SEXUAL REPRODUCTION IN FLOWERING PLANTS

Flowers are morphological and embryological marvels; sites of sexual reproduction.
• Hormonal & structural changes → development of floral primordium → inflorescences → floral buds → flowers.
• A flower is a modified shoot meant for reproduction.
• Androecium (stamens) = male reproductive organ; Gynoecium (pistils) = female reproductive organ.

A    STAMEN — TWO PARTS

*Number & length of stamens are variable across species.*

B    ANGIOSPERM ANTHER — KEY CHARACTERS

• A longitudinal groove runs lengthwise separating the thecae.
• Microsporangia develop further and become pollen sacs; they extend longitudinally through the entire length of the anther and are packed with pollen grains.

TAPETUM — HIGH-YIELD DETAILS ⭐⭐

• Innermost wall layer of microsporangium
• Cells have dense cytoplasm
• Generally more than one nucleus (multinucleate — can become binucleate through endomitosis) ⭐
• Nutritive function ONLY — does NOT help in dehiscence ⭐

SPOROGENOUS TISSUE ⭐

• A group of compactly arranged homogenous cells occupying the CENTRE of each microsporangium.
• Diploid (2n) in nature.
• Each cell of sporogenous tissue is a potential Pollen Mother Cell (PMC) / Microspore Mother Cell.

A    MICROSPOROGENESIS

Process of Microsporogenesis ⭐⭐

• Inside each microsporangium → several thousands of pollen grains are formed.
• Released with dehiscence of anther.

B    POLLEN GRAIN — BASIC FEATURES

C    POLLEN WALL — TWO LAYERS

• Cytoplasm of pollen grain is surrounded by a plasma membrane (inside intine).
• GERM PORE FUNCTION ⭐: Pollen tube emerges through the germ pore during germination.

• In 2-celled pollen: generative cell divides by MITOSIS inside the pollen tube (during growth through stigma/style) → 2 male gametes
• In 3-celled pollen: generative cell divides by MITOSIS before pollen is shed → pollen tube carries 2 male gametes from the beginning
• A mature male gametophyte produces two sperms (male gametes) and a vegetative cell.

A    POLLEN ALLERGY

• Can cause severe allergies, bronchial afflictions → chronic respiratory disorders: Asthma, Bronchitis
• ⭐ KEY: Parthenium (Carrot grass) — came to India as contaminant with imported wheat; ubiquitous in occurrence; major cause of pollen allergy.

B    POLLEN AS FOOD

• Rich in nutrients
• Used as pollen tablets / syrups / food supplements (especially in western countries)
• Claimed to enhance performance of athletes & race horses.

C    POLLEN VIABILITY

• Viability depends on prevailing temperature & humidity.
• Stored pollen can be used as pollen banks (similar to seed banks) in crop breeding programmes.
• Pollen grains must land on stigma before losing viability for fertilisation to occur.

• Gynoecium = female reproductive part of flower
• May consist of a single pistil (monocarpellary) or more than one pistil (multicarpellary)
• Multicarpellary pistils may be fused (syncarpous; e.g., Papaver) or free (apocarpous; e.g., Michelia) ⭐⭐

THREE PARTS (TOP → BOTTOM) ⭐

• Placenta is located inside the ovarian cavity.
• Arising from the placenta are the megasporangia, commonly called ovules.

NUMBER OF OVULES IN OVARY (frequently asked) ⭐⭐

• An ovule generally has a single embryo sac formed from a megaspore.
• ⭐ KEY: Ovule = Megasporangium (they are equivalent).

OVULE — Structural Arrangement (tip → base) ⭐⭐

KEY DETAILS ABOUT MMC ⭐⭐

• Single MMC differentiates in the micropylar region of nucellus
• It is a large cell with dense cytoplasm and prominent nucleus
• MMC differentiates from the archesporium (a cell of the archesporium → MMC)

Process of Megasporogenesis ⭐⭐

• In majority of flowering plants — only ONE megaspore is functional; other three degenerate.
• ⭐ KEY: This method of embryo sac formation from a single megaspore is called monosporic development.

PLOIDY CHECK (frequently asked) ⭐⭐⭐

Development Steps ⭐⭐⭐

CELL DISTRIBUTION IN MATURE EMBRYO SAC ⭐⭐⭐

THREE TYPES — COMPARISON ⭐⭐⭐

• AUTOGAMY — ADDITIONAL DETAILS: In normal open flowers, complete autogamy is rather rare. Requires: (1) Synchrony in pollen release & stigma receptivity, and (2) Anthers & stigma should lie close to each other.

CHASMOGAMOUS vs CLEISTOGAMOUS FLOWERS ⭐⭐⭐

*Plants like Viola, Oxalis, Commelina produce BOTH types of flowers.*

A    WIND POLLINATION (ANEMOPHILY)

• ⭐ Corn cob: the 'ears/silk' you see are the stigma and style waving in the wind to trap pollen grains.

