Study the following notes and follow the given you tube link to understand the chapter better.

NOTES

SEXUAL REPRODUCTION IN FLOWERING PLANTS

(CHAPTER- 2)

Flowers are objects of aesthetic, ornamental, social, religious and cultural value. A few ornamental flowers are: Orchids, Tulip, Petunia, Daffodils, Lilies A few flowers used in social and cultural celebrations: Rose, Rajnigandha, Marigold, Chrysanthamum, Gladioli.

Floriculture is growing of ornamental flowers. Horticulture is the art of cultivating and management of gardens.

I. PRE-FERTILISATION: STRUCTURES AND EVENTS

Anthesis- is the opening of floral buds.

- Several hormonal and structural changes are initiated which lead to the differentiation and development of the flower. - Androecium is the male reproductive structure which consists of a whorl of stamens. - Gynoecium is the female reproductive part which is made up of pistils/carpels.

STAMEN, MICROSPORANGIUM AND POLLEN GRAIN

STAMEN

- It is the male reproductive part of a flower and consists of two parts- A long slender stalk called the filament and a terminal generally bilobed structure called as the anther. - A typical anther is bilobed with each lobe having two theca, that is, it is dithecous. - The two lobes are separated in the front by a deep groove and are attached to each other at the back by a sterile tissue called the connective. - Each anther lobe has 2 microsporangia. Hence the whole anther comes to have 4 microsporangia. This makes the anther a 4 sided tetragonal structure consisting of 4 microsporangia located at the corners. - The microsporangia further develops to form the Pollen sacs which extends longitudinally through the length of the anther and are packed with pollen grains.

STRUCTURE OF MICROSPORANGIUM

- It is generally surrounded by four wall layers- 1) the epidermis 2) endothecium 3) middle layer 4) tapetum - The outer three layers perform the function of protection and help in the dehiscence of anther to release the pollen. - The innermost wall layer, Tapetum, helps to nourish the developing pollen grains. - The cells of the tapetum possesses dense cytoplasm and they have generally more than one nucleus due to endomitosis and endopolyploidy.

SPOROGENESIS

- At centre of each microsp0rangia, a group of homogenous compactly arranged cells called the sporogenous tissue, is present. - Each sporogenous cell divides mitotically to form diploid microspore mother cell (MMC) or the p0llen mother cell (PMC). - A Pollen mother cell undergoes meiosis to form Four microspores and this process is called as microsporogenesis . - As the microspores are arranged in a cluster of four cells, they are called as microspore tetrad. - As the anthers mature and dehydrate, the microspores dissociate from each other and develop into pollen grains. - Inside each microsporangium, several thousands of microspores or pollen grains are formed which are released when the anther dehiscicates.

POLLEN GRAIN

- A pollen grain represents the male gametophyte. - Pollen grains are generally spherical in shape and they have a prominent two layered walls. - The hard outer wall is called as the Exine and it is made up of sporopollenin which is one of the most resistant organic material known. It can withstand high temperature and strong acids and alkali. No enzyme can degrade sporopollenin. Hence because of the presence of sporopollenin, pollen grains can be well preserved as fossils. - The exine has prominent apertures called as Germ pores where the sporopollenin is absent. It is from here that the pollen tube germinates. - Hence the exine helps the pollen grains to with stand unfavourable conditions. - The inner wall of the Pollen Grain is called as Intine. It is a thin and continuous layer made up of cellulose and pectin. - The cytoplasm of pollen grain is surrounded by a plasma membrane. - When the Pollen Grain is mature, it contains two cells- the vegetative cell and the generative cell. - The vegetative cell is bigger and has abundant food reserve and a large irregularly shaped nucleus. - The generative cell is small and it floats in the cytoplasm of the vegetative cell. It is spindle- shaped with dense cytoplasm and a nucleus. - In most of the angiosperms, pollen grains are shed at this two celled stage. - In the remaining species, the generative cell divide mitotically to give rise to 2 male gametes before the pollen grains are shed and hence these are at the three celled stage.

Pollen viability - The period for which pollen grains remain viable is highly variable. - It depends on the prevailing temperature and humidity. - In some cereals such as rice and wheat, the pollen grains lose viability within 30 minutes of their release. - In some members of Rosaceae, leguminoseae and solanaceae, they maintain viability for months. - the pollen grains of a large number of species can be stored for years in liquid nitrogen (at – 196 degree C) and such stored Pollens can be used as Pollen banks similar to seed banks and can be used in crop breeding programs.

Pollen allergy - In many cases, the pollen grains of many species causes severe allergies and Bronchital problems in people leading to chronic respiratory disorders like Asthama, Bronchitis etc. - In many of the cases the Pollen allergy is called as hay fever which is caused by pollens from the weed Parthenium (carrot grass), which came to India as a contaminant with imported wheat. - Other plants which can cause Pollen allergy are Amaranthus, Chenopodium and Sorghum.

Pollen Products - Pollen grains are rich in nutrients like carbohydrates, proteins and unsaturated fats. Hence they are also used as food supplement. They are sold in the form of tablets and syrup. - It increases the performance of athletes and race horses. - Since pollen grains contain unsaturated fats which protects them from UV rays, therefore they are used in the manufacture of creams and lotions which helps to protect the skin.

PISTIL, MEGASPORANGIUM(OVULE) and EMBRYO SAC

PISTIL

- The gynoecium represents the female reproductive part. - The gynoecium may consist of a single pistil (monocarpellary) or it may have more than one pistil (multicarpellary). - When there are more than one pistil they may be fused together (syncarpous) or they may be free (apocarpous). - Each pistil has three parts- the stigma, style and ovary. - Stigma serves as the landing platform for pollen grains. - Style is the elongated slender part beneath the stigma. - Ovary is the basal buldged part of the pistil. - Inside each ovary there are present several Chambers called as locules. - The Placenta is located inside the ovarian cavity. - Arising from the Placenta are the megasporangia which are commonly called as ovules. - The number of ovules in an ovary maybe one (wheat, paddy and mango) to many (papaya watermelon and Orchids)

THE MEGASPORANGIUM (OVULE)

- The ovule is a small structure attached to the Placenta by means of a stock called as the funicle. - The body of the ovule fuses with the funicle in the region called as hilum. - Hilum represents the junction between ovule and funicle. - Each ovule has one or two protective envelops called as the integuments. - Within the integument is a mass of cells called as the nucellus, which contains abundant reserve food materials. - The integuments encircle the nucellus except at the tip where a small opening called as the micropyle, is present. - Opposite the micropylar end, is the Chalaza, representing the basal part of the ovule. - Located in the nucellus is the embryo sac of female gametophyte and a ovule generally has a single embryo sac formed from a megaspore.

MEGASPOROGENESIS - It is the process of formation of megaspores from the megaspore mother cell. - Generally a single megaspore mother cell differentiates in the micropylar region of the nucellus. it is a large cell containing dense cytoplasm and a prominent nucleus. - The megaspore mother cell undergoes meiosis which results in the production of four megaspores.

FEMALE GAMETOPHYTE (EMBRYO SAC}

- In most of the flowering plants, only one of the megaspore is functional while the other three degenerate. - Only the functional megaspore develops into a female gametophyte (embryo sac). This method of embryo sac formation from a single megaspore is termed as monosporic development.

Formation of the embryo sac - The nucleus of the functional megaspore divides mitotically to form two nuclei which moves to the opposite poles, forming 2- nucleate embryo sac. - Two more sequential mitotic nuclear divisions result in the formation of 4-nucleate and later 8-nucleate stages of the embryo sac. - The nuclear divisions are not followed immediately by the cell wall formation. - After the 8-nucleate stage, the cell walls are laid down. - Six of the 8 nuclei are surrounded by cell walls and these 6 cells are organised into 2 groups- 1) One group of 3 cells are present at the micropylar end and this forms the egg apparatus. It consist of two synergids and one egg cell.  Synergids have special cellular thickenings at the micropylar tip called filiform apparatus which helps to guide the pollen tube into the synergids. 2) The other group of three cells located at the chalazal end are called as the antipodals. - The remaining two nuclei, called the polar nuclei, forms one large central cell and is situated below the egg apparatus. - Thus a typical angiosperm embryo sac, at maturity, though 8-nucleate is 7-celled. (Three cells in the egg apparatus, three antipodal cells and one large central cell)

POLLINATION

- As both types of gametes are non-motile, they have to be brought together for fertilization to occur. - This is achieved through the process of pollination in which the pollen grains are transferred from the anther to the stigma of a pistil.

