Development of the megaspore within the ovule (megasporangium) is known as megasporogenesis.
Definitions [37]
Definition: Thalamus (Receptacle)
The swollen terminal part of the pedicel on which all floral whorls are arranged is called the thalamus or receptacle.
Definition: Whorl
A circular arrangement of floral organs at the same level on the thalamus is called a whorl.
Definition: Perianth
When calyx and corolla are not differentiated, the floral envelope is called the perianth.
Definition: Tepals
Individual members of the perianth are called tepals.
Definition: Flower
A modified, compressed reproductive shoot of angiosperms bearing sepals, petals, stamens, and carpels, meant for sexual reproduction, is called a flower.
Definition: Compound Pollen Grains
When microspores of a tetrad do not separate and remain attached together in groups, they are called compound pollen grains.
Definition: Microsporogenesis
The process of formation of microspores from the sporogenous tissue is said to be microsporogenesis.
Definition: Pollinium
A compact mass formed when all microspores within a pollen sac remain united as a single structure is called a pollinium.
Definition: Germ Pore
The region where the exine is absent and through which the pollen tube emerges is called a germ pore.
Definition: Palynology
The study of external morphology of mature pollen grain is called palynology.
Definition: Pollenkitt
A yellowish, sticky, oily substance covering the exine of insect-pollinated pollen grains is called pollenkitt.
Definition: Sporoderm
The protective wall or covering of a pollen grain is called the sporoderm.
Definition: Ovule (Megasporangium)
The structure in flowering plants that develops into a seed after fertilization is called the ovule.
Definition: Megasporogenesis
Definition: Pollination
Transfer of pollen grains from the anther to the stigma of a flower is called pollination.
or
Pollination is the transfer of pollen grains from the anther of a flower to the stigma of the same flower or of another flower of usually the same species.
Definition: Autogamy
When pollen grains from the anther are transferred to the stigma of the same flower, it is known as autogamy.
Definition: Geitonogamy
When pollens of a flower pollinate any other flower present on the same plant, it is said to be geitonogamy
Definition: Xenogamy
Xenogamy is the transfer of pollen grains from the anther of one plant to the stigma of another plant of the same species, resulting in genetic variation.
Define ornithophily.
Ornithophily is bird pollination, where the pollen grains of the flower are distributed by specialised birds for pollination. They usually have small sizes and long beaks e.g. Sun birds and humming birds. Some ornithophilous plants are Bombax, Callistemon (Bottle Brush), Butea, etc.
Definition: Anemophily
Transfer of pollen grains through wind is known as anemophily.
Definition: Epi-hydrophily
When pollination of flowers occurs at the surface of water, it is called epi-hydrophily.
Definition: Hypo-hydrophily
When pollination of flowers occurs below water, it is said to be hypo-hydrophily.
Definition: Hydrophily
Transfer of pollen grains through water is known as hydrophily.
Definition: Zoophily
Pollination in which pollen grains are transferred from the anther to the stigma with the help of animals such as insects, birds, bats and other animals.
Definition: Pollen–pistil interaction
The recognition and acceptance or rejection of pollen by the pistil, from pollen deposition on the stigma to pollen tube entry into the ovule, is called pollen–pistil interaction.
Definition: Artificial Hybridization (Artificial Fertilization)
The controlled crossing of selected male and female parent plants to produce genetically superior offspring is called artificial hybridization.
Definition: Double Fertilization
Union of the second male gamete with two polar nuclei to form endosperm, along with fertilization, is called double fertilization.
Definition: Zygotic Meiosis
The process in which a zygote undergoes meiosis to form haploid spores in some algae and fungi is called zygotic meiosis.
Definition: Post-fertilization Events
All events occurring after fertilization and formation of the zygote during sexual reproduction are called post-fertilization events.
Definition: Germination
The process by which the embryo present in the seed develops into a seedling under appropriate conditions is called germination.