B    WATER POLLINATION (HYDROPHILY)

VALLISNERIA — SPECIFIC MECHANISM ⭐⭐

• Female flowers reach the surface of water by a long stalk.
• Male flowers / pollen grains are released onto the water surface.
• Carried passively by water currents → some reach female flowers.

SUBMERGED POLLINATION (SEA-GRASSES) ⭐⭐

• Female flowers remain submerged in water.
• Pollen released inside water; long, ribbon-like; carried passively.

C    ANIMAL POLLINATION (ZOOPHILY)

Pollinators ⭐

Adaptations of Insect/Animal-Pollinated Flowers ⭐⭐

• Floral Rewards: Nectar (usual reward), Pollen grains (usual reward), Safe places to lay eggs (e.g., Amorphophallus).
• ⭐ ROBBERS: Many insects consume pollen or nectar without bringing about pollination — such visitors are called pollen/nectar robbers.

Special Facts ⭐⭐⭐

MONOECIOUS vs DIOECIOUS ⭐⭐⭐

RECOGNITION & RESPONSE ⭐⭐

• Pollination does NOT guarantee transfer of right type of pollen.
• Recognition involves a continuous dialogue between pollen & pistil, mediated by chemical components of both.
• This knowledge helps plant breeders manipulate pollen-pistil interaction even in incompatible pollinations to get desired hybrids.

EVENTS OF POLLEN-PISTIL INTERACTION (Sequence) ⭐⭐⭐

LAB TECHNIQUE — POLLEN GERMINATION ⭐

• Dust pollen on ~10% sugar solution on a glass slide
• Observe after 15–30 minutes under low power lens of microscope
• Pollen tubes coming out of pollen grains can be seen

Major approach in crop improvement programmes — to combine desirable characters for commercially superior varieties.

Steps of Artificial Hybridisation ⭐⭐

Double Fertilisation — Events ⭐⭐⭐

• Central cell after triple fusion becomes the Primary Endosperm Cell (PEC) → develops into endosperm.

Following double fertilisation → endosperm development, embryo development, ovule → seed, ovary → fruit = collectively called post-fertilisation events.

COCONUT — CLASSIC EXAMPLE ⭐⭐⭐

ENDOSPERMIC vs NON-ENDOSPERMIC SEEDS ⭐⭐⭐

• ⭐ KEY: Zygote divides ONLY AFTER a certain amount of endosperm is formed → adaptation to ensure assured nutrition for developing embryo.
• Embryo develops at the micropylar end of the embryo sac (where zygote is situated).
• Early stages of embryogeny are similar in both monocots & dicots.

STAGES OF EMBRYOGENY (DICOT) ⭐⭐

DICOT EMBRYO — STRUCTURE ⭐⭐

*Dicot embryo = Embryonal axis + Two cotyledons*

MONOCOT EMBRYO (GRASS FAMILY) — STRUCTURE ⭐⭐⭐

KEY STRUCTURES ⭐⭐

SEED MATURATION ⭐

• Water content reduces → seed becomes relatively dry (10–15% moisture by mass).
• General metabolic activity of embryo slows down.
• Embryo may enter a state of inactivity called dormancy.
• If favourable conditions (adequate moisture, oxygen, suitable temperature) are available → germination.
• Simultaneous Transformation: Ovules → Seeds and Ovary → Fruit proceed simultaneously.

IMPORTANCE OF SEEDS ⭐⭐

• Seed formation is more dependable — pollination & fertilisation are independent of water.
• Better dispersal strategies to new habitats → colonisation.
• Sufficient food reserves → nourish seedlings until capable of photosynthesis.
• Hard seed coat → protection to young embryo.
• Products of sexual reproduction → new genetic combinations → variations.
• Seed = basis of agriculture — dehydration & dormancy crucial for storage (food throughout year + raise crop next season).

• ⭐ Parthenocarpy can be induced through application of growth hormones → such fruits are seedless.

RELATIONSHIP: NUMBER OF OVULES & SEEDS ⭐

• The number of seeds in a fruit is related to the number of ovules in the ovary.
• Orchid fruits — contain thousands of tiny seeds.
• Parasitic species Orobanche and Striga — also produce very large number of seeds.

• Some seeds lose viability within a few months; many live for several years; some for hundreds of years.

RECORD-BREAKING DORMANCY ⭐⭐⭐

*The Lupinus seed germinated and flowered after 10,000 years of dormancy!*

MECHANISMS OF APOMIXIS ⭐⭐

• In these species, nucellar cells surrounding the embryo sac develop into additional embryos → each ovule contains many embryos.

*(Rarely asked in NEET, but high-yield historical enrichment)*