Sexual Reproduction in Flowering Plants PDF

Sexual reproduction in flowering plants © 2022, Aakash BYJU'S. All rights reserved Key Takeaways Male reproductive structures Stamen 1 2 Journey to pollen grain Anther Microsporogenesis Microsporangium Pollen grain maturation Structure of pollen grain 3 4 Female reproductive structures Ovary Ovule ©...

Sexual reproduction in flowering plants © 2022, Aakash BYJU'S. All rights reserved Key Takeaways Male reproductive structures Stamen 1 2 Journey to pollen grain Anther Microsporogenesis Microsporangium Pollen grain maturation Structure of pollen grain 3 4 Female reproductive structures Ovary Ovule © 2022, Aakash BYJU'S. All rights reserved Journey to female gametophyte Megasporogenesis 5 6 Megagametogenesis Pollination Self-pollination 7 8 Cross-pollination Outbreeding © 2022, Aakash BYJU'S. All rights reserved Structure of female gametophyte Agents of pollination Abiotic 9 Biotic 10 Artificial hybridisation Fertilisation Double fertilisation Types of seed 11 12 13 Post-fertilisation events Endosperm Embryo Apomixis 14 Polyembryony 15 16 Summary © 2022, Aakash BYJU'S. All rights reserved Seed Hybrid seeds Plants Algae Bryophytes Kingdom plantae The word comes from two Greek words ○ Angeion which means vessel ○ Sperma which means seed They are plants which bear seeds enclosed in fruits. Pteridophytes Gymnosperms Angiosperms © 2022, Aakash BYJU'S. All rights reserved Angiosperms Flowering plants belong to angiosperms. Sexual Reproduction in Flowering Plants In angiosperms, before the flower is formed, several changes occur in the plant. Formation of the floral primordium. It is the tissue which develops to form the flower. © 2022, Aakash BYJU'S. All rights reserved Inflorescences are formed which bear the buds. Flowering takes place Parts of Typical Flower A typical angiospermic flower consists of 4 whorls: Petal Whorl of stamens (Androecium) Whorl of carpels (Gynoecium) Sepal The sepals combine together to form the calyx and petals combine together to form corolla. Petal and sepal are accessory whorls. Androecium and gynoecium are essential whorls. © 2022, Aakash BYJU'S. All rights reserved The Stamen Stamen Thalamus Anther and filament combine together to form stamen. The stamen is a long slender structure which forms the male reproductive organs in flowers. © 2022, Aakash BYJU'S. All rights reserved The Stamen Stamen Anther Filament Thalamus Anther is the lobed terminal structure that contains and produces the pollen grains. Filament is the long and slender stalk that connects the anther to the thalamus or the petal. The proximal end of the filament is attached to the thalamus or the petal. © 2022, Aakash BYJU'S. All rights reserved Structure of the Anther X Anther X Filament Transverse section of a young anther If the anther is cut along X - X, the transverse section of an anther can be seen. © 2022, Aakash BYJU'S. All rights reserved Structure of the Anther A typical angiospermic anther has two lobes, i.e., it is bilobed. Each lobe has two chamber-like structures known as the theca, i.e., they are dithecous. Bilobed Dithecous A longitudinal groove runs lengthwise separating the theca. Transverse section of a young anther © 2022, Aakash BYJU'S. All rights reserved Structure of the Anther Bilobed Each theca consists of a microsporangia at the corner. Microsporangia In total contain 4 sporangium hence tetrasporangiate. Dithecous Transverse section of a young anther © 2022, Aakash BYJU'S. All rights reserved Structure of Microsporangium Endothecium Middle layers Endothecium Middle layers Transverse section of a young anther Cellular structure of the wall layers Each microsporangium is covered by 4 wall layers made up of cells. The outermost layer is known as the epidermis. Next layer is the endothecium. Next to it lies the middle layers. The epidermis, endothecium and the middle layers are protective in nature. These layers also help in dehiscence of anther to release the pollen. © 2022, Aakash BYJU'S. All rights reserved Structure of Microsporangium Endothecium Middle layers Transverse section of a young anther Endothecium Middle layers Cellular structure of the wall layers The innermost wall layer is known as tapetum. It plays a role in nourishing the developing pollen grains. Cells of the tapetum possess dense cytoplasm and generally have more than one nucleus. The tissue present in the microsporangium which undergoes meiosis to produce gametes is known as the sporogenous tissue. © 2022, Aakash BYJU'S. All rights reserved Microsporogenesis Microsporogenesis is the process of formation of microspores from microspore mother cells through meiosis. It occurs inside the sporogenous tissue at the center of each microsporangia. Microsporangia (Sporogenous tissue) © 2022, Aakash BYJU'S. All rights reserved Microsporogenesis Pollen mother cell (2n) Each pollen mother cell is diploid or 2n. This pollen mother cell undergoes meiotic division. As a result two haploid cells are formed. Meiosis I 2 haploid cells Meiosis II © 2022, Aakash BYJU'S. All rights reserved Microsporogenesis Meiosis II After meiosis II, four haploid cells (microspores) are formed which are arranged in a cluster. This cluster is known as the microspore tetrad. Each of these microspores form pollen grains. 