Chapter 1: Sexual Reproduction in Flowering Plants

Introduction

Sexual reproduction in flowering plants (angiosperms) involves the formation of male and female gametes and their fusion to form a zygote, which develops into an embryo. The flower is the fascinating reproductive organ of angiosperms.

Flower Structure

A typical flower consists of four main whorls arranged on a swollen end of the stalk (receptacle):

1. Calyx: The outermost whorl consisting of sepals.
2. Corolla: The whorl of petals, typically brightly colored to attract pollinators.
3. Androecium: The male reproductive whorl, consisting of stamens.
4. Gynoecium: The female reproductive whorl, consisting of carpels (pistils).

Development of Male Gametophyte

The stamen consists of a long, slender stalk called the filament and a bilobed, pollen-bearing structure called the anther. Inside the anther, microsporangia develop and become pollen sacs containing pollen grains. The process of formation of microspores from a pollen mother cell (PMC) through meiosis is called microsporogenesis.

Pollen grains represent the male gametophytes. They have a tough outer layer (exine) and an inner layer (intine). A mature pollen grain contains two cells: the vegetative cell and the generative cell.

Development of Female Gametophyte

The gynoecium represents the female reproductive part. A single pistil consists of three parts: stigma, style, and ovary. Inside the ovarian cavity, ovules (megasporangia) are present.

The formation of megaspores from the megaspore mother cell (MMC) is called megasporogenesis. Usually, one of the four megaspores remains functional and develops into the female gametophyte (embryo sac), while the other three degenerate. A typical mature angiosperm embryo sac is 8-nucleate and 7-celled.

Pollination

Pollination is the transfer of pollen grains to the stigma of a pistil.

• Autogamy: Transfer of pollen within the same flower.
• Geitonogamy: Transfer of pollen from the anther of one flower to the stigma of another flower on the same plant.
• Xenogamy: Transfer of pollen from the anther to the stigma of a different plant (cross-pollination).

Agents of Pollination

• Abiotic: Wind (anemophily), Water (hydrophily)
• Biotic: Insects (entomophily), Birds (ornithophily), Bats (chiropterophily), etc.

Outbreeding Devices

Plants have developed several mechanisms to discourage self-pollination and encourage cross-pollination, such as non-synchronization in pollen release and stigma receptivity, self-incompatibility, and production of unisexual flowers.

Pollen-Pistil Interaction

It encompasses all the events from pollen deposition on the stigma until the pollen tube enters the ovule. This is a dynamic process involving pollen recognition followed by promotion or inhibition of the pollen.

Double Fertilization

After entering one of the synergids, the pollen tube releases the two male gametes.

1. Syngamy: One male gamete fuses with the egg cell to form the zygote (diploid, \(2n\)).
2. Triple Fusion: The other male gamete fuses with the two polar nuclei in the central cell to produce a Primary Endosperm Nucleus (PEN) (triploid, \(3n\)).

Since two types of fusions take place in an embryo sac, the phenomenon is termed Double Fertilization.

Post-Fertilization Events

Development of Endosperm and Embryo

The PEN develops into the endosperm, which provides nourishment to the developing embryo. The zygote divides to give rise to the embryo (proembryo \(\rightarrow\) globular \(\rightarrow\) heart-shaped \(\rightarrow\) mature embryo).

Development of Seed and Formation of Fruit

The ovules mature into seeds, and the ovary develops into a fruit. The transformation of ovules into seeds and ovary into fruit proceeds simultaneously. The wall of the ovary develops into the fruit wall called pericarp.

Significance of Seed and Fruit Formation

Seed formation guarantees the continuation of the species. Fruit formation protects the seeds and plays a vital role in their dispersal.

Special Modes of Reproduction

• Apomixis: The production of seeds without fertilization. It is a form of asexual reproduction that mimics sexual reproduction.
• Parthenocarpy: The development of fruit without fertilization (e.g., banana). These fruits are seedless.
• Polyembryony: The phenomenon of the occurrence of more than one embryo in a seed (e.g., Citrus, Mango).

Competency Based Questions

Q1. A plant has 24 chromosomes in its microspore mother cell. Calculate the number of chromosomes in its endosperm and syngids, respectively.

(A) 36 and 12
(B) 12 and 12
(C) 36 and 24
(D) 24 and 12

Q2. How many meiotic divisions are required to produce 200 seeds in a typical angiosperm? Explain with mathematical steps.

Q3. If a plant exhibits self-incompatibility, which of the following processes is directly hindered?

(A) Megasporogenesis
(B) Microsporogenesis
(C) Pollen-pistil interaction leading to pollen tube growth
(D) Syngamy of female gametes with male gametes from a different plant

Q4. A farmer observed that his Citrus crop produced seeds that gave rise to multiple seedlings from a single seed. Identify the phenomenon and state its genetic consequence on the offspring.

Q5. The graph below shows the rate of pollen tube growth for two different pollen grains (Pollen A and Pollen B) on the same stigma over time. Based on your understanding of pollen-pistil interaction, what can you infer?

(Assume Pollen A shows continuous rapid growth, while Pollen B germinates but stops growing shortly after).

(A) Pollen A is incompatible, while Pollen B is compatible.
(B) Both Pollen A and Pollen B are from the same plant (self-pollen) in a self-incompatible species.
(C) Pollen A is from a compatible species, while Pollen B is from an incompatible species or rejected due to self-incompatibility.
(D) Both Pollen A and Pollen B lack a vegetative cell.