals (called in the case of plants) without a fertilization event. These haploid individuals give rise to gametes through . Meiosis and formation therefore occur in separate generations or "phases" of the life cycle, referred to as alternation of generations. Since sexual 3 | Page ​ ​ reproduction is often more narrowly defined as the fusion of gametes (fertilization), spore formation in plant and algae might be considered a form of asexual reproduction (agamogenesis) despite being the result of meiosis and undergoing a reduction in . However, both events (spore formation and fertilization) are necessary to complete sexual reproduction in the plant life cycle.

Fungi and some algae can also utilize true asexual spore formation, which involves mitosis giving rise to reproductive cells called mitospores that develop into a new organism after dispersal. This method of reproduction is found for example in conidial fungi and the red algaPolysiphonia, and involves sporogenesis without meiosis. Thus the number of the spore cell is the same as that of the parent producing the spores. However, mitotic sporogenesis is an exception and most spores, such as those of plants, most , and many algae, are produced by meiosis.

5. Fragmentation ​ Fragmentation is a form of asexual reproduction where a new organism grows from a fragment of the parent. Each fragment develops into a mature, fully grown individual. Fragmentation is seen in many organisms such as animals (some annelid worms and sea stars), fungi, and plants. Some plants have specialized structures for reproduction by fragmentation, such as gemmae in liverworts. Most lichens, which are a symbiotic union of a and photosynthetic algae or bacteria, reproduce through fragmentation to ensure that new individuals contain both symbionts. These fragments can take the form of soredia, dust-like particles consisting of fungal hyphae wrapped around photobiont cells.

6. Parthenogenesis ​ Parthenogenesis is a form of agamogenesis in which an unfertilized egg develops into a new individual. Parthenogenesis occurs naturally in many plants, invertebrates (e.g. water , , stick insects, some ants, bees and parasitic wasps), and vertebrates (e.g. some reptiles, amphibians, fish, very rarely birds). In plants, apomixis may or may not involve parthenogenesis.

7. Agamogenesis ​ Agamogenesis is any form of reproduction that does not involve a male gamete. Examples are parthenogenesis and apomixis.

8. Apomixis and nucellar embryony

Apomixis in plants is the formation of a new without fertilization. It is important in and in flowering plants, but is very rare in other seed plants. In flowering plants, the term "apomixis" is now most often used for agamospermy, the formation of seeds without fertilization, but was once used to include 4 | Page ​ ​ vegetative reproduction. An example of an apomictic plant would be the triploid ​ ​ European dandelion. Apomixis mainly occurs in two forms: In gametophytic apomixis, the embryo arises from an unfertilized egg within a diploid embryo sac that was formed without completing meiosis. In nucellar embryony, the embryo is formed from the diploid nucellus tissue surrounding the embryo sac. Nucellar embryony occurs in some citrus seeds. Male apomixis can occur in rare cases, such as the Saharan Cypress where the genetic material of the embryo are derived entirely from pollen. The term "apomixis" is also used for asexual reproduction in some animals, notably water-fleas, .

Alternation between sexual and asexual reproduction

Some alternate between the sexual and asexual strategies, an ability known as heterogamy, depending on conditions. For example, the freshwater crustacean Daphnia reproduces by parthenogenesis in the spring to rapidly populate ponds, then switches to sexual reproduction as the intensity of competition and predation increases. Many protists and fungi alternate between sexual and asexual reproduction.

For example, the slime mold undergoes binary fission (mitosis) as single-celled under favorable conditions. However, when conditions turn unfavorable, the cells aggregate and follow one of two different developmental pathways, depending on conditions. In the social pathway, they form a multicellular slug which then forms a fruiting body with asexually generated spores. In the sexual pathway, two cells fuse to form a giant cell that develops into a large cyst. When this macrocyst germinates, it releases hundreds of amoebic cells that are the product of meiotic recombination between the original two cells.

The hyphae of the common mold (Rhizopus) are capable of producing both mitotic as well as meiotic spores. Many algae similarly switch between sexual and asexual reproduction. A number of plants use both sexual and asexual means to produce new plants, some species alter their primary modes of reproduction from sexual to asexual under varying environmental conditions.

Examples in animals

A number of invertebrates and some less advanced vertebrates are known to alternate between sexual and asexual reproduction, or be exclusively asexual. Alternation is observed in a few types of insects, such as aphids (which will, under favourable conditions, produce eggs that have not gone through meiosis, essentially cloning themselves) and the cape bee Apis Mellifera Capensis (which can reproduce asexually through a process called thelytoky). A few species of amphibians and reptiles have the same ability (see parthenogenesis for concrete examples). A very unusual case among more advanced vertebrates is the female turkey's ability to produce fertile eggs in the absence of a male. The eggs result in 5 | Page ​ ​ often sickly and nearly always male turkeys. This behaviour can interfere with the incubation of eggs in turkey farming.

There are examples of parthenogenesis in the hammerhead shark and the blacktip shark. In both cases, the sharks had reached sexual maturity in captivity in the absence of males, and in both cases the offspring were shown to be genetically identical to the mothers.

Polyembryony is a widespread form of asexual reproduction in animals, whereby the fertilized egg or a later stage of splits to form genetically identical clones. Within animals, this phenomenon has been best studied in the parasitic Hymenoptera. In the 9-banded armadillos, this process is obligatory and usually gives rise to genetically identical quadruplets. In other mammals, monozygotic twinning has no apparent genetic basis, though its occurrence is common. There are at least 10 million identical human twins and triplets in the world today.

