Pre-Fertilisation: Stamen, Microsporangium and Pollen Grain
Microsporangium Structure
Surrounded by four wall layers:
- Epidermis, endothecium, middle layers: Protection and anther dehiscence
- Tapetum (innermost): Nourishes developing pollen grains NEET 2013, 2024 Re
Crucial detail: Inside young anther, sporogenous tissue occupies center and is diploid NEET 2013. It undergoes meiosis to form haploid microspore tetrads.
Pollen Grain (Male Gametophyte)
- Shed at 2-celled stage (Vegetative + Generative cell) in 60% of angiosperms
- Vegetative cell: Larger, abundant food reserve Mains 2012
- Generative cell: Divides mitotically to form two male gametes 2007
Pollen Wall (Sporoderm)
- Exine: Hard outer layer made of Sporopollenin - most resistant organic material known. Withstands high temperatures, strong acids, alkalis; cannot be degraded by any enzyme 2012, Karnataka NEET 2013, 2008. Allows pollen to be well preserved as fossils NEET 2018, 2024 Re
- Germ Pores: Prominent apertures on exine where sporopollenin is absent; site for pollen tube initiation Mains 2012
- Intine: Inner continuous wall made of cellulose and pectin
Pollen Viability & Usage
- Viability varies from 30 mins (wheat, rice) to months in families like Rosaceae, Leguminosae, and Solanaceae 2021, 2023-Manipur
- Can be stored for years in liquid nitrogen at -196°C (cryopreservation) for crop breeding NEET 2018, NEET-I 2016
- Pollen tablets used as food supplements 2014
- Some pollen causes severe allergies and bronchial afflictions
Pre-Fertilisation: Pistil, Megasporangium (Ovule) and Embryo Sac
Megasporangium (Ovule)
Technically equivalent to the ovule NEET 2013, 2014
- Attached to placenta by funicle
- Body of ovule fuses with funicle at Hilum - remains as scar on seed 2020, 2015
Ovule Types
- Anatropous: Inverted, most common, micropyle directed downwards 2002
- Orthotropous: Straight, with funiculus, chalaza, and micropyle in one straight line 1993
Megasporogenesis
Diploid Megaspore Mother Cell (MMC) undergoes meiotic division to form four haploid megaspores Karnataka NEET 2013. Three degenerate.
Embryo Sac (Female Gametophyte)
Develops from single functional megaspore (monosporic development) NEET 2017, Mains 2011, 1994, 1990
Formation Sequence
3 successive free nuclear mitotic divisions → migration of nuclei to poles → cell wall formation Odisha NEET 2019, 2023-Manipur
- Results in 8-nucleate, 7-celled structure at maturity
- Arrangement is 3+2+3 2006
Egg Apparatus
At micropylar end, consists of one egg cell and two synergids 2022 Re
Filiform Apparatus
Special cellular thickenings at micropylar tip of synergids
Key Function: Recognises suitable pollen and guides entry of pollen tube into synergid 2015, 2014, 2011, 2008
Pollination
Transfer of pollen grains to stigma. Categorized into three types:
Types of Pollination
- Autogamy: Pollination within same flower
Crucial detail: Cleistogamous flowers do not open at all; invariably autogamous and assure seed set even in absence of pollinators, but lack genetic variability NEET 2013, 2015, 2022
- Geitonogamy: Transfer of pollen to another flower of same plant. Functionally cross-pollination (requires agent), but genetically similar to autogamy 2015, 2014, 2023-Manipur, 1994
- Xenogamy: Transfer of pollen to different plant. Only type that brings genetically different pollen types to stigma 2021
Outbreeding Devices
- Monoecious plants (e.g., castor, maize) prevent autogamy but NOT geitonogamy 2012
- Dioecious plants (e.g., papaya) prevent both autogamy and geitonogamy NEET 2017, 2012, 2008
Artificial Hybridisation
- Emasculation (removal of anthers) done in bisexual flowers at bud stage