KINDS OF POLLINATION There are three types of pollination depending upon the source of the Pollen.

1) Autogamy - In this the pollination is achieved within the same flower that is, transfer of pollen grains occur from the anther to the stigma of the same flower. Requirements for autogamy 1. The flowers should be bisexual 2. Synchrony in Pollen release and stigma receptivity. 3. Anther and stigma should be placed very close to each other.

Autogamy occurs in two types of flowers: 1. Chasmogamous flower- These are open flowers which have the anther and stigma exposed and they lie very close to each other. 2. Cleistogamous flower- These flowers do not open at all and the anther and stigma lie close to each other and when the anthers dehisce in the flower buds, the pollen grains come in contact with the stigma and pollination takes place. Advantages of cleistogamous flowers 1. There is no chance of cross pollen landing on the stigma. 2. Produces assured seed set even in the absence of pollinators.

Example of plants which show both type of flowers: Viola (Common pansy), Oxalis, Commelina

Chasmogamous flowers Cleistogamousflowers 1. The mature flowers are open The flowers remain closed throughout 2. The flowers are conspicuous The flowers are small and inconspicuous 3. Both self and cross pollination can occur Only self-pollination occurs

2) Geitonogamy - It is the transfer of pollen grains from the anther to the stigma of another flower of the same plant. - Although geitonogamy is functionally cross pollination involving a pollinating agent but genetically it is similar to autogamy since the Pollen grains come from the same plant.

Autogamy Geitonogamy 1. It is the transfer of pollen grains from It is the transfer of pollen grains from the anther the anther to the stigma of the same to the stigma of another flower of the same flower plant 2. The flowers are hermaphrodite The flowers are generally unisexual 3. No external agent is required for An external agent is required for pollination pollination

3) Xenogamy - It is a type of pollination in which the transfer of pollen grains occur from anther to the stigma of a different plant’ - This is the only type of pollination which brings genetically different types of pollen grains to the stigma.

Geitonogamy Xenogamy 1. It is a kind of self-pollination It is cross pollination 2. Occurs between two flowers of the Occurs between two flowers of different same plant plants 3. Pollination occurs between Pollination occurs between genetically genetically similar flowers dissimilar flowers 4. It produces fewer variations It produces a lot of variations

AGENTS OF POLLINATION - Plants use two types of agents- Biotic (animals) and Abiotic (wind and water)- for pollination. - Majority of plants used biotic agents. - Only a small proportion of plants use abiotic agent because the pollen grains coming in contact with the stigma is a chance factor in wind and water pollination. - To compensate for this uncertainty and associated loss of pollen grains, the flowers produce enormous amount of pollen as compared to the number of ovules available for pollination.

1) Wind pollination (Anemophily) Pollination is carried out by wind.

Characteristic features of wind pollinated flowers 1) The flowers are generally colourless, odourless and nectarless. 2) Flowers are generally small and inconspicuous and packed into inflorescence. (Eg: Corn cobs- the tassels hanging out of it are the stigma and style to trap the pollen) 3) Pollen grains are light and non-sticky so that they can be transported by the wind currents. 4) They possess well exposed stamens so that the pollens are easily dispersed into the wind currents. 5) The stigma are often large and feathery to easily trap the airborne pollen. 6) The flowers generally have a single ovule in each ovary. Eg: Maize, Coconut, Date, Grasses, Mulberry

Disadvantages of wind pollination 1. It is a highly wasteful phenomena because it is non directional. 2. Rate of successful pollination is low. 3. Airborne Pollen of some plants cause hay feverand bronchial allergy.