Bdelloid reproduce exclusively asexually, and all individuals in the class Bdelloidea are females. Asexuality evolved in these animals millions of years ago and has persisted since. There is evidence to suggest that asexual reproduction has allowed the animals to evolve new proteins through the Meselson effect that have allowed them to survive better in periods of dehydration.

❖​ Vegetative reproduction

Vegetative reproduction (vegetative propagation, vegetative multiplication, vegetative cloning) is a form of asexual reproduction in plants. It is a process by which new individuals arise without production of seeds or spores. It can occur naturally or be induced by horticulturists, in which case the technical process may require special care.

Although most plants normally reproduce sexually, they all have the ability for vegetative propagation. This is because meristematic cells can differentiate according to stage of growth, location on the plant and environmental conditions. Horticulturalists are interested in understanding and affecting those factors in order to optimize the process.

Success rates and difficulty of propagation vary greatly. For example willow and coleus can be propagated merely by inserting a stem in water or moist . On the other hand, monocotyledons, unlike dicotyledons, typically lack a vascular cambium and therefore are harder to propagate.

❖​ Types of Vegetative Propagation

In a wide sense, methods of vegetative propagation include cutting, Vegetative apomixis, layering, division, budding, grafting and tissue culture. Cutting is the most common artificial vegetative propagation method, where pieces of the 6 | Page ​ ​

"parent" plant are removed and placed in a suitable environment so that they can grow into a whole new plant, the "clone", which is genetically identical to the parent. Cutting exploits the ability of plants to grow adventitious roots (i.e. root material that can generate from a location other than the existing or primary root system, as in from a leaf or cut stem) under certain conditions.

Vegetative Propagation is usually considered a cloning method. However, there are several cases where vegetatively propagated plants are not genetically identical. Rooted stem cuttings of thornless blackberries will revert to thorny type because the adventitious shoot develops from a cell that is genetically thorny. Thornless blackberry is a chimera, with the epidermal layers genetically thornless but the tissue beneath it genetically thorny. Leaf cutting propagation of certain chimeral variegated plants, such as snake plant, will produce mainly non variegated plants.

Grafting is often not a complete cloning method because sexual seedlings are used as rootstocks. In that case only the top of the plant is clonal. In some crops, particularly apples, the rootstocks are vegetatively propagated so the entire graft can be clonal if the scion and rootstock are both clones.

Apomixis is a type of asexual reproduction involving unfertilized seeds. Hawkweed (Hieracium), dandelion (Taraxacum), some Citrus (Citrus) and Kentucky blue grass (Poapratensis) all use this form of asexual reproduction. Bulbils are sometimes formed in the flowers of garlic. The leafy crown of a pineapple fruit will root to form a new plant. These cases would not be vegetative reproduction because normally reproductive parts were involved. They would be considered asexual reproduction however. Vegetative reproduction involves only vegetative structures, i.e. roots, stems or leaves.

Vegetativestructures

Virtually all types of shoots and roots are capable of vegetative propagation, including stems, basal shoots, tubers, rhizomes, stolons, corms, bulbs and buds.

• The rhizome is a modified underground stem serving as an organ of vegetative reproduction, e. g. Polypody, Iris, Couch Grass and Nettles.

• aerial stems, called runners or stolons are important vegetative reproduction organs in some species, such as the strawberry, numerous grasses, and some ferns.

• Adventitious buds form on roots near the ground surface, on damaged stems (as on the stumps of cut trees), or on old roots. These develop into above-ground stems and leaves.

• A form of budding called suckering is the reproduction or regeneration of a plant by shoots that arise from an existing root system. Species that characteristically 7 | Page ​ ​ produce suckers include Elm (Ulmus), Dandelion (Taraxacum), and members of the Rose Family (Rosa).

• Another type of a vegetative reproduction is the production of bulbs. Plants like onion (Allium cepa), hyacinth (Hyacinth), narcissus (Narcissus) and tulips (Tulipa) reproduce by forming bulbs.

• Other plants like potatoes (Solanum Tuberosum) and dahlia (Dahlia) reproduce by a method similar to bulbs: they produce tubers.

• Gladioli and crocuses (Crocus) reproduce by forming a bulb-like structure called a corm.

• Some orchids reproduce by the growth of keikis from the stem or cane of the parent plant.

Natural vegetative propagation

Natural vegetative reproduction is mostly a process found in herbaceous and woody perennial plants, and typically involves structural modifications of the stem, although any horizontal, underground part of a plant (whether stem or a root) can contribute to vegetative reproduction of a plant. And, in a few species (such as Kalanchoë), leaves are involved in vegetative reproduction. Most plant species that survive and significantly expand by vegetative reproduction would be perennial almost by definition, since specialized organs of vegetative reproduction, like seeds of annuals, serve to survive seasonally harsh conditions. A plant that persists in a location through vegetative reproduction of individuals over a long period of time constitutes a clonal colony.

In a sense, this process is not one of "reproduction" but one of survival and expansion of biomass of the individual. When an individual organism increases in size via cell multiplication and remains intact, the process is called "vegetative growth". However, in vegetative reproduction, the new plants that result are new individuals in almost every respect except genetic. Of considerable interest is how this process appears to reset the aging clock.

Artificial vegetative propagation

Man-made methods of vegetative reproduction are usually enhancements of natural processes, but range from simple cloning such as rooting of cuttings to grafting and artificial propagation by laboratory tissue cloning. It is very commonly practised to propagate cultivars with individual desirable characteristics. Fruit tree propagation is frequently performed by budding or grafting desirable cultivars (clones), onto rootstocks that are also clones, propagated by layering.