Definition: Embryogenesis
The process by which the zygote develops into an embryo through mitotic divisions and cell differentiation is called embryogenesis.
Definition: Endosperm
Endosperm is a nutritive tissue formed after fertilisation in flowering plants. It is usually triploid and provides food to the developing embryo.
Definition: Embryo
The embryo is the young developing plant formed from the zygote after fertilisation. It develops at the micropylar end of the embryo sac.
Define the following:
Seed
Seed is defined as a fertilized mature ovule which possesses an inactive embryo and reserve food for its further development.
Definition: Apomixis
Apomixis is the production of seeds without fertilisation and is a form of asexual reproduction that mimics sexual reproduction.
Definition: Polyembryony
Presence of more than one embryo within a single seed is called polyembryony.
Define Apomixis.
Apomixis is a type of asexual reproduction that mimics sexual reproduction, i.e., the production of seeds without pollination and fertilization.
Key Points
Key Points: The Flower
- Flower: A modified shoot and the reproductive unit of angiosperms.
- Structure: Consists of four whorls—calyx, corolla (accessory), androecium, and gynoecium (reproductive).
- Types by sex: Flowers may be unisexual (male/female) or bisexual (both present).
- Symmetry: Actinomorphic (radial), zygomorphic (bilateral), or asymmetric.
- Based on floral parts: Flowers can be trimerous, tetramerous, or pentamerous depending on the number of parts.
- Position of ovary: Hypogynous (superior), perigynous (half inferior), epigynous (inferior).
Key Points: Parts of Flower
| Part of Flower | Whorl Type | Main Components | Function |
|---|---|---|---|
| Calyx | Accessory whorl | Sepals | Protect inner floral parts |
| Corolla | Accessory whorl | Petals | Attract insects for pollination |
| Androecium | Essential whorl | Stamens (filament + anther) | Male reproduction; pollen formation |
| Gynoecium | Essential whorl | Carpels (stigma, style, ovary) | Female reproduction; seed and fruit formation |
Key Points: Structure and Development of Anther
- A typical anther is dithecous and tetrasporangiate, having two lobes, each with two microsporangia (pollen sacs).
- Microsporangia contain sporogenous tissue, which develops into microspore mother cells that form pollen grains.
- The anther wall has four layers: epidermis, endothecium, middle layers, and tapetum.
- The tapetum provides nutrition to developing pollen, and microspore mother cells undergo meiosis to form haploid microspores.
- During anther dehiscence, the endothecium helps in rupture at the stomium, releasing pollen grains for pollination.
Key Points: Microsporogenesis
- Microsporogenesis is the process in which pollen mother cells (PMC) undergo meiosis to form haploid microspores (pollen grains).
- Each PMC divides meiotically to produce a tetrad of four haploid microspores.
- Microspores separate at maturity and develop into pollen grains.
- A pollen grain has a two-layered wall called sporoderm: outer exine and inner intine.
- Exine is made of sporopollenin, a tough and resistant substance, and contains germ pores for pollen tube growth.
- Intine is the inner layer made of cellulose and pectin.
- Pollen viability (ability to germinate) depends on temperature and humidity; it is short (about 30 minutes) in crops like rice and wheat but can last for months in some plant families.
Key Points: Types of Pollen Tetrads
| Type of Pollen Tetrad | Arrangement of Microspores | Distinct Feature | Example |
|---|---|---|---|
| Tetrahedral | Four microspores at the corners of a tetrahedron | Only three microspores visible from one side | Rhododendron, Nicotiana |
| Isobilateral | All four microspores arranged in one plane | Microspores lie opposite each other | Cucurbita |
| Decussate | Microspores arranged in two crossed pairs | Cross-like arrangement | Magnolia |
| T-shaped | Three microspores in a row and one at right angle | Forms a T-shape | Aristolochia |
| Linear | Four microspores arranged in a straight line | All microspores in one row | Halophila |
Key Points: Structure and Development of Male Gametophyte
- Pollen grains act as male gametophytes and are usually spherical, measuring about 25–50 μm in diameter.
- Each pollen grain has a two-layered wall (sporoderm) consisting of a thick outer exine and a thin inner intine.
- The exine contains sporopollenin, which makes pollen grains highly resistant and helps in their fossil preservation.
- Germ pores are present where the exine is absent; dicots usually have three pores, while monocots have one pore.
- The intine protrudes through the germ pore during germination to form the pollen tube.
- In insect-pollinated plants, pollen grains are coated with pollenkitt, which aids in insect attraction and pollination.
- Pollen viability varies widely, lasting minutes in cereals and months in some dicots; pollen can be preserved at –196°C in pollen banks.
- After release, the pollen grain divides into a vegetative cell and a generative cell, and the generative cell later forms two male gametes.
Key Points: Structure and Development of Ovule
- The ovule is an integumented megasporangium present inside the ovary and is attached to the placenta by a stalk called the funicle.
- The nucellus is the central part containing reserve food, surrounded by one or two integuments for protection.
- The integuments leave a small opening called the micropyle at the apex, while the basal region is called the chalaza, and the attachment point is the hilum.
- The ovule is commonly anatropous, and contains an embryo sac (female gametophyte) with structures like egg cell, synergids, and polar nuclei.
- During development, the ovule arises from the placenta, and integuments grow around the nucellus to form a mature ovule.
Key Points: Megasporogenesis
- Megasporogenesis is the process of formation of megaspores from a diploid megaspore mother cell (MMC) in the ovule.
- It occurs inside the nucellus of the ovule, usually near the micropylar region.
- The MMC undergoes meiosis to form four haploid megaspores arranged in a linear tetrad.
- Out of the four megaspores, three degenerate, and only one functional megaspore remains.
- The functional megaspore undergoes three mitotic divisions to form an 8-nucleate, 7-celled embryo sac (female gametophyte).
- The embryo sac contains one egg cell, two synergids, three antipodal cells, and one central cell with two polar nuclei.
Key Points: Development of Female Gametophyte or Embryo Sac
- The female gametophyte (embryo sac) develops from the functional megaspore through mitotic divisions.
- The nucleus divides to form 8 nuclei, arranged with four at each pole, and one from each pole moves to the centre as polar nuclei.
- At the micropylar end, three nuclei form the egg apparatus (one egg cell and two synergids), which help in guiding the pollen tube.
- At the chalazal end, three nuclei form the antipodal cells, while the two polar nuclei fuse to form a diploid secondary nucleus.
- The mature embryo sac is monosporic, 7-celled and 8-nucleated (Polygonum type), which is the most common in angiosperms.
Agencies of Pollination
| Agency | Agent | Type | Examples |
|---|---|---|---|
| Anemophily | Wind | Abiotic | Wheat, rice, maize, grass |
| Hydrophily (Hypo) | Water (submerged) | Abiotic | Zostera |
| Hydrophily (Epi) | Water (surface) | Abiotic | Vallisneria |
| Entomophily | Insects (e.g., bumblebee) | Biotic | Rose, Jasmine, Salvia, Lotus |
| Ornithophily | Birds (e.g., sunbird) | Biotic | Bombax, Butea, Callistemon |
| Chiropterophily | Bats | Biotic | Anthocephalus, Kigelia, Adansonia |
Key Points: Pollination
- Pollination is the transfer of pollen grains from anther to stigma and is essential for fertilisation since gametes are non-motile.
- Pollination occurs through external agents such as wind, water, and animals because pollen grains cannot move on their own.
- There are three types of pollination: autogamy (same flower), geitonogamy (same plant), and xenogamy (different plants of the same species).
- Self-pollination leads to inbreeding, while cross-pollination increases genetic variation.
- Abiotic agents of pollination include wind (anemophily) and water (hydrophily).