2) Water Pollination (Hydrophily) - Pollination is carried out by water. - It is very rare in flowering plants and limited to only 30 genera, mostly monocotyledons. - It is a regular mode of transport of male gametes in lower plants like algae, bryophytes and pteridophytes. It is believed particularly for some bryophytes and pteridophytes that their distribution is limited because of the need for water for the transport of male gametes and fertilization Eg: Vallisneria, Hydrilla and Zostera (sea grass)

- Not all aquatic plants use water for pollination. In a majority of aquatic plants such as water hyacinth and water lilies, the flowers emerge above the water level and are pollinated by insects and wind as in most land plants. - in Vallisneria, the female flower reaches the surface of water by a long stalk and the male flowers or pollen grains are released on the surface of water and they are carried passively by water currents and some of them eventually reach the female flowers and the stigma. - In another group of water pollinated plants such a seagrasses (Zostera), the female Flowers remain submerged in water and the pollen grains are released inside the water.

Characteristics of water pollinated flowers 1) Flowers are small, inconspicuous and light. 2) Pollen grains are long, ribbon like and carried passively by water 3) Pollen grains are protected from wetting by a mucilaginous covering. 4) They have long sticky and unwettable stigma 5) Flowers are without nectar and odour.

3) Pollination By Animals - Many of the flowering plants use a variety of animals as their pollinating agents like bee, butterflies, flies, Beetles, wasps, ants, moths, birds like sunbirds and humming birds and larger animals like lemurs, rodents and reptiles - Among animals, insects, specially the bees are the most common pollinating agents. - The animal visitors come in contact with the anther and stigma when they come to these plants for their floral rewards- Nectar and pollen grains. - The body of the animal gets a coating of the pollen grains and when this animal comes in contact with the stigma, it brings about pollination.

Examples of special floral rewards: 1) The tallest flower of Amorphophallus, provides a safe place to the insects to lay their eggs. 2) The plant Yucca and a species of moth cannot complete their life cycle without each other. The moth lays its eggs in the locules of the ovary and in turn the flower gets pollinated by the moth.

Characteristics of animal pollinated flowers 1) Flowers are large, colourful, fragrant and rich in nectar. 2) When the flowers are small, a number of flowers are clustered into an inflorescence to make them conspicuous. 3) The flowers pollinated by flies and beetles, secrete foul odour to attract these animals. 4) The pollen grains are usually sticky so that they can stick to the body of the animal.

OUTBREEDING DEVICES

- These are the devices for promoting cross pollination. - Majority of the flowering plants produce hermaphrodite flowers and hence the pollen grains are likely to come in contact with the stigma of the same flower resulting in self- pollination. - Continued self-pollination results in inbreeding depression. - Flowering plants have developed many devices to discourage self-pollination and to encourage cross pollination. A few of them are: 1) Pollen release and stigma receptivity are not synchronised, that is, either the pollen is released before the stigma becomes receptive or the stigma becomes receptive much before the release of the pollen. 2) In some other species, the anther and stigma are placed at different positions so that the pollen cannot come in contact with the stigma of the same flower. 3) Self-incompatibility is a genetic mechanism that prevent self-pollen (from the same flower or other flowers of the same plant) from fertilizing the ovules by inhibiting pollen germination or pollen tube growth in the pistil. 4) Production of unisexual flowers. -In monoecious plants (eg: Castor and Maize)- Both male and female flowers are present on the same plant. It prevents autogamy but not geitonogamy. -In dioecious plants (Eg: Papaya and water melon)-male and female flowers are present on different plants such a plant is either male or female. This condition prevents both autogamy and geitonogamy.

Advantages of cross pollination 1) Maintains the variability of the race. 2) It makes the offsprings adaptable to changes in the environment. 3) Production of better offspring due to phenomena of hybrid vigour. 4) It does not allow harmful and Lethal recessive alleles to become homozygous.