In horticulture, a "cutting" is a branch that has been cut off from a mother plant below an internode and then rooted, often with the help of a rooting liquid or 8 | Page ​ ​ powder containing . When a full root has formed and leaves begin to sprout anew, the clone is a self-sufficient plant, genetically identical to the mother plant. Examples are cutting from the stems of blackberries (Rubusoccidentalis), cutting from leaves of African violets (Saintpaulia), and cutting the stems of verbenas (Verbena) to create new plants. A related form of regeneration is that of grafting. This is a process of taking a bud and grafting onto a plants stem. Many nurseries now sell trees that can produce four or more varieties of apples (Malus spp.) from stems grafted to a common rootstock.

Cultivated plants propagated by vegetative methods:-A number of commonly cultivated plants are propagated by vegetative means rather than by seeds. This is a listing of such plants: Apple, Avocado, Peach, Canna etc

❖​ Micropropagation

Micropropagation is the practice of rapidly multiplying stock plant material to produce a large number of progeny plants, using modern plant tissue culture methods.

Micropropagation is used to multiply novel plants, such as those that have been genetically modified or bred through conventional plant breeding methods. It is also used to provide a sufficient number of plantlets for planting from a stock plant which does not produce seeds, or does not respond well to vegetative reproduction.

Methods

Establishment

Micropropagation begins with the selection of plant material to be propagated. Clean stock materials that are free of viruses and fungi are important in the production of the healthiest plants. Once the plant material is chosen for culture, the collection of explant(s) begins and is dependent on the type of tissue to be used; including stem tips, anthers, petals, pollen and others plant tissues. The explant material is then surface sterilized, usually in multiple courses of bleach and alcohol washes and finally rinsed in sterilized water. This small portion of plant tissue, sometimes only a single cell, is placed on a growth medium, typically containing sucrose as an energy source and one or more plant growth regulators (plant hormones). Usually the medium is thickened with agar to create a gel which supports the explant during growth. Some plants are easily grown on simple media but others require more complicated media for successful growth; The plant tissue grows and differentiates into new tissues depending on the medium. For example, media containing cytokinins are used to create branched shoots from plant buds. and it happens in a vegetative form.

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Multiplication

Multiplication is the taking of tissue samples produced during the first stage and increasing their number. Following the successful introduction and growth of plant tissue, the establishment stage is followed by multiplication. Through repeated cycles of this process, a single explants sample may be increased from one to hundreds or thousands of plants. Depending on the type of tissue grown, multiplication can involve different methods and media. If the plant material grown is callus tissue, it can be placed in a blender and cut into smaller pieces and recultured on the same type of culture medium to grow more callus tissue. If the tissue is grown as small plants called plantlets, hormones are often added that cause the plantlets to produce many small offshoots that can be removed and recultured.

Pretransplant

This stage involves treating the plantlets/shoots produced to encourage root growth and "hardening." It is performed in vitro, or in a sterile "test tube" environment.

"Hardening" refers to the preparation of the plants for a natural growth environment. Until this stage, the plantlets have been grown in "ideal" conditions, designed to encourage rapid growth. Due to lack of necessity, the plants are likely to be highly susceptible to disease and often do not have fully functional dermal coverings and will be inefficient in their use of water and energy. In vitro conditions are high in humidity and plants grown under these conditions do not form a working cuticle and stomata that keep the plant from drying out, when taken out of culture the plantlets need time to adjust to more natural environmental conditions. Hardening typically involves slowly weaning the plantlets from a high-humidity, low light, warm environment to what would be considered a normal growth environment for the species in question. This is done by moving the plants to a location high in humidity.

Transfer from culture

In the final stage of plant micropropagation, the plantlets are removed from the plant media and transferred to soil or (more commonly) potting compost for continued growth by conventional methods.

This stage is often combined with the "pretransplant" stage.

Advantages of Micropropagation

Micropropagation has a number of advantages over traditional plant propagation techniques: 10 | Page ​ ​

• The main advantage of micropropagation is the production of many plants that are clones of each other.

• Micropropagation can be used to produce disease-free plants.

• Micropropagation produces rooted plantlets ready for growth, saving time for the grower when seeds or cuttings are slow to establish or grow.

• It can have an extraordinarily high fecundity rate, producing thousands of propagules while conventional techniques might only produce a fraction of this a number.

• It is the only viable method of regenerating genetically modified cells or cells after protoplast fusion.

• It is useful in multiplying plants which produce seeds in uneconomical amounts, or when plants are sterile and do not produce viable seeds or when seed can't be stored (vgr. recalcitrant seeds).

• Micropropagation often produces more robust plants, leading to accelerated growth compared to similar plants produced by conventional methods - like seeds or cuttings.

• Some plants with very small seeds, including most orchids, are most reliably grown from seed in sterile culture.

• A greater number of plants can be produced per square meter and the propagules can be stored longer and in a smaller area.

Disadvantages of Micropropagation.

Micropropagation is not always the perfect means of multiplying plants, conditions that limits its use include:

• It is very expensive, and can have a labor cost of more than 70%

• A monoculture is produced after micropropagation, leading to a lack of overall disease resilience, as all progeny plants may be vulnerable to the same infections.

• An infected plant sample can produce infected progeny. This is uncommon if the stock plants are carefully screened and vetted to prevent culturing plants infected with virus or fungus.