- Crucial exception: Female parent producing unisexual flowers does NOT require emasculation, just bagging before flower opens 2022 Re
Pollination Agents
| Agent | Characteristics | High-Yield Examples & Features |
|---|---|---|
| Wind (Anemophily) | Small, colourless, nectarless flowers. Single ovule per ovary, packed into inflorescence. Abundant, light, non-sticky pollen | Grasses 2011. Corn cob tassels represent stigma and style that wave in wind to trap pollen 2023 |
| Water (Hydrophily) | Pollen grains often long, ribbon-like, protected from wetting by mucilaginous covering 2024 Re | Vallisneria: Pollen carried passively by water currents to surface Odisha 2019, 2024 Re. Zostera (Seagrass) |
| Animals (Zoophily) | Large, colourful, fragrant flowers. Offer nectar and pollen as floral rewards 2015 | Exception: Flowers pollinated by flies and bats secrete foul odours to attract them 2015. Water lily and water hyacinth are aquatic but pollinated by insects or wind NEET 2020, NEET-II 2016 |
Double Fertilisation
Mechanism: Phenomenon unique to Angiosperms NEET 2017, 1993. Discovered by Nawaschin and Guignard (1999)
Pollen tube enters ovule through micropyle and liberates two male gametes into one of synergids 2002
Step 1 - Syngamy (True Fertilisation)
One male gamete fuses with egg cell nucleus to form diploid zygote (2n) NEET 2019, 2018
Step 2 - Triple Fusion
Second male gamete moves to central cell and fuses with two polar nuclei (or secondary nucleus) to produce triploid Primary Endosperm Nucleus (PEN) (3n) NEET 2019, 2018
Primary Role: Main role of double fertilisation is to produce endosperm to nourish embryo 1996
Post-Fertilisation: Structures and Events
Endosperm and Embryo
Endosperm
Develops from PEN and precedes embryo development to ensure constant nutrient supply
High-yield example: Coconut water from tender coconut is free-nuclear endosperm (thousands of nuclei), surrounding white kernel is cellular endosperm NEET-I 2016, 2015
Embryo
Develops at micropylar end from zygote
- Monocot Embryo: Possesses only one, large, shield-shaped cotyledon called scutellum, situated laterally 2016, 2006, 2015
- Shoot apex and leaf primordia enclosed in hollow foliar structure called coleoptile 2017
- Radicle enclosed in coleorhiza
Seed and Fruit
Seed
Fertilised, mature ovule
- Non-albuminous (ex-albuminous) seeds: No residual endosperm as completely consumed during embryo development (e.g., Pea, groundnut, gram) 2014, 2020
- Albuminous seeds: Retain part of endosperm (e.g., Wheat, maize, castor) Karnataka NEET 2013
- Perisperm: Persistent, residual nucellus found in some seeds (e.g., Black pepper, beet) NEET 2019, 2013, 2022 Re, 1989
- Micropyle remains as small pore in seed coat to facilitate entry of oxygen and water during germination 1988
Fruit Types
| Fruit Type | Definition | Key Examples |
|---|---|---|
| True Fruit | Develops strictly from ovary 2019 | Mango, Coconut |
| False Fruit | Thalamus contributes to fruit formation 2020, 2022 | Apple, Strawberry, Cashew 2015 Re, 2022 |
| Parthenocarpic Fruit | Fruit develops without fertilisation; seedless | Banana 2015 Re |
Apomixis and Polyembryony
Apomixis
Form of asexual reproduction that mimics sexual reproduction, resulting in formation of seeds without fertilisation NEET-I 2016
Agricultural benefit: Apomictic genes in hybrid crops prevent segregation of desired characters in progeny. Farmers can keep using seeds to raise new crops year after year without buying new hybrid seeds 2022 Re
Polyembryony
Occurrence of more than one embryo in a seed
Adventive Embryony
Nucellar cells or integuments (maternal sporophytic tissue) surrounding embryo sac start dividing, protrude into embryo sac, and develop into multiple embryos
High-yield examples: Citrus and Mango 2011, 2010, 2005, 2001