Self-Pollination Cross-pollination 1. It is the transfer of pollen grains from It is the transfer of pollen grains from anther to the anther to the stigma of the same or the stigma of another flower of another plant of genetically different flowers of the same the same species plant 2. N o agents required for pollination External agents required for pollination 3. Cannot produce new variety Formation of new varieties occurs 4. Flowers are bisexual Flowers maybe unisexual or bisexual 5. Both anther and stigma mature at the Anthers and stigma generally mature at different same time times 6. Flowers can be open or closed Flowers are always open

POLLEN-PISTIL INTERRACTION

- Pollination does not guarantee the transfer of right type of pollen (compatible pollen from the same species as the stigma) - The pistil has the ability to recognise the pollen, whether it is of the right type (compatible) or of the wrong type (incompatible). - If it is of the right type, the pistil accepts the pollen and promotes post-pollination events that leads to fertilization. - If the pollen is of the wrong type, the pistil rejects the pollen by preventing pollen germination on the stigma or the pollen tube growth in the style. - The ability of the pistil to recognise the pollen followed by its acceptance or rejection is the result of a continuous interaction between the pistil and the pollen grain which is mediated by chemical components of the pollen interacting with those of the pistil. - All the events from pollen deposition on the stigma until the Pollen tube enters the ovule- are together referred to as Pollen-pistil interaction.

Events following compatible pollination

1) The Pollen grain germinates on the stigma to produce a Pollen tube through one of the grem pore. 2) The contents of the pollen grain moves into the pollen tube. 3) The Pollen tube grows through the tissues of the stigma and style and reaches the ovary. 4) If the pollen grains are shed at two celled condition then the generative cell divides and forms the two male gametes during the growth of the pollen tube in the stigma. 5) In plants which shed the pollen in the 3 celled condition, the pollen tubes carry the two male gametes from the beginning. 6) The pollen tube after reaching the ovary, enters the ovule through the micropyle. 7) It then enters one of the synergids through the filiform apparatus present at the micropylar end.

Plant breeders use this knowledge in manufacturing Pollen-pistil interaction, even in incompatible pollination, to get desired hybrids.

ARTIFICIAL HYBRIDISATION

- It is one of the major approaches of Crop improvement programs. Requirements: 1. Only the desired pollen grains are used for pollination. 2. Stigma is protected from contamination by unwanted Poland - This is achieved by Emasculation and Bagging techniques.

If the female parent bears bisexual flowers 1) Anthers are removed from the flower bud before the anther dehisces using a pair of forceps. This step is called as emasculation. 2) Emasculated flowers have to be covered with a bag of suitable size generally made up of butter paper to prevent contamination of the stigma with unwanted pollen and this process is called as bagging. 3) When the stigma of the bagged flower attains receptivity, mature pollen grains collected from and anthers of the male parent are dusted on the stigma and the flowers are rebagged and the fruits are allowed to develop.

If the female parent produces unisexual flowers: There is no need for emasculation. The female flower buds are bagged before the flowers Open. When the stigma becomes receptive, pollination is carried out using the desired pollen and the flower is rebagged.

II. DOUBLE FERTILISATION

- After entering one of the synergids, the pollen tube releases the two male gametes into the cytoplasm of the synergid.. - One of the male gametes move towards the egg cell and fuses with its nucleus thus completing the Syngamy. - Syngamy results in the formation of a diploid cell, that is, the zygote. - The other male gamete moves towards the two polar nuclei located in the centre of the cell and fuses with them to produce a triploid primary endosperm nucleus (PEN). - As this involves the fusion of three haploid nuclei, it is termed as triple fusion. - Since two types of fusions, syngamy and triple fusion takes place in an embryo sac, the phenomenon is termed as double fertilization. - The central cell after triple fusion becomes the primary endosperm cell (PEC) and develops into the endosperm while the zygote develops into an embryo.

III. POST-FERTILISATION: STRUCTURES AND EVENTS

- Flowers begin to fade after fertilization. The sepals, petals, stamens and style wither away and are shed. - After double fertilization the events of endosperm and embryo development, maturation of ovules into seeds and ovary into fruit are collectively termed as post fertilization events.