• Not all plants can be successfully tissue cultured, often because the proper medium for growth is not known or the plants produce secondary metabolic chemicals that stunt or kill the explant. 11 | Page ​ ​

• Sometimes plants or cultivars do not come true to type after being tissue cultured, this is often dependent on the type of explant material utilized during the initiation phase or the result of the age of the cell or propagule line.

• Some plants are very difficult to disinfect fungal organisms.

The major limitation in the use of Micropropagation for many plants is the cost of production; for many plants the use of seeds, which are normally disease free and produced in good numbers, readily produce plants (see orthodox seed) in good numbers at a lower cost. For this reason, many plant breeders do not utilize micropropagation because the cost is prohibitive, other breeders use it to produce stock plants that are then used for seed multiplication.

Mechanization of the process could reduce labour costs, but has proven difficult to achieve, despite active attempts to develop technological solutions.

❖​ Sexual reproduction in Plants

Sexual reproduction is the creation of a new organism by combining the genetic material of two organisms. The two main processes are: meiosis, involving the halving of the number of ; and fertilization, involving the fusion of two gametes and the restoration of the original number of chromosomes. During meiosis, the chromosomes of each pair usually cross over to achieve .

The of sexual reproduction is a major puzzle. The first fossilized evidence of sexually reproducing organisms is from eukaryotes of the Stenian period, about 1 to 1.2 billion years ago. Sexual reproduction is the primary method of reproduction for the vast majority of macroscopic organisms, including almost all animals and plants. Bacterial conjugation, the transfer of DNA between two bacteria, is often mistakenly confused with sexual reproduction, because the mechanics are similar.

A major question is why sexual reproduction persists when parthenogenesis appears in some ways to be a superior form of reproduction. Contemporary evolutionary thought proposes some explanations. It may be due to selection pressure on the itself—the ability for a population to radiate more rapidly in response to a changing environment through sexual recombination than parthenogenesis allows. Alternatively, sexual reproduction may allow for the "ratcheting" of evolutionary speed as one clade competes with another for a limited resource.

In the first stage of sexual reproduction, "meiosis," the number of chromosomes is reduced from a diploid number (2n) to a haploid number (n). During "fertilization," haploid gametes come together to form a diploid and the original number of chromosomes (2n) is restored. 12 | Page ​ ​

Plants

Animals typically produce male gametes called sperm, and female gametes called eggs and ova, following immediately after meiosis, with the gametes produced directly by meiosis. Plants on the other hand have mitosis occurring in spores, which are produced by meiosis. The spores germinate into the phase. The gametophytes of different groups of plants vary in size; angiosperms have as few as three cells in pollen, and and other so called primitive plants may have several million cells. Plants have an alternation of generations where the saprophyte phase is succeeded by the gametophyte phase. The saprophytes phase produces spores within the sporangium by meiosis.

Flowering plants

Flowering plants are the dominant plant form on land and they reproduce by sexual and asexual means. Often their most distinguishing feature is their reproductive organs, commonly called flowers. The anther produces male gametophytes, the sperm is produced in pollen grains, which attach to the stigma on top of a carpel, in which the female gametophytes (inside ovules) are located. After the pollen tube grows through the carpel's style, the sex cell nuclei from the pollen grain migrate into the ovule to fertilize the egg cell and endosperm nuclei within the female gametophyte in a process termed double fertilization. The resulting zygote develops into an embryo, while the triploid endosperm (one sperm cell plus two female cells) and female tissues of the ovule give rise to the surrounding tissues in the developing seed. The ovary, which produced the female gametophyte(s), then grows into a fruit, which surrounds the seed(s). Plants may either self-pollinate or cross-pollinate. Nonflowering plants like ferns, and liverworts use other means of sexual reproduction.

Ferns

Ferns mostly produce large diploid sporophytes with rhizomes, roots and leaves; and on fertile leaves called sporangium, spores are produced. The spores are released and germinate to produce short, thin gametophytes that are typically heart shaped, small and green in color. The gametophytes or thallus, produce both motile sperm in the antheridia and egg cells in separate archegonia. After rains or when dew deposits a film of water, the motile sperm are splashed away from the antheridia, which are normally produced on the top side of the thallus, and swim in the film of water to the archegonia where they fertilize the egg. To promote outcrossing or cross fertilization the sperm are released before the eggs are receptive of the sperm, making it more likely that the sperm will fertilize the eggs of different thallus. A zygote is formed after fertilization, which grows into a new sporophytic plant. The condition of having separate sporophyte and gametophyte plants is called alternation of generations. Other plants with similar 13 | Page ​ ​ reproductive means include the Psilotum, Lycopodium, Selaginella and Equisetum.

Bryophytes

The , which include liverworts, hornworts and mosses, reproduce both sexually and vegetatively. They are small plants found growing in moist locations and like ferns, have motile sperm with flagella and need water to facilitate sexual reproduction. These plants start as a haploid spore that grows into the dominate form, which is a multicellular haploid body with leaf-like structures that photosynthesize. Haploid gametes are produced in antheridia and archegonia by mitosis. The sperm released from the antheridia respond to chemicals released by ripe archegonia and swim to them in a film of water and fertilize the egg cells thus producing a zygote. The zygote divides by mitotic division and grows into a sporophyte that is diploid. The multicellular diploid sporophyte produces structures called spore capsules, which are connected by seta to the archegonia. The spore capsules produce spores by meiosis, when ripe the capsules burst open and the spores are released. Bryophytes show considerable variation in their breeding structures and the above is a basic outline. Also in some species each plant is one sex while other species produce both sexes on the same plant.