4 haploid cells Microspore Tetrad © 2022, Aakash BYJU'S. All rights reserved Microsporogenesis Microsporangium Each of the pollen mother cells undergo similar meiotic divisions. Inside each microsporangium, several thousands of microspores are formed, which develop to form pollen grains that are released with the dehiscence of anther. Pollen mother cells (2n) Microsporangium Microspores © 2022, Aakash BYJU'S. All rights reserved Formation of Pollen Grain 1 Vacuole A pollen is formed from the microspore. The microspore expands and a single large vacuole is formed. Nucleus Vacuoles grow in the microspore 1st mitosis 2 The nucleus divides asymmetrically and the spindle holds the chromosomes as studied in the cell division. A single large vacuole is formed and cell division begins © 2022, Aakash BYJU'S. All rights reserved 1st mitosis Formation of Pollen Grain Consequently, one large cell (vegetative cell) and a small cell (generative cell) are formed. 3 Vegetative cell This is the 2-celled stage of the pollen grain. Slowly, the generative cell detaches from the wall of the pollen grain. In over 60 percent of angiosperms, pollen grains are shed at this 2-celled stage. Generative cell Mitotic division results in 2-celled stage of the pollen grain 2nd mitosis 4 The generative cell divides mitotically to form the two male gametes. The pollen is completely matured and is 3-celled. Male gametes 3-celled stage © 2022, Aakash BYJU'S. All rights reserved Vegetative Cell and Generative Cell 2-celled stage of the pollen grain Vegetative cell Large size Abundant food reserve Smaller size Irregularly shaped nucleus Floats in the cytoplasm of vegetative cell Spindle-shaped Dense cytoplasm and nucleus Generative cell © 2022, Aakash BYJU'S. All rights reserved Pollen Structure Pollen has a prominent two-layered wall. Exine Intine  Hard outer layer  Thin inner layer  Made of sporopollenin- most resistant organic material known  Continuous layer  Made of: ○ Cellulose ○ Pectin  Has apertures called germ pores Germ Pore © 2022, Aakash BYJU'S. All rights reserved It is a pore in the exine through which the pollen tube germinates. Pollen Structure The final pollen grain has 3 nuclei ○ 2 male gametes ○ Vegetative nucleus  Vacuoles Vegetative nucleus  Vegetative nucleus is also called tube nucleus that later helps form the pollen tube through a germ pore Male gametes © 2022, Aakash BYJU'S. All rights reserved The single large nucleus of the immature pollen grain breaks down into multiple small vacuoles after the first mitosis.  Male gametes are formed by the mitosis of the generative cell.  One of them later fuses with the egg cell to form the zygote.  The other gamete fuses with the polar nuclei to form the primary endosperm nucleus. Sporopollenin A lot of plant fossils are of pollen. This is due to the presence of sporopollenin in pollen grains. It is one of the most resistant organic materials on this planet. It forms the major component of exine. It can withstand high temperatures. It can withstand strong acids and alkalis. It protects pollen grain from external damage. © 2022, Aakash BYJU'S. All rights reserved Dehiscence Dehiscence is the process by which the pollen grains are released from the anther. Pollen grains are present inside pollen sacs, which are nothing but microsporangia. Pollen sacs are in turn present inside the anther. For dehiscence, pollen loses water. A strip between the pollen sacs disintegrates, resulting in the release of pollen grains. This powdery deposit of pollen grains accumulates on the anther. It is then picked up by a different agent. © 2022, Aakash BYJU'S. All rights reserved Pollen Allergy Pollens can cause allergy Cause respiratory disorders ○ Asthma ○ Bronchitis Parthenium is an example Contaminant of imported wheat Widespread in India now © 2022, Aakash BYJU'S. All rights reserved Parthenium (carrot grass) Pollen Viability The period for which the pollen grains retain the ability to germinate after landing on the stigma is called pollen viability. Differs amongst species: ○ 30 minutes : Rice and wheat ○ Few months: Members of rosaceae, leguminoseae and solanaceae Can be stored at -1960C in liquid nitrogen in pollen banks ○ This method of storing pollen is called cryopreservation and is also used in the case of humans and higher animals to store gametes for later use. © 2022, Aakash BYJU'S. All rights reserved Structure of Ovary Pistil Stigma Style Ovary © 2022, Aakash BYJU'S. All rights