ENDOSPERM

- The endosperm development precedes embryo development because the endosperm is filled with reserve food material which is used for the nutrition of the developing embryo. - Steps of Endosperm Development: 1. The primary endosperm nucleus (PEN) undergoes successive nuclear division to give rise to free nuclei. This stage of endosperm development is called as free nuclear endosperm. 2. Subsequently the cell wall formation occurs and the endosperm becomes cellular. 3. The number of free nuclei formed before cellularization varies greatly. Eg: The coconut water from tender coconut, is nothing but free nuclear endosperm made up of thousands of nuclei and the surrounding white kernel is the Cellular endosperm.

Depending upon the fate of the endosperm, there are two types of seeds: 1. Non-endospermic or Ex-albuminous seeds: In such seeds the food reserve is completely consumed by the developing embryo before seed maturation. Eg: Pea, Groundnut, Beans 2. Endospermic or Albuminous seeds: In such seeds the food reserve in the endosperm may persist in the mature seed and used up during seed germination. Eg: Coconut, Castor

EMBRYO

- The embryo develops at the micropylar end of the embryo sac where the zygote is situated. - Most of the zygotes start dividing only after certain amount of endosperm is formed. This is an adaptation to provide assured nutrition to the developing embryo. - Early stages of embryo development (Embryogeny) is similar in both monocotyledons and dicotyledons. - The zygote gives rise to the proembryo and subsequently to the globular, heart-shaped and mature embryo.

In Dicotyledons - The embryo consists of an embryonal axis and two cotyledons. - The portion of the embryonal axis above the level of cotyledon is the Epicotyl which terminates with the plumule or stem tip. - The cylindrical portion below the level of cotyledons is the Hypocotyl that terminates at its lower end in the radicle or the root tip. The root tip is covered with a root cap.

In Monocotyledons - They possess only one cotyledon. - In the grass family the cotyledon is called scutellum that is situated towards one side of the embryonal axis. - At its lower end, the embryonal axis has a radicle and root cap enclosed in undifferentiated sheath called the Coleorrhiza. - The portion of the embryonal axis above the level of the attachment of the scutellum is the epicotyl. The epicotyl has a shoot apex and a few leaf primordia enclosed in a hollow foliar structure called the Coleoptile.

SEED

- Seed is the final product of sexual reproduction it is often described as a fertilised ovule. - Seeds are formed inside fruits. - A seed typically consists of- seed coat, cotyledons and an embryo axis. - The cotyledons are simple structures and are generally thick and swollen due to storage of food reserves. Depending upon the type of food reserve present in the cotyledon, there are two types of seeds: 1) Non-albuminous or ex-albuminous These seeds have no residual endosperm as it is completely consumed during embryo development. Eg: Pea and groundnut 2) Albuminous seeds These seeds retain a part of the endosperm as it is not completely used up during embryo development. Eg: Wheat, Maize Barley, Castor 3) Occasionally in some seeds the remnants of nucellus are also persistent and this residual, persistent nucellus is called as the perisperm. Eg: Black pepper and Beet.

Non-endospermic/Ex-albuminous seed Endospermic/Albuminous seed 1. An endosperm is absent The seed containsa special tissue called endosperm 2. The food stored in the The food is retained in the endosperm endosperm is all used up during embryo development 3. The cotyledons are often thick The cotyledons are thin as it stores the food Eg: Pea, groundnut, Bean Wheat, maize, castor

Development of seed 1) Integuments of ovules harden as tough protective seed coat. 2) The micropyle remains as a small pore In The seed coat which facilitates the entry of oxygen and water into the seed during germination. 3) As the seed matures, its water content is reduced and the seed become relatively dry. 4) The general metabolic activity of the embryo slows down and the embryo may enter a state of inactivity called dormancy. 5) If favourable conditions are available like adequate moisture, Oxygen and suitable temperature, then they germinate.

Development of Fruit 1) As ovules mature into seeds, the ovary develops into a fruit and both these transformations proceed simultaneously. 2) The wall of the ovary develops into the wall of the fruit called as pericarp.