Fungi

Fungi are classified by the methods of sexual reproduction they employ. The outcome of sexual reproduction most often is the production of resting spores that are used to survive inclement times and to spread. There are typically three phases in the sexual reproduction of fungi: plasmogamy, and meiosis.

❖​ 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 but also as objects of romance, ritual, religion, medicine and as a source of food.

❖​ Morphology of Flower 14 | Page ​ ​

Diagram showing the main parts of a mature flower

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).

A flower is a modified stem tip with compressed internodes, bearing structures that are highly modified leaves.[1] 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).

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. The parts of a flower are arranged in whorls 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 androsoikia: 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 15 | Page ​ ​ pollen. The supportive stalk, the style, becomes the pathway for pollen tubes to grow from pollen grains adhering to the stigma.

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 unisexualIf 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.

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, the collection 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).

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.

❖​ Development of Flower

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.[2] Many perennial and most biennial plants require vernalization to flower. The molecular interpretation of these signals is through the 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.[3] The first step is the transformation of the vegetative stem primordia into floral primordia. This occurs as biochemical changes take place to change 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. 16 | Page ​ ​

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.[4] Once the process begins, even if that cue is removed the stem will continue to develop a flower.

Organ development of Flower

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.

❖​ Flower specialization and pollination

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.

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 17 | Page ​ ​ look for nectar. Flowers also attract pollinators by scent and color. Still other flowers use 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.

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 - ∞G1

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.[5] 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 18 | Page ​ ​ 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.

❖​ Pollination in Flower

The primary purpose of a flower is reproduction. Since the flowers are the reproductive organs of plants, 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 a specific design 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 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. Birds and bees 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 (Asiminatriloba). Flowers pollinated by night visitors, including bats and moths, are likely to concentrate on scent to attract pollinators and most such flowers are white. 19 | Page ​ ​

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 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.

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.

Types of Pollination

Pollination is of two main types i.e., Self Pollination & Cross Pollination

Self Pollination: - It is transfer of Pollen grain from Anther to stigma of same or genetically similar flower. It occurs only when anther & stigma mature at same time. It is of two types- Autogamy & Goitnogamy.

A) Autogamy: - It results when a flower is pollinated by its own pollen. An autogamous flower is pollinated by its pollen. An autogamous flower is always intersexual as the whole process occurs within the same flower. E.g. Rice, Wheat, etc. 20 | Page ​ ​

B) Geitonogamy:- In this type of pollination pollen of one flower is deposited on the stigma of another flower borne on the same plant. I t is also k/as marriage b/w neighbours. It resembles cross pollination as pollen transfer occurs through pollinating agencies.

Importance of self Pollination:-

1). It eliminates some harmful and useless recessive traits.

2). There is not any need for a pollinating agency.

3). It will maintain the pure lines, purity of race & superiority of variety.

Cross pollination: - It is migration of pollen grain from one flower to the stigma of genetically different flowers. This type of pollination is also k/as Xenogamy i.e., Marriage b/w strange. Both biotic and abiotic agents are required. The various factors or agencies required for cross pollination are

A). Anemophily or Wind Pollination: - A large number of plants such as coconut, palm, date palm, maize and numerous grasses are wind pollinated. These flowers are unisexual with the anther and stigma exposed. Their pollen grains are smooth and dry. There is much wastage of pollen grain as they are produced in enough quantities. Eg, A single flower of Cannabis produces over 5, 00,000 pollen grains.

B). Hydrophily or Water Pollination: - This type of pollination is brought about by the agency of water. It occurs in aquatic plants like Lemma, Vallisneria and is of two types- Hypohydrophily & Epihydrophily. Hypohydrophily takes place below the surface of water. Eg.Ceratophyllum whereas Epihydrophily occurs on the surface of water. E.gLemma.

C). Entomophily or Insect pollination: -It is the transfer of pollen grain from one flower to the stigma of another flower with the help of insects like moth, bees , wasp, butterfly, etc.

D). Ornithophily or Bird pollination: - It is the pollination that is carried out with the help of birds which get their staple food from flowers in the form of nectar. The nectar is chiefly composed of sugar. It is reported that a humming bird can take half of its body weight of sugar in a single day. Birds which have a variety of flowers like red silk, cotton, coral tree, bottle brush etc. Birds like Mynah, Crow, Parrot, and Bulbils perform this type of pollination.

E). Chiropterophily or Pollination by Bat: -This act of pollination is performed by Bats. Bats perform pollination during night as it is a nocturnal animal. It can transport pollen grain upto a distance of 30km. Bats visit large numbers of flowers that have a strong scent. They move swiftly and transport pollen over long distances. The Sausage tree, Kigeliapinnata is bat pollinated.

Importance of Cross Pollination or Significance of Pollination:- 21 | Page ​ ​

1). It is highly useful for self sterile & proponent plants. 2). It increases the yield as well as adaptability. 3). It provides better adaptability to offspring. 4). It eliminates defective traits.

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.

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

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 of Flower

Land plants have existed for about 425 million years, the first ones reproduced by a simple adaptation of their aquatic counterparts: spores. 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 , 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. 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 . Recently discovered 22 | Page ​ ​ 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.

Recent DNA analysis (molecular ) shows that Amborellatrichopoda, found on the Pacific island of New Caledonia, is the sister group to the rest of the flowering plants, and morphological studies suggest that it has features which may have been characteristic of the earliest flowering plants.