reserved Transverse section of ovary Structure of Ovary Locule Chamber/ cavity inside ovary Ovules Structures which develop into seeds Placenta Tissue to which ovules are attached Funicle Thin stalk that attaches ovule to placenta Transverse section of ovary © 2022, Aakash BYJU'S. All rights reserved Recall! More Than one Locules Locule = 1 © 2022, Aakash BYJU'S. All rights reserved Locule = 1 Locules = 3 Locules = 5 Structure of Ovule Chalazal end Chalaza Hilum Nucellus Integuments Funicle Embryo sac Micropyle Micropylar end © 2022, Aakash BYJU'S. All rights reserved Structure of an ovule Structure of Ovule Hilum Integuments - One or more protective layers within ovule Nucellus - Mass of parenchymal cells within integuments Has abundant food reserves Micropyle - Tip of the ovule not covered by integuments Small opening for pollen tube penetration Embryo sac - Located inside nucellus Female gametophyte Micropylar end - Region of ovule near micropyle Chalazal end - Region of ovule near chalaza Opposite to micropylar end © 2022, Aakash BYJU'S. All rights reserved - Point of attachment of funicle with ovule Megasporogenesis It is the process of formation of a megaspore from a megaspore mother cell. Megaspore mother cell/MMC (2n) At the micropylar end, one of the cells from the nucellus with a prominent nucleus grows in size to form a megaspore mother cell (MMC). Megaspore dyad (n) It is formed after meiosis - I. MMC undergoes the first meiotic division to give rise to megaspore dyad (haploid). © 2022, Aakash BYJU'S. All rights reserved Megasporogenesis Megaspore tetrad (n) After meiosis I, MMC undergoes meiosis II. This results in the formation of megaspore tetrad (four megaspores), which are also haploid. Megaspore tetrad (n) degeneration Generally, only one out of the four megaspores remains functional, while the other three cells degenerate. At the end of megasporogenesis, only a single haploid functional megaspore is left behind. This megaspore undergoes development to form the female gametophyte. © 2022, Aakash BYJU'S. All rights reserved Megagametogenesis It is the process of maturation of megaspore into a female gametophyte. Functional megaspore The haploid functional megaspore undergoes mitosis, to mature into a female gametophyte or the embryo sac. Two-nucleate stage The nucleus of the megaspore divides mitotically to form two nuclei. These nuclei move towards the opposite poles. © 2022, Aakash BYJU'S. All rights reserved Megagametogenesis Four-nucleate stage The binucleate embryo sac undergoes second mitosis to form a four-nucleate embryo sac. Eight-nucleate stage Finally, the nuclei undergoes third mitosis to give an eightnucleate embryo sac. Till this stage, mitosis is strictly free nuclear, that is, the nuclear division is not followed by the cell wall formation. Female gametophyte Six of the eight nuclei are surrounded by cell walls and organised into cells. © 2022, Aakash BYJU'S. All rights reserved Female Gametophyte It is a seven celled, eight nucleate structure that contains the female gamete, i.e., the egg cell. Chalazal end Antipodal cells Polar nuclei Synergids Egg cell Filiform apparatus Micropylar end © 2022, Aakash BYJU'S. All rights reserved Egg apparatus Female Gametophyte Parts of female gametophyte Antipodal cells Three antipodal cells are formed towards the chalazal end. The functions of antipodal cells in female gametophytes are still not clear. Synergids Synergids are present beside the egg cell. Synergids and egg cells are grouped together at the micropylar end. These constitute the egg apparatus. Polar nuclei These fuse with the male gamete, undergo triple fusion, to form an endosperm. © 2022, Aakash BYJU'S. All rights reserved Megagametogenesis Filiform apparatus They are the cellular thickenings present at the micropylar tip of the synergids. They play an important role in guiding the pollen tube into the embryo sac. Central cell Six of the eight nuclei are surrounded by cell walls and organised into cells. The two polar nuclei are present in the largest cell of embryo sac - central cell. Egg cell The egg cell is the female gamete. It fuses with the male gamete to form the zygote, which later develops into an embryo. © 2022, Aakash BYJU'S. All rights reserved Pollination Pollination is the transfer of pollen grains from the anther to the stigma of a pistil. Pollination Self pollination Cross pollination Autogamy Geltonogamy Xenogamy Self-pollination with the same flower Pollination from same plant but different flower Pollination from different plant © 2022, Aakash BYJU'S. All rights reserved Self-Pollination Self pollination is the transfer of pollen from anther to stigma of genetically similar flower. Self pollination Autogamy Transfer of pollen from anther to stigma of same flower © 2022, Aakash BYJU'S. All rights reserved Geitonogamy Transfer of pollen from anther to stigma of genetically similar flower from the same plant Autogamy  Autogamy requires synchrony in pollen release and stigma receptivity.  