Types of fruits Depending upon whether the mature pericarp is fleshy or dry, there are two types of fruits: 1) Fleshy- The pericarp is fleshy. Eg: Guava, Mango, Orange 2) Dry- The pericarp is dry. Eg: Groundnut, Mustard Depending upon which parts of the flower contributes to the development of the fruit, there are two types of fruits: 1) False fruits- The thalamus contributes to the fruit formation along with the ovary and such fruits are called as false fruits. Eg: Apple, Strawberry, Cashew 2) True Fruits- Most fruits develop only from the ovary and these are called as true fruits. Eg: Tomato, Grapes, Mango

Parthenocarpic fruits: - There are a few species of fruits in which the fruit develops without fertilisation and such fruits are called as parthenocarpic fruits. Eg: Banana - Parthenocarpy or formation of seedless fruits can also be induced through the application of growth hormones.

Advantages of seed production 1) Since reproductive processes such as pollination and fertilization are independent of water, seed formation is much more dependable. 2) Seeds have better adaptive strategies for dispersal to new habitat and help the species to colonize in other new areas. 3) As they have sufficient food reserves, young seedlings are nourished until they are capable of photosynthesis on their own. 4) The hard seed coat provides protection to the young embryo. 5) Being products of sexual reproduction, they generate new genetic combinations leading to variations. 6) To humans - Seeds are a major source of oils, spices, beverages etc. - They are crucial for storage which can be used throughout the year. - They are helpful to raise crops for the next season. 7) In Agriculture- Discovery of germination and sowing of seeds by early humans gave rise to agriculture which helped in the development of civilisation and knowledge.

Seed viability- - It is the time period for which seeds retain the ability to germinate after their dispersal. - This period varies greatly. - In few species the seeds loose ability within a few months. - Seeds of a large number of species live for several years. some seeds can remain alive for hundreds of years. Eg: 1) The oldest is that of Lupine (Lupinus arcticus) excavated from the Arctic Tundra. This seed germinated and flowered after an estimated record of 10000 years of dormancy. 2) A recent record of 2000 year old viable seed of the Date Palm (Phoenix dactylifera) was discovered during an archaeological excavation at King Herod’s Palace near the Dead Sea .

Some plants produce a very large number of tiny seeds for easy dispersal by wind and birds. Eg: Orchids, Ficus and few parasitic plants like Orobanche and Striga.

APOMIXIS AND POLYEMBRYONY

- Apomixis is the production of seeds without the fusion of gametes. It is a form of asexual reproduction that mimics sexual reproduction. Eg: Some species of Asteraceae and Grasses

Apomixis can occur by several methods 1) In some species the diploid egg cell is formed without reduction division which develops into an embryo without fertilization. 2) In many of the citrus and mango varieties some of the nuceller cells surrounding the embryo sac start dividing, protrudes into the embryo sac and develops into embryos. In such species each ovule contains many embryos and this occurrence of more than one embryo in a seed is referred to as polyembryony.

Parthenocarpy Apomixsis 1. It is the production of seedless The fruit contains a seed but the seed is fruits which develops without formed without the fusion of gametes fertilisation 2. It has no role in reproduction It takes part in reproduction 3. It is economically important It is agriculturally important Eg: Banana Eg: Oranges, mango

Significance of Apomixis - Hybrid varieties of several food and vegetable crops are being extensively cultivated because of their higher yield, vigour and resistance to diseases. - Cultivation of hybrids have tremendously increased productivity. - However there is one major problem in using hybrid varieties. - Hybrid vigour is not maintained beyond a single generation as segregation of genes begin to occur in the second generation. - In order to maintain higher yield, hybrid seeds have to be produced every year which is very costly. Hence it becomes expensive for the farmers. - But in apomixis there is no segregation of characters in the new seeds as the embryo inside the seed develops from a diploid cell. - Hence the farmers can keep on using the hybrid seeds to raise new crops year after year and he does not have to buy the hybrid seeds every year. - Hence research is going on in many Laboratories around the world to understand the genetics of apomixis to transfer the apomictic genes into the hybrid varieties.

YOU TUBE LINK

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