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 , especially when it comes to radical which seem to have required inferior transitional forms. Note that the example of wasp 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, including gigantopterids, 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.

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. The most primitive flowers are thought to have had a variable number of flower parts, often 23 | Page ​ ​ 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. 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. 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 . In the United Kingdom, New Zealand, Australia and Canada, red poppies are worn to commemorate soldiers who have died in times of war.

• 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 has inspired the works of numerous poets, 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. 24 | Page ​ ​

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 a lotus flower. Apart from the association with Vishnu, the Hindu tradition also considers the lotus to have spiritual significance. For example, it figures in the Hindu stories of creation.

Usage of Flower

In modern times, people have sought ways to cultivate, 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

• For funeral flowers and expressions of sympathy for the grieving

• For worshiping goddesses. inHindu culture it is very common to bring flowers as a gift to temples.

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 plant 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 25 | Page ​ ​ 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.

❖​ Gametophyte of Flower

A gametophyte is the haploid, multicellular phase of plants and algae that undergo alternation of generations, with each of its cells containing only a single set of chromosomes.

The gametophyte produces male or female gametes (or both), by a process of called mitosis. The fusion of male and female gametes produces a diploidzygote, which develops by repeated mitotic cell divisions into a multicellular sporophyte. Because the sporophyte is the product of the fusion of two haploid gametes, its cells are diploid, containing two sets of chromosomes. The mature sporophyte produces spores by a process called meiosis, sometimes referred to as "reduction division" because the chromosome pairs are separated once again to form single sets. The spores are therefore once again haploid and develop into a haploid gametophyte.

In mosses, liverworts and hornworts (bryophytes), the gametophyte is the commonly known phase of the plant. An early developmental stage in the gametophyte of mosses (immediately following germination of the meiospore) is called the protonema. The adult gametophyte of mosses is called the gametophore as it carries the gamete-producing sex organs.

In most other land plants, the gametophyte is very small. In ferns the gametophyte is a free living organism called the prothallus, in contrast to angiosperms.

In gymnosperms and angiosperms, the gametophyte are reduced to only a few cells; in angiosperms the female gametophyte (embryo sac) is known as a megagametophyte and the male gametophyte (pollen) is called a microgametophyte. 26 | Page ​ ​

In some multicellular , , or (Ulva is one example), the sporophytes and gametophytes are often isomorphic, but in some species the gametophyte may be reduced.

❖​ Double fertilization

Double fertilization is a complex fertilization mechanism that has evolved in flowering plants, known as angiosperms. This process involves the joining of a female gametophyte (embryo sac) with two male gametes (sperm). It begins when a pollen grain adheres to the stigma of the carpel, the female reproductive structure of a flower. After a pollen grain has landed on an accessible stigma, the pollen grain takes in moisture and begins to germinate, forming a pollen tube that extends down toward the ovary through the style. The tip of the pollen tube then enters the ovary and penetrates through the micropyle. The micropyle is an opening in the protective layers of the ovule. The pollen tube proceeds to release the two sperm in or near the embryo sac.

One sperm fertilizes the egg cell and the other sperm combines with the two polar nuclei of the large central cell of the embryo sac. The sperm and haploid egg combine to form a diploid zygote, while the other sperm and two haploid polar nuclei form a triploid nucleus (some plants may form polyploid nuclei). The large cell of the embryo sac will then form the endosperm, a nutrient-rich tissue which provides nourishment to the developing embryo. The ovary, surrounding the ovules, develops into the fruit, which is used for protection and dispersion of the seeds.

The two central cell maternal nuclei (polar nuclei) that contribute to the endosperm, arise by mitosis from a single meiotic product. Therefore, maternal contribution to the genetic constitution of the triploid endosperm is different from that of the embryo.

In a recent study done of the plant Arabidopsis thaliana, the migration of male nuclei inside the female gamete, in fusion with the female nuclei, has been documented for the first time using in vivo imaging. Identification of the genes involved in the migration and fusion process has also been determined. For the complete study and the steps captured by in vivo imaging please refer to the article "Double Fertilization - Caught in the Act".

Brief history

Double fertilization was discovered more than a century ago by SergiusNawaschin in St. Petersburg, Russia, and Léon Guignard in France. Each made the discovery independently of the other. Lilium Martagon and Fritillariatenella were used in the first observations of double fertilization, which were made using the classical light microscope. Due to the limitations of the light microscope, there were many unanswered questions regarding the process of double fertilization. However, with 27 | Page ​ ​ the development of the electron microscope, many of the questions were answered. Most notably, the observations made by the group of W. Jensen showed that the male gametes did not have any cell walls and that the plasma membrane of the gametes is close to the plasma membrane of the cell that surrounds them inside the pollen grain.

In vitro double fertilization

In vitro double fertilization is often used to study the molecular interactions as well as other aspects of gamete fusion in flowering plants. One of the major obstacles in developing an in vitro double fertilization between male and female gametes is the confinement of the sperm in the pollen tube and the egg in the embryo sac. A controlled fusion of the egg and sperm has already been achieved with poppy plants. Pollen germination, pollen tube entry, and double fertilization processes have all been observed to proceed normally. In fact, this technique has already been used to obtain seeds in various flowering plants and was named “test-tube fertilization”.