Also, the anthers and the stigma should lie close to each other so that self-pollination can occur.  Complete autogamy is very rare in flowers which have stamens and stigmas are exposed, i.e. in an open flower  For this reason, some plants produce 2 types flowers ○ Chasmogamous flower ○ Cleistogamous flower © 2022, Aakash BYJU'S. All rights reserved AUTO GAMOS Self Marriage Autogamy Chasmogamous flower  Chasmogamous flowers are the flowers with exposed anthers and stigma  Anther and stigma need to be close Stigma Bent filament Curving of filaments over stigma in Mirabills jalapa © 2022, Aakash BYJU'S. All rights reserved Autogamy Cleistogamous flowers  Plants such as Viola (common pansy), Oxalis, and Commelina produce these flowers.  Pollination : 1 Anthers dehisce in the flower buds 2 Pollen grains come in contact with the stigma 3 Production of assured seed-set © 2022, Aakash BYJU'S. All rights reserved Cleistogamous flower of Viola Geitonogamy Geitonogamy  Transfer of pollen grains to another flower of same plant  Geitonogamy is functionally cross-pollination involving a pollinating agent  Genetically similar to autogamy © 2022, Aakash BYJU'S. All rights reserved GEITON GAMOS Neighbour Marriage Advantages of Self-pollination Advantages of self - pollination It maintains the parental characters/purity of the race indefinitely It is useful is maintaining pure lines for hybridization experiments The plants do not need to produce large quantities of pollen grains The flowers do not need to develop characteristics to attract pollinators. Seed production is assured It eliminates bad recessive characters © 2022, Aakash BYJU'S. All rights reserved Disadvantages of Self-pollination Disadvantages of self-pollination No introduction of new characters Decrease in adaptability to change in environment Decreased immunity to disease Decrease in variability © 2022, Aakash BYJU'S. All rights reserved Cross Pollination Cross pollination is the transfer of pollen from anther of one flower to stigma of genetically different flower. Cross pollination Xenogamy Xenogamy is the transfer of pollen from anther of one flower to stigma of genetically different flower. Needs help of external agencies © 2022, Aakash BYJU'S. All rights reserved XENOS GAMOS Strange Marriage Cross Pollination Advantages of cross pollination Disadvantages of cross pollination Higher yield Large number of pollen needs to be produced Increased adaptability and resistance to diseases Less chance of successful pollination Production of new and useful varieties Good characters might be lost Elimination/ replacement of defective characters Bad characters might be introduced © 2022, Aakash BYJU'S. All rights reserved Agents of Pollination Types of agents Biotic Abiotic Anemophily Hydrophilly Entomophily Ornithophily Chiropterophily © 2022, Aakash BYJU'S. All rights reserved Abiotic agents - Anemophily © 2022, Aakash BYJU'S. All rights reserved  Anemos means wind and philein means to love.  Pollination by wind is more common amongst abiotic pollinations.  The transfer of pollen grains from the anther to the stigma occurs by wind as an agent.  Examples: Coconut palm, date palm, grasses, etc. Characteristics of Anemophilous Flowers Flowers are packed into inflorescence Pollen grains are light Pollen grains are non-sticky Anemophilous flowers Well exposed stamens © 2022, Aakash BYJU'S. All rights reserved Flowers are packed into inflorescence Single ovule in a ovary Feathery large stigma Abiotic agents - Hydrophily  Pollination by water is quite rare in flowering plants and is limited to about 30 genera, mostly monocotyledons.  Not all aquatic plants use water for pollination. o  In most others, the flowers emerge above the level of water and are pollinated by insects or wind. E.g. Vallisneria, Hydrilla, marine sea grasses (Zostera) © 2022, Aakash BYJU'S. All rights reserved Sea grasses Characteristics of Hydrophilous Flowers In Vallisneria Sea grasses  Female flower reaches the surface of water  Female flowers remain submerged in water  Male flowers or pollen grains are released on to the surface of water  The pollen grains are released inside the water  They are carried passively by water currents  Some of them eventually reach the female flowers and the stigma © 2022, Aakash BYJU'S. All rights reserved Pollens In most water pollinated plants, the pollen grains are long, ribbon like pollen grains with mucilaginous covering Entomophily Entomon – insect; philein – to love It is the most common type of pollination Pollinator-insects ○ Moths, butterflies, wasps, bees, beetles, etc. Plants provide nectar, edible pollen grains or shelter (to lay eggs). © 2022, Aakash BYJU'S. All rights reserved Entomophily Characteristics of