Pre-fertilisation: Structures and Events

Setting the Stage for Reproduction

Long before a flower becomes visible on a plant, the decision to flower has already been made. Hormonal signals trigger a cascade of structural changes deep within the plant’s tissues. Specialised regions called floral primordia (the earliest recognisable stages of flower development) begin to differentiate. These primordia develop into inflorescences (clusters of flowers on a stem), which in turn bear floral buds that eventually open into fully formed flowers.

Once a flower is ready, its two reproductive whorls take centre stage. The androecium (the male reproductive whorl, made up of stamens) and the gynoecium (the female reproductive whorl, made up of one or more pistils/carpels) must each build their respective structures and produce gametes before fertilisation can happen.

Let us trace this journey step by step, starting with the male side.

The Male Side: Stamen, Microsporangium, and Pollen Grain

Stamen Structure

Each stamen (the individual unit of the androecium) has two parts:

Fig 1.2: (a) A typical stamen; (b) Three-dimensional cut section of an anther

Filament — a long, slender stalk whose lower end attaches to the thalamus (the receptacle at the base of the flower) or to a petal
Anther — the terminal, typically bilobed (two-lobed) structure sitting at the top of the filament

The number and length of stamens vary widely from species to species. If you were to collect one stamen from each of ten different flowering species and line them up, the range in size, shape, and how the anther attaches to the filament would be striking.

Inside the Anther: Microsporangia

A typical angiosperm anther is bilobed, and each lobe contains two theca (pollen-producing compartments). Because there are two theca per lobe, the anther is said to be dithecous (having two theca per lobe). A lengthwise groove on the outside often marks the boundary between the two theca. Altogether, the anther has four microsporangia (the structures where microspores develop), one at each corner of its roughly four-sided (tetragonal) cross-section.

Fig 1.3: (a) Transverse section of a young anther; (b) Enlarged view of one microsporangium showing wall layers; (c) A mature dehisced anther

As the anther matures, each microsporangium becomes a pollen sac packed with pollen grains. These pollen sacs run the full length of the anther.

Microsporangium Wall Layers

If you look at a cross-section through a young microsporangium, it appears roughly circular in outline and is wrapped by four distinct wall layers, from outside to inside:

Layer	Position	Main role
Epidermis	Outermost	Protection
Endothecium	Below epidermis	Helps the anther split open (dehisce) to release pollen
Middle layers	Between endothecium and tapetum	Protection and structural support
Tapetum	Innermost	Nourishes the developing pollen grains

The outer three layers mainly protect the developing pollen and assist in dehiscence. The tapetum deserves special attention: its cells are packed with dense cytoplasm and frequently become binucleate (containing two nuclei) or even multinucleate. How does a tapetal cell end up with two nuclei? The nucleus divides by mitosis, but the cell itself does not split into two daughter cells, so both nuclei remain together in a single enlarged cell. This extra nuclear material helps the tapetum meet the heavy nutritional demands of thousands of developing pollen grains.

At the centre of each young microsporangium sits the sporogenous tissue, a compact group of similar-looking cells that will eventually give rise to pollen.

Microsporogenesis: From Sporogenous Tissue to Microspore Tetrads

As the anther develops, each cell of the sporogenous tissue has the potential to become a microspore mother cell (also called a pollen mother cell, or PMC). Every PMC undergoes meiosis (the type of cell division that halves the chromosome number), producing a cluster of four haploid cells called a microspore tetrad. The entire process of forming microspores from a PMC through meiosis is called microsporogenesis (micro = small; sporo = spore; genesis = origin).

Since the PMC is diploid ( $2n$ ) and meiosis halves the chromosome number, each microspore in the tetrad is haploid ( $n$ ).

As the anther matures and starts to dry out, the four microspores in each tetrad separate from one another and individually develop into pollen grains. A single microsporangium can produce several thousand pollen grains, all of which are eventually released when the anther dehisces (splits open along a line of weakness).

Pollen Grain: The Male Gametophyte

Each pollen grain is a tiny, self-contained male gametophyte (the gamete-producing generation in the plant’s life cycle). If you dust pollen from an open Hibiