Flowering plants (Angiosperms) show a complex and fascinating method of sexual reproduction. This chapter is a "score booster" for NEET aspirants, usually contributing 12-16 marks in the Biology section.
Pre-Fertilization: Structure and Events
Before fertilization occurs, several structural changes take place in the floral primordium to form male and female reproductive organs.
Stamen, Microsporangium, and Pollen Grain
The Stamen: Consists of a long stalk (filament) and a terminal bilobed structure called the Anther.
Anther Structure: A typical angiosperm anther is bilobed and dithecous.
Wall Layers: The microsporangium is surrounded by four layers: Epidermis, Endothecium, Middle layers, and Tapetum (the innermost layer that provides nutrition).
Sporopollenin: The outer layer of pollen (exine) is made of this highly resistant organic material.
The Pistil, Megasporangium (Ovule), and Embryo Sac
Gynoecium: Represents the female reproductive part. It can be monocarpellary or multicarpellary.
The Ovule: Attached to the placenta by a funicle. The point of attachment is the hilum.
Female Gametophyte: A mature embryo sac in angiosperms is 7-celled and 8-nucleate.
Pollination: Agents and Types
Pollination is the transfer of pollen grains from the anther to the stigma.
Autogamy, Geitonogamy, and Xenogamy
Autogamy: Transfer within the same flower.
Geitonogamy: Transfer between different flowers of the same plant (genetically similar to autogamy).
Xenogamy: Transfer between flowers of two different plants (brings genetic variation).
Biotic and Abiotic Agents
Wind (Anemophily): Pollen is light and non-sticky. Stigmas are feathery.
Water (Hydrophily): Rare, seen in plants like Vallisneria and Zostera.
Insects (Entomophily): Flowers are colorful, fragrant, and rich in nectar.
Double Fertilization: The Core Event
Double fertilization is a unique characteristic found only in Angiosperms.
Syngamy and Triple Fusion
Syngamy: One male gamete fuses with the egg cell to form a diploid Zygote ($2n$).
Triple Fusion: The second male gamete fuses with two polar nuclei to form a triploid Primary Endosperm Nucleus ($3n$).
Post-Fertilization: Structures and Functions
Following double fertilization, the ovule develops into a seed and the ovary matures into a fruit.
Endosperm and Embryo Development
Endosperm: Develops before the embryo to provide assured nutrition. It can be nuclear (Coconut water) or cellular (White kernel).
Dicot vs Monocots: Dicot embryos have two cotyledons, while monocots (like Grass) have one called Scutellum.
Seeds and Fruits
Perisperm: Remnants of nucellus (e.g., Black pepper, Beet).
Dormancy: A state of inactivity in seeds during unfavorable conditions.
False Fruits: When parts like the thalamus contribute to fruit formation (e.g., Apple, Strawberry).
Special Modes: Apomixis and Polyembryony
Apomixis: Production of seeds without fertilization (mimics sexual reproduction).
Polyembryony: Occurrence of more than one embryo in a seed (e.g., Citrus, Mango).
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