entomophilous flowers Showy and brightly coloured Small flowers grouped to be conspicuous Lily Sunflower © 2022, Aakash BYJU'S. All rights reserved Landing platform Honey/ nectar guides Landing platform in Viola Nectar guides in Mimulus Entomophily Characteristics of entomophilous flowers Produce pleasant odour Produce foul odour Secretion of nectar Pleasant odour Foul odour in rafflesia Nectar glands in Magnolia in Jasmine attracts flies and beetles © 2022, Aakash BYJU'S. All rights reserved Produce edible pollens Edible pollens Magnolia Entomophily Characteristics of entomophilous flowers Inserted stamens Inserted and sticky stigma Insterted stamens in Petunia Sticky stigma in Lillies © 2022, Aakash BYJU'S. All rights reserved Pollensspiny, heavy, surrounded by pollen kit Safe place to lay eggs Pollen grains Amorphophallus Ornithophily Ornis – bird; philein – to love It is performed by birds. Bird pollinators are small in size and have long beaks. ○ Eg., sun birds, hummingbirds Ornithophilous plants - Bombax, lobelia, etc Other bird pollinators include crows, bulbil, parrots. Sun bird Hummingbird Bird pollinators © 2022, Aakash BYJU'S. All rights reserved Bombax Lobelia Ornithophilous plants Ornithophily Characteristics of ornithoophilous flowers Abundant watery, sugary nectar Edible parts © 2022, Aakash BYJU'S. All rights reserved Brightly coloured flowers Funnel shaped corolla Leathery floral parts Chiropterophily Cheir – hand; pteros – wing; philein – to love It is performed by bats Facilitate long distance pollen transfer E.g. Agave palmeri, Anthocephalus, Adansonia, etc Agave palmeri © 2022, Aakash BYJU'S. All rights reserved Anthocephalus Adansonia Bat Chiropterophily Characteristics of chiropterophilous flowers Dull coloured © 2022, Aakash BYJU'S. All rights reserved Large and stout Strong fermenting/ fruity smell Abundant pollens and nectar Inbreeding Depression Inbreeding depression is the reduced biological fitness in a given population as a result of inbreeding, or breeding of related individuals. © 2022, Aakash BYJU'S. All rights reserved Outbreeding Outbreeding - Breeding between unrelated organisms Prevents inbreeding depression Outbreeding devices - Devices that discourage self-pollination and encourage cross-pollination 1 Unisexuality 2 Dichogamy 3 Self-incompatibility 4 Heterostyly © 2022, Aakash BYJU'S. All rights reserved Outbreeding Devices Unisexuality Unisexuality Monoecious plants Dioecious plants Unisexual flowers (male/ female) present on the same plant Unisexual flowers (male/ female) present on different plants Prevents autogamy but not geitonogamy Prevents autogamy and geitonogamy E.g., Maize, Castor, etc. E.g., Papaya © 2022, Aakash BYJU'S. All rights reserved Outbreeding Devices Dichogamy Dichogamy Protandry Protogyny Anthers mature before the stigma of the same flower Stigma matures before anthers of the same flower Prevents autogamy Prevents autogamy E.g., Salvia, Sunflower E.g., Mirabilis jalapa, Gloriosa, etc. Anthers and stigma mature at different times in bisexual flowers © 2022, Aakash BYJU'S. All rights reserved Outbreeding Devices Self-incompatibility Self-incompatibility acts at the genetic level. Pollen grains of a flower do not germinate on stigma of the same flower or flowers of same plant Prevents both autogamy and geitonogamy E.g., Often observed in tobacco, potato, crucifers Tobacco © 2022, Aakash BYJU'S. All rights reserved Potato Crucifers Outbreeding Devices Haterostyly Haterostyly Style longer than stamen Stamen longer than style E.g. Primrose, Jasmine, Lythrum, etc Anther and stigma are placed at different locations; prevents autogamy © 2022, Aakash BYJU'S. All rights reserved Artificial Hybridisation Artificial hybridisation The crossing between two different species Offspring produced are called hybrids In plant breeding, the pollen grains from species that have the desired characteristics are carefully chosen. Species 1 Species 2 Artificial hybridisation Benefits Tremendous growth Development of disease resistance Crops that can sustain extreme temperatures © 2022, Aakash BYJU'S. All rights reserved Hybrid Steps of Artificial Hybridisation Emasculation : Removal of anther before dehiscence Bagging : Covering of stigma before it reaches receptivity Rebagging : Covering of stigma after dusting pollen grains In bisexual flowers In unisexual flowers Emasculation Emasculation Bagging Bagging Emasculation Dusting pollen grains Rebaging © 2022, Aakash BYJU'S. All rights reserved Rebaging Bagging Pollen – Pistil Interaction The entire process from pollen deposition to the formation of pollen tube and entering of the pollen tube into the ovule. © 2022, Aakash BYJU'S. All rights reserved Different species /selfincompatible pollen grain Same species/compatible pollen grain Stigma Stigma Pollen tube formation Pollen tube formation Pollen – Pistil Interaction Post