- Biotic agents include insects, birds, and bats, which help in effective pollen transfer between flowers.
Key Points: Outbreeding Devices
| Outbreeding Devices | Mechanism | Role | Examples |
|---|---|---|---|
| Self-sterility (Self-incompatibility) | Pollen fails to germinate on own stigma | Prevents self-fertilization | Petunia, Brassica |
| Dichogamy – Protogyny | Stigma matures before anthers | Avoids autogamy | Magnolia, Aristolochia |
| Dichogamy – Protandry | Anthers mature before stigma | Avoids autogamy | Salvia, Helianthus |
| Herkogamy | Spatial separation of sex organs | Mechanical prevention of autogamy | Gloriosa, Caryophyllaceae |
| Heterostyly | Different style lengths | Promotes cross-pollination | Primula |
| Unisexuality – Monoecious | Male and female flowers on same plant | Prevents autogamy | Maize, Castor |
| Unisexuality – Dioecious | Male and female flowers on different plants | Prevents autogamy & geitonogamy | Papaya, Date palm |
Key Points: Pollen Pistil Interaction
- Pollen-pistil interaction is the series of events from pollen landing on the stigma to fertilisation.
- The pistil recognises compatible pollen and rejects incompatible or foreign pollen using specific proteins.
- Compatible pollen germinates on the stigma and forms a pollen tube by absorbing nutrients.
- The pollen tube grows through the style and reaches the ovule, entering through a synergid in the embryo sac.
- The pollen tube releases male gametes, enabling fertilisation and seed formation.
Key Points: Double Fertilization and Triple Fusion
- Double fertilisation is a unique feature of angiosperms where two fusion events occur inside the embryo sac.
- The pollen tube enters the ovule usually through the micropyle (porogamy), but sometimes through the chalaza (chalazogamy) or integuments (mesogamy).
- The pollen tube releases two male gametes into the embryo sac after entering a synergid.
- Syngamy: One male gamete fuses with the egg to form a diploid zygote.
- Triple fusion: The second male gamete fuses with two polar nuclei to form a triploid primary endosperm nucleus (PEN).
- Significance: It forms both embryo and endosperm, restores the diploid condition, prevents polyembryony, and ensures proper seed development.
Key Points: Post-fertilization Events
- Post-fertilization events are the processes that occur after fertilization, beginning with the formation of a diploid zygote.
- The development of the zygote into an embryo through mitotic divisions and cell differentiation is called embryogenesis.
- In flowering plants, the ovary develops into a fruit and ovules form seeds, while in animals, the zygote undergoes cleavage to form stages like morula or blastula.
- Viviparous animals provide better protection and care to the embryo than oviparous animals, increasing the survival chances of offspring.
Key Points: Endosperm
- Endosperm is a nutritive tissue formed from the triploid primary endosperm nucleus (PEN) after fertilisation.
- The PEN undergoes repeated mitotic divisions, and the embryo and endosperm develop simultaneously inside the ovule.
- Other cells of the embryo sac degenerate, while endosperm formation provides nourishment to the developing embryo.
- Nuclear type is the most common; divisions occur without wall formation initially, forming free nuclei, and walls develop later.
- Cellular type shows immediate wall formation after each division, making it cellular from the beginning.
- Helobial type is intermediate; first division forms two unequal cells (micropylar and chalazal), followed by nuclear divisions and later wall formation.
Key Points: Embryo
- Embryogenesis is the process of development of the zygote into an embryo, which begins after fertilisation and some endosperm formation.
- The embryo develops at the micropylar end of the embryo sac.
- The zygote divides to form a two-celled proembryo: a terminal (embryonal) cell and a basal (suspensor) cell.
- The suspensor pushes the developing embryo into the endosperm and helps in nutrient supply, while the embryonal cell forms the main embryo.
- The embryo passes through stages like the octant and heart-shaped stage, forming structures such as cotyledons, plumule, radicle, and hypocotyl.