❖​ Structures and functions related to Double fertilization

➢​ Megagametophyte (Female Gametophyte)

The female gametophyte, or megagametophyte, used in double fertilization is the embryo sac. This develops within an ovule, enclosed by the ovary at the base of a carpel. Surrounding the embryo sac are two integuments, which form an opening called the micropyle. The embryo sac, which is primarily haploid, originates from the diploid megaspore mother cell within the ovule. The megasporocyte undergoes a meiotic cell division, producing four haploid megaspores. The next sequence of events varies, depending on the particular species. However, in most angiosperm species, only one of the four resulting megaspores survives. This megaspore undergoes three rounds of mitotic division, resulting in one large cell with eight haploid nuclei. Membranes then divide this mass into a multicellular female gametophyte, called the embryo sac. The lower end of the embryo sac consists of the haploid egg cell positioned in the middle of two other haploid cells, called synergids. The synergids function in the attraction and guidance of the pollen tube to the embryo sac through the micropyle. At the upper end of the embryo sac are three antipodal cells. The remaining two nuclei, called polar nuclei, share the cytoplasm of the large central cell of the embryo sac, rather than being partitioned into separate cells.

➢​ Microgametophyte (Male Gametophyte)

The male gametophytes, or microgametophytes, used in double fertilization are pollen grains. They develop within the microsporangia, or pollen sacs, of anthers at the tips of the stamens. Each microsporangia contains diploid microspore mother cells, or microsporocytes. Each microsporocyte undergoes meiosis, 28 | Page ​ ​ forming four haploid microspores, each of which can eventually develop into a pollen grain. A microspore undergoes mitosis and cytokinesis in order to produce two separate cells, the generative cell and the tube cell. These two cells in addition to the spore wall make up an immature pollen grain. As the male gametophyte matures, the generative cell passes into the tube cell, and the tube cell produces the pollen tube. Once the pollen grain has matured, the anthers break open, releasing the pollen. The pollen is carried to the stigma of another flower, by wind or animal pollinators, and deposited on the pistil. As the pollen grain germinates, the pollen tube elongates, and the generative cell undergoes mitosis, producing two sperm cells. The pollen tube extends down the long style of the carpel and into the ovary, where its sperm cells are released near the embryo sac. Double fertilization proceeds from here

Seed Development

The development of the fruit from flower starts from the stage of fertilization and continues which is described as below: Flowers are the true reproductive organs of flowering plants. The "male" part is the stamen or androecium, which produces pollen (male gametes) in anthers. The "female" organ is the carpel or gynoecium, which contains eggs (female gamete) and is the site of fertilization. While the majority of flowers are perfect and hermaphrodite (having both male and female parts in the same flower structure), flowering plants have developed numerous morphological and also physiological mechanisms actually to reduce or prevent self-fertilization. Heteromorphic flowers have short carpals and long stamens, or otherwise vice versa, so animal pollinators cannot easily transfer pollen to the pistil (receptive part of the carpel). Homomorphic flowers could employ a biochemical (physiological) mechanism called self-incompatibility to discriminate between self and non-self pollen grains. In other species, the male and female parts are morphologically separated, developing on different flowers.

The main growth of the fruits from the seeds include three main parts which includes:

Fertilization Embryology Fruits and Seeds

➢​ Fertilization

During the period of the fertilization the embryo-sac lies in a close proximity to the opening of micro Pyle, into which the pollen-tube has penetrated, the separating cell-wall becomes absorbed, and the male or sperm-cells are ejected into the embryo-sac. Guided by the synergetic one male-cell passes into the oosphere with which it fuses, the two nuclei uniting, while the other fuses with the definitive nucleus, or, as it is also called, the endosperm nucleus. This is remarkable double fertilization as it has been known, although only recently discovered, has been proved to take part in widely-separated families, and both in Monocotyledons and of a prothallium after a cause following the reinvigorating 29 | Page ​ ​ union of the polar nuclei. This view is still maintained by those who are differentiate two acts of fertilization within the embryo-sac, and regard that of egg by the first male-cell, as the true or generative fertilization, and that of polar nuclei by the second male gamete as a vegetative fertilization which gives a stimulus to development in correlation with the other.

If, on the other hand, the endosperm is the product of an act of fertilization as definite as that giving rise to the embryo itself, we have to recognize that twin-plants are produced within the embryo-sac—one, the embryo, which becomes the angiosperm us plant, the other, the endosperm, a short-lived, undifferentiated nurse to assist in the nutrition of the former, even as the subsidiary embryos in a pluri-embryonic Gymnosperm may facilitate the nutrition of the dominant one. If this is so, and the endosperm like the embryo is normally the product of a sexual act, hybridization will give a endosperm as it does a hybrid embryo, and herein (it is suggested) we may have the explanation of the phenomenon of xenia observed in the mixed endosperms of hybrid races of maize and other plants, regarding which it has only been possible hitherto to assert that they were indications of the extension of the influence of the pollen beyond the egg and its product. This would not, however, explain the formation of fruits intermediate in size and colour between those of crossed parents. The significance of the coalescence of the polar nuclei is not explained by these new facts, but it is noteworthy that the second male-cell is said to unite sometimes with the apical polar nucleus, the sister of the egg, before the union of this with the basal polar one.

➢​ Embryology

The result of fertilization is the development of the ovule into the seed. By the segmentation of the fertilized egg, now invested by cell-membrane, the embryo-plant arises. A varying number of transverse segment-walls transform it into a pro-embryo—a cellular row of which the cell nearest the micropyle becomes attached to the apex of the embryo-sac, and thus fixes the position of the developing embryo, while the terminal cell is projected into its cavity. In Dicotyledons the shoot of the embryo is wholly derived from the terminal cell of the pro-embryo, from the next cell the root arises, and the remaining ones form the suspensor. In many Monocotyledons the terminal cell forms the cotyledonary portion alone of the shoot of the embryo, its axial part and the root being derived from the adjacent cell; the cotyledon is thus a terminal structure and the apex of the primary stem a lateral one—a condition in marked contrast with that of the Dicotyledons. In some Monocotyledons, however, the cotyledon is not really terminal. The primary root of the embryo in all Angiosperms points towards the micropyle. The developing embryo at the end of the suspensor grows out to a varying extent into the forming endosperm, from which by surface absorption it derives good material for growth; at the same time the suspensor plays a direct part as a carrier of nutrition, and may even develop, where perhaps no endosperm 30 | Page ​ ​ is formed, special absorptive "suspensor roots" which invest the developing embryo, or pass out into the body and coats of the ovule, or even into the placenta. The formation of endosperm starts, as has been stated, from the endosperm nucleus. Its segmentation always begins before that of the egg, and thus there is timely preparation for the nursing of the young embryo. If in its extension to contain the new formations within it the embryo-sac remains narrow, endosperm formation proceeds upon the lines of a cell-division, but in wide embryo-sacs the endosperm is first of all formed as a layer of naked cells around the wall of the sac, and only gradually acquires a pluricellular character, forming a tissue filling the sac. The function of the endosperm is primarily that of nourishing the embryo, and its basal position in the embryo-sac places it favourably for the absorption of food material entering the ovule. Its duration varies with the precocity of the embryo. The picturization of the fruit from flower of tomato is explained below

❖​ APOMIXIS AND POLYEMBRYONY

A normal sexual reproduction in flowering plants involves transformation of diploid sporophytic cells into haploid gametes by meiosis and fusion of resultant haploid gametes (i.e.,syn-gamy) of opposite sex to form diploid zygote. The zygote then develops into an embryo and the ovule is transformed into a seed. However, there are some plants (e.g., species of Asteraceae and grasses) which have ability to develop seeds without fertilization. Thus, development of reproductive propagules without meiosis and syngamy is called apomixis (GK.apo = without mixis = mixing). The apomixis is also called asexual reproduction. There are two kinds of apomixis (or asexual reproduction) in flowering plants:Agamospermy and vegetative propagation. Agamospermy is further divided into three types: (i) Adventive embryony: Formation of embryo directly from the diploid sporophytic cells (nucellus or integument) of ovule (other than zygote) is called adventive embryony, e.g., Citrus. (ii) Parthenogenesis : Formation of embryo from unfertilized egg. (iii) Apospory and Apogamy: Formation of embryo from any other cell of embryo sac (other than egg) without fertilization. During embryogenesis, an embryo develops from zygote inside the embryo sac and the embryo sac becomes an endosperm. Apomictic embryos, if developed, increase the number of embryos inside the seed. Occurrence of more than one embryo in a seed is called polyembryony.

❖​ Outbreeding depression

A concept in selective breeding and zoology, outbreeding depression refers to cases when offspring from crosses between individuals from different populations have lower fitness than progeny from crosses between individuals from the same population. (In selective breeding, the concept is opposed to inbreeding depression). This phenomenon can occur in two ways. 31 | Page ​ ​

First, selection in one population might produce a large body size, whereas in another population small body size might be more advantageous. between these populations may lead to individuals with intermediate body sizes, which may not be adaptive in either population. It might be that, in a certain environment, having either a large or small body is advantageous, whereas an intermediate-sized body is comparatively disadvantageous.

A second way outbreeding depression can occur is by the breakdown of biochemical or physiological compatibilities between genes in the different breeding populations. Within local, isolated breeding populations, are selected for their positive, overall effects on the local genetic background. Due to non additive gene action, the same genes may have rather different average effects in different genetic backgrounds--hence, the potential evolution of locally coadapted gene complexes. However, on the other hand, according to the overdominance hypothesis in genetics, it is believed that certain combinations of alleles (which can only be obtained by outbreeding) are especially advantageous when paired in a heterozygous individual, which is one explanation for the existence of hybrid vigor.

A third, but neutral, effect of outbreeding is the loss of of a particular group that lends its distinctness and contributes to the diversity of said types in either group by the exclusive retention of select traits.

❖​ Pollen- Pistil Interaction: -

This process is of great importance in sexual reproduction. The success of hybridization programs of crop improvement depends largely on the pollen- pistil interaction. When the pollen grains land on the stigma, the process of hydration occurs. By this process exine and intine proteins are released on the stigmatic surface. The pollen wall proteins bind to the pellicle within a few minutes of the contact and thereafter cannot be released readily by leaching. The stigma surface pellicle thus forms a receptor site for the pollen wall proteins. In wet stigma the cuticle generally raptures during the deposition of exudates and the pollen tube enters through the pectin cellulose layer. Pollination initiates many physiological and biochemical changes in the pistil. Signals of compatible and incompatible pollination are transferred to the ovule even before the pollen tube is reached there. Following pollination, signals are evidently passed to the style and ovary to initiate responses in accordance with the compatible or incompatible pollination. As the pollen tube enters one of the synergids, low oxygen tension in the embryo sac causes the tip of the pollen tube to rapture. Consequently, the male gametes are released in the embryo sac. One of the male gametes fuses with the egg to form the zygote and the other fuses with the secondary nucleus and forms the primary endosperm nucleus.