pollination, if the pollen is compatible with the stigma, the following are expected to happen: ○ Pollen absorbs water and nutrients from the stigma surface ○ In 2-celled pollen the generative cell divides to form two male gametes ○ Pollen tube formation begins and grows down towards the ovary, through the style Pollen tube (Germinating from vegetative cell) Male gametes move towards ovule Pollen grain Vegetative cell Male gametes - Male gametes enter embryo sac through micropylar end © 2022, Aakash BYJU'S. All rights reserved Fertilisation Process of formation of zygote by the fusion of male and female gametes Occurs in the embryo sac Male gamete (n) Female gamete (n) Fertilisation Zygote (2n) © 2022, Aakash BYJU'S. All rights reserved Fertilisation Navigation of pollen tube Discharge of male gametes Pollen tube growth stops Male gametes move © 2022, Aakash BYJU'S. All rights reserved Double Fertilisation One male gamete fuses with the egg nucleus to form the zygote - Syngamy. Other male gamete fuses with the 2 polar nuclei forming the primary endosperm nucleus (PEN) - Triple fusion. Male gamete (n) Polar nucleus (n) Triple fusion Primary endosperm nucleus (PEN) (3n) Endosperm (3n) © 2022, Aakash BYJU'S. All rights reserved Polar nucleus (n) Double Fertilisation Changes in embryo sac Antipodal cells PEN (3n) PEC Zygote (2n) Polar nuclei Fertilisation Synergids Filiform apparatus © 2022, Aakash BYJU'S. All rights reserved Egg cell Double Fertilisation Fertilisation based on pollen tube entry Porogamy Mesogamy Chalazogamy Pollen tube enters via the micropylar end Pollen tube enters via the chalazal end Pollen tube Pollen tube enters via the integuments Chalaza Micropyle Pollen tube Pollen tube © 2022, Aakash BYJU'S. All rights reserved Integuments Post Fertilization Events Endosperm development Embryo development © 2022, Aakash BYJU'S. All rights reserved Ovule Seed Ovary Fruit Endosperm Development Endosperm Main source of nutrition for embryo in the seed Endosperm development should begin before embryo development starts Primary endosperm cell (PEC) PEN (3n) Zygote (2n) Endosperm development Nuclear type © 2022, Aakash BYJU'S. All rights reserved Cellular type Helobial type Endosperm Development Nuclear endosperm development The PEN divides repeatedly (mitotic division) without cytokinesis. It results in the formation of a large number of free nuclei in the cell. A large central vacuole is formed and nuclei get arranged at the periphery. Later, cell wall formation takes place from the periphery towards the centre and multicellular endosperm is formed. Examples: maize, rice, wheat, cotton, sunflower © 2022, Aakash BYJU'S. All rights reserved Endosperm Development Nuclear endosperm development Primary endosperm nucleus © 2022, Aakash BYJU'S. All rights reserved Free nuclei Endosperm Development Cellular endosperm development Division of PEN (karyokinesis) is followed by cytokinesis and two cells are formed due to transverse division. Further division is similar, which leads to the formation of the cellular endosperm. It is not very common. Examples: Petunia, Balsam, Datura. © 2022, Aakash BYJU'S. All rights reserved Endosperm Development Cellular endosperm development Primary endosperm nucleus Endosperm © 2022, Aakash BYJU'S. All rights reserved Endosperm Development Helobial endosperm development The first division is like cellular endosperm and results in a large micropylar cell and small chalazal cell. The chalazal cell usually does not divide further and functions as a base cell. The micropylar cell divides further, like nuclear endosperm. It is an intermediate type, a combination of both nuclear and cellular endosperm. This type of endosperm development is common in monocotyledons. Examples: Eremurus © 2022, Aakash BYJU'S. All rights reserved Endosperm Development Helobial endosperm development Primary endosperm nucleus Chalazal chamber Cell wall formation Micropylar chamber © 2022, Aakash BYJU'S. All rights reserved Free nuclei Endosperm Embryo Development Happens at micropylar end Endosperm provides nutrition for development Zygote Embryogeny Seeds Monocots Dicots One cotyledon Two cotyledons © 2022, Aakash BYJU'S. All rights reserved Embryo Embryogenesis in Dicots The zygote gives rise to the proembryo and subsequently to the globular, heart-shaped and mature embryo The zygote undergoes unequal division to form a terminal and a basal cell. ○ Terminal cell (Apical cell) ○ Basal cell Embryo 6-10 celled suspensor filament Mature embryo suspensor Zygote Heart-shaped embryo Globular embryo © 2022, Aakash BYJU'S. All rights reserved Embryogenesis in dicots Dicot Embryo Portion of embryonal axis above cotyledons : Epicotyl Epicotyl terminates with stem tip/plumule Portion below cotyledons : Hypocotyl Hypocotyl terminates with root tip/radicle The root tip is covered with a root cap Plumule Hypocotyl Cotyledons Radicle © 2022, Aakash BYJU'S. All rights reserved Dicot embryo Embryogenesis in Monocots Embryogenesis in monocots takes place by the following steps: Zygote Divides transversely Terminal cell Divides transversely forming 2 cells. This series of division leads to the quadrant stage which divides transversely forming octants arranged in 2 tiers of 4 cells each © 2022, Aakash BYJU'S. All rights reserved Basal cell Larger and lies towards the micropylar end, does not divide again but becomes transformed directly into a large vesicular cell Embryogenesis in Monocots Embryos of monocotyledons possess only one cotyledon. In the grass family, the cotyledon is called scutellum that is situated towards one side (lateral) of the embryonal axis. At its lower end, the embryonal axis has the radical and root cap enclosed in an undifferentiated sheath called coleorrhiza. The portion of the embryonal axis above the level of attachment of scutellum is the epicotyl. Epicotyl has a shoot apex and a few leaf primordia enclosed in a hollow foliar structure, the coleoptile. © 2022, Aakash BYJU'S. All rights reserved Monocot Embryo Coleorhiza - Undifferentiated sheath that encloses radical and root cap. Epicotyl - Portion of the embryonic axis above the level of attachment of scutellum. Epicotyl has a shoot apex and a few leaf primordia enclosed in a hollow foliar structure, the coleoptile. Scutellum (Cotyledon) Coleoptile Shoot apex Epiblast Radicle Root cap Coleorhiza © 2022, Aakash BYJU'S. All rights reserved Monocot embryo Seed Ovule Integuments Micropyle Nucellus © 2022, Aakash BYJU'S. All rights reserved Seed Seed coats Micropyle Disappears or persists (Perisperm) Seed Embryo Embryo Develops from a zygote Cotyledon Cotyledon Food reserves Used by the embryo to grow Seed coat Outermost part of a seed Protects the developing embryo © 2022, Aakash BYJU'S. All rights reserved Seed coat Seed Fruit Parts of a fruit Seed Endocarp Mesocarp Epicarp Pericarp Epicarp - Skin Mesocarp - Tissue between epicarp and endocarp Endocarp - Covering of the seed Fruit True fruit © 2022, Aakash BYJU'S. All rights reserved False fruit Parthenocarpic fruit Fruit False fruit True fruit Develops from mature ovary Example : Mango Develops from parts of the flower other than the ovary Example : Apple, strawberry, cashew, etc thalamus also contributes to fruit formation Thalamus Epicarp Mesocarp Seed Endocarp Mango © 2022, Aakash BYJU'S. All rights reserved Seed Apple Seed Dormancy Seed dormancy During certain unfavourable conditions (temperature, humidity, etc.), the embryo becomes inactive, i.e., the metabolic activities (release of energy, consumption of energy, etc.) slow down. This state is known as seed dormancy. Seed dormancy - Duration Date palm (Phoenix dactylifera) - 2,000 years © 2022, Aakash BYJU'S. All rights reserved Arctic lupine (Lupinus arcticus) – 10,000 years No Seed Dormancy! If there was no seed dormancy, then seeds would germinate soon after their formation. As a result, they would have no shelf life. It helps farmers to store the seeds and sow whenever required. © 2022, Aakash BYJU'S. All rights reserved Advantage of Seeds Advantages of having seeds for Angiosperms: Dependable seed formation as pollination and fertilisation are independent of water Nourishment to embryo Dispersion of seeds by various agents Protection to embryo Genetic variation © 2022, Aakash BYJU'S. All rights reserved Seed - Conditions for Germination Conditions required by the seed to germinate: ○ suitable temperature ○ adequate moisture ○ proper supply of oxygen Once all the conditions are met, the seed germinates into a small plant. Suitable temperature © 2022, Aakash BYJU'S. All rights reserved Proper oxygen supply Adequate moisture Journey of a Seed If the fruit would fall beside the tree and the seed would start germinating there itself © 2022, Aakash BYJU'S. All rights reserved The new plant and the old plant would have to compete for: a) Water b) Nutrition c) Space d) Sunlight Seed Dispersal Seed dispersal is the process by which seeds are dispersed to different places through agents like wind, water, animal, and explosion. Agents of seed dispersal Seeds dispersed by wind are light with wings or feathery structures. E.g., Dandelion, swan plant Seeds dispersed by water should be able to float on water. E.g., Lotus, coconut Seeds present under pressure inside the fruit explode which helps in their dispersal. E.g., Exploding cucumber, pea Seeds dispersed by animals are present in edible fruits. They remain undigested and are passed out through the faeces. E.g., Watermelon, cherries. © 2022, Aakash BYJU'S. All rights reserved Wind Explosion Water Animals Types of Seeds Seeds (Based on cotyledons) Monocotyledonous Dicotyledonous Seeds which have a single cotyledon Seeds which have two cotyledons e.g.: Maize, wheat © 2022, Aakash BYJU'S. All rights reserved e.g.: Beans, maple Types of Seeds Seeds (Based on endosperm) Albuminous Non - albuminous Endosperm present Endosperm absent Endosperm not fully consumed during embryo development Endosperm fully consumed during embryo development Eg. wheat, maize, barley, and castor. Eg. pea and groundnut © 2022, Aakash BYJU'S. All rights reserved Types of Seeds Seeds (Based on endosperm) Albuminous Non - albuminous Embryo Embryo Endosperm Cotyledon In non albuminous seeds, the endosperm is usually consumed and the food is stored in the cotyledons. © 2022, Aakash BYJU'S. All rights reserved Seed Advantages for Angiosperms Dependable seed formation as pollination and fertilisation are independent of water Nourishment to embryo Dispersion of seeds by various agents Protection of embryo Genetic variation © 2022, Aakash BYJU'S. All rights reserved Apomixis Term coined by Hans Karl Albert Winkler. Process of production of seeds without fertilisation. Apomixis = Apo (without) + mixis (mingling). Commonly found in some species of Asteraceae and grasses. Apomixis Type 1 © 2022, Aakash BYJU'S. All rights reserved Type 2 Apomixis – Type 1 1 The egg cell is diploid (2n). 4 5 2n 2 2n The diploid cell divides mitotically. 6 3 4 2n Multiple egg cells give rise to the zygotic embryo. The embryo is formed. © 2022, Aakash BYJU'S. All rights reserved 7 Zygotic Embryo 2n 2n 2n 2n Apomixis – Type 2 1 Female gametophyte 2 Nucellus cells are somatic diploid cells. 3 Nucellus cells penetrate into the embryo sac. 4 Some of the nucellar cells penetrate into the embryo sac and then start dividing inside. 5 Thus, the nucellar embryo is formed. © 2022, Aakash BYJU'S. All rights reserved 1 2 5 4 3 2n 2n 2n 2n 2n 2n 2n 2n 2 n Polyembryony Polyembryony - Occurrence of more than one Nucellar embryo in seeds 2n embryo Egg cell Examples: Mango seed types 1 Embryo More than 1 embryo Polyembryony © 2022, Aakash BYJU'S. All rights reserved 2n Hybrid Seeds Hybrid seeds are produced by cross-pollination. They contain characteristics of diverse plant species. They show extensive growth and productivity. To produce hybrid seeds, there are two methods. Two solutions Solution 1 Solution 2 Create hybrid seeds every year Produce hybrid seeds by apomixis © 2022, Aakash BYJU'S. All rights reserved Hybrid Seeds Production of hybrid seeds through apomixis is better because No meiosis No segregation of chromosomes in gametes Hybrid nature is maintained Transferring apomictic genes to hybrid varieties makes it easy to produce large numbers of hybrid seeds. A1 A2 B1 B2 A1 A2 B1 B2 Apomixis Hybrid Parent (2n) © 2022, Aakash BYJU'S. All rights reserved Hybrid Zygote (2n) Summary Structure of flower Petal Sepal © 2022, Aakash BYJU'S. All rights reserved Whorl of stamens (Androecium) Whorl of carpels (Gynoecium) Summary Structure of anther Epidermis Protective role Endothecium Middle layer Formation of microspore Sporogenous tissue Tapetum helps in nourishment of microspore © 2022, Aakash BYJU'S. All rights reserved Summary Pollen formation Pollen mother cell is diploid or 2n Pollen mother cell undergoes meiotic division First it undergoes meiosis 1, forms two haploid cells After meiosis II, four haploid cells are formed known as the microspore tetrad These microspores form pollen grains © 2022, Aakash BYJU'S. All rights reserved Summary Vacuole Pollen maturation Vegetative cell 1st Mitosis Generative cell Nucleus Vacuoles grow in the microspore © 2022, Aakash BYJU'S. All rights reserved A single large vacuole is formed and cell division begins Mitotic division results in 2-celled stage of the pollen grain Summary Chalazal end Chalaza Hilum Nucellus Integuments Funicle Embryo sac Micropyle Micropylar end © 2022, Aakash BYJU'S. All rights reserved Structure of an ovule Summary Megasporogenesis Megaspore mother cell Megagametogenesis Involves Involves meiosis mitosis Haploid megaspore © 2022, Aakash BYJU'S. All rights reserved Female gametophyte Summary Agents of cross pollination Pollination Self-pollination Autogamy Geitonogamy Cross pollination Abiotic agents Xenogamy Wind Insects © 2022, Aakash BYJU'S. All rights reserved Biotic agents Water Birds Bats Summary Pollination Self pollination Autogamy Geltonogamy Cross pollination Xenogamy Chasmogamous flower – open flowers with exposed anther and stigma. Cleistogamous flowers – closed flowers that do not open at all and produce assured set of seeds. © 2022, Aakash BYJU'S. All rights reserved Summary Male gamete (n) Polar nucleus (n) Polar nucleus (n) Triple Fusion Primary Endosperm Nucleus (PEN) (3n) Endosperm (3n) Fertilisation based on pollen tube entry Porogamy © 2022, Aakash BYJU'S. All rights reserved Chalazogamy Mesogamy Summary Types of seeds Based on number of cotyledons Monocotyledonous © 2022, Aakash BYJU'S. All rights reserved Dicotyledonous Based on presence or absence of endosperm Albuminous Non-albuminous