- In monocots, a single cotyledon (scutellum) is present with protective sheaths, the coleoptile (plumule) and coleorhiza (radicle), while in dicots, two cotyledons are formed.
Key Points: The Seed
- Seed is the fertilised ovule and final product of sexual reproduction in angiosperms.
- It consists of seed coat, cotyledons and embryo axis; food is stored mainly in cotyledons or endosperm.
- Seeds may be albuminous (endosperm present) or non-albuminous (endosperm absent); perisperm may persist in some seeds.
- Dormancy and dehydration help seeds survive adverse conditions and allow long-term storage.
- Seeds ensure dispersal, protection, nourishment of embryo, genetic variation and agricultural sustainability.
Key Points: Apomixis
- Apomixis is the formation of seeds without fertilisation, where embryos develop without meiosis and syngamy.
- In apomixis, the embryo develops inside the ovule, and the ovule forms a seed without gamete fusion.
- Apogamy is the formation of an embryo from a gametophyte cell without fertilisation, while apospory is the formation of a diploid gametophyte from a sporophyte cell without meiosis.
- Recurrent apomixis produces a diploid embryo sac (e.g., diplospory and apospory), while non-recurrent apomixis forms haploid embryos that are usually sterile.
- Adventive embryony involves the formation of embryos from the nucellus or integuments along with a normal embryo, leading to polyembryony (e.g., mango, orange).
- Apomixis helps in producing genetically identical plants quickly and efficiently.
Key Points: Polyembryony
- Polyembryony is the formation of more than one embryo in a single seed, leading to multiple seedlings.
- It was first observed by Leeuwenhoek in Citrus (orange) seeds.
- Polyembryony can occur due to development from the zygote or other tissues like the nucellus and integuments.
- Types include true polyembryony (embryos in the same embryo sac) and false polyembryony (embryos in different embryo sacs).
- Cleavage polyembryony occurs when the zygote splits into many parts, while adventive polyembryony occurs from nucellus or integument cells.
- It is important in horticulture and increases chances of survival by producing multiple seedlings.
Important Questions [47]
- Explain the Events Upto Double Fertilisation After the Pollen Tube Enters One of the Synergids in an Ovule of an Angiosperm.
- Read the statement and answer the questions that follow. A flower of brinjal has 520 ovules in its ovary. However, it produces a fruit with only 480 viable seeds. (a) What could have prevented the rest of the 40 ovules from maturing into viable seeds? Explain giving a reason.
- In a flowering plant a microspore mother cell produces four male gametophytes, while a megaspore mother cell forms only one female gametophyte. Explain.
- A Capsicum Flower Has 240 Ovules in Its Ovary. But, It Produces a Fruit with Only 180 Viable Seeds. Explain Giving a Reason that Could Be Responsible for Such a Result.
- List the three statges the annuals and biennial angiosperms have to pass through during their life cycle.
- Give an Example of an Angiosperm Seed that Has a Perisperm. Name the Part the Perisperm Develops From.
- Draw a diagram of the mature embryo sac of an angiospermic ovule and label any four parts in it.
- Explain the monosporic development of the embryo sac in the ovule of an angiosperm.
- Name the Part of the Flower Which the Tassels of the Corn- Cob Represent.
- Make a list of any three outbreeding devices that flowering plants have developed and explain how they help to encourage cross-pollination.
- Why Angiosperm Anthers Are Called Dithecous?
- As a senior biology student you have been asked to demonstrate to the students of secondary level in your school, the procedure(s) that shall ensure cross-pollination in a hermaphrodite flower.
- Why do moss plants produce very large number of male gametes? Provide one reason. What are these gametes called?
- Geitonogamous Flowering Plants Are Genetically Autogamous but Functionally Cross-pollinated. Justify.
- Describe the Characteristic Features of Wind Pollinated Flowers.
- Explain the Post-pollination Events Leading to Seed Production in Angiosperms.
- List the different types of pollination depending upon the source of pollen gain.
- A flower of tomato plant following the process of sexual reproduction produces 240 viable seeds.
- A flower of tomato plant following the process of sexual reproduction produces 200 viable seeds.
- Draw a Diagram of a Section of a Megasporangium of an Angiosperm and Label Funiculus, Micropyle, Embryosac and Nuclleus.
- Describe the Structure of Its Microsporangium.
- Compare the Characteristic Features of Insect Pollinated and Wind Pollinated Flowers. Explain How the Respective Features Assist in Pollination.
- Answer the Following Question. Express the Process of Pollination In Vallisneria.
- Differentiate between geitonogamy and xenogamy (allogamy).
- Answer the Following Question. Write the Difference in the Characteristics of the Progeny Produced as a Result of the Two Processes.
- Can a plant flowering in Mumbai be pollinated by pollen grains of the same species growing in New Delhi? Provide explanations to your answer.
- Coelacanth was caught in South Africa. State the significance of the discovery of Coelacanth in the evolutionary history of vertebrates.
- Draw an L.S. of pistil showing pollen tube entering into the embryo sac. Label the following: Nucellus Antipodals Synergids Micropyle
- Flowers which have single ovule in the ovary and are packed into inflorescence are usually pollinated by ______.
- One of the major approaches of crop improvement programme is Artificial Hybridisation.
- Answer the Following Question. You Are Conducting Artificial Hybridization on Papaya and Potato. Which One of Them Would Require the Step of Emasculation and Why? However, for Both, You Will Use the
- Emasculation and bagging are the two important steps carried during artificial hybridization to obtain superior varieties of desired plants. Explain giving reasons, in which types of flowers
- Describe the Development of an Endosperm in a Viable Seed. Why Does Endosperm Development Precedes Embryo Development?
- Some Organisms Suspend Their Metabolic Activities to Survive in Unfavourable Conditions. Explain with the Help of Any Four Examples.
- Why does endosperm development precede embryo development in angiosperm seeds?
- Draw a labelled mature stage of a dicotyledonous embryo.
- Describe the Development of Endosperm After Double Fertilisation in an Angiosperm.
- A non biology person is quite shocked to know that apple is a false fruit, mango is a true fruit and banana is a seedless fruit. As a biology student how would you satisfy this person?
- Angiosperms Bearing Unisexual Flowers Are Said to Be Either Monoecious Or Dioecious. Explain with the Help of One Example Each.
- Why Does Endosperm Development Preceeds that of Zygote ?
- State what is apomixis. Comment on its significance. How can it be commercially used?
- Why Are Some Seeds Referred to as Apomictic Seeds?
- State what is apomixis. Write its significance. How can it be commercially used ?
- Explain Any Three Advantages that Seeds Offer to Angiosperms.
- Mention one advantage of apomictic seeds to farmers.
- Answer the Following Question. When a Seed of Orange is Squeezed, Many Embryos, Instead of One Are Observed. Explain How It is Possible.
- Explain the different ways apomictic seeds can develop, Give an example of each.
Concepts [27]
- Flower
- Pre-fertilisation in Flowering Plant: Structures and Events
- Structure and Development of Anther
- Microsporogenesis
- Structure and Development of Male Gametophyte
- Pollen Products
- Structure and Development of Ovule
- Megasporogenesis
- Development of Female Gametophyte or Embryo Sac
- Pollination
- Autogamy
- Geitonogamy
- Xenogamy
- Agents of Pollination
- Anemophily
- Hydrophily
- Animal-Mediated Pollination (Zoophily)
- Outbreeding Devices
- Pollen Pistil Interaction
- Artificial Hybridization or Artificial Fertilization
- Double Fertilization and Triple Fusion
- Post-Fertilization Structures and Events
- Endosperm
- Embryo
- The Seed
- Apomixis
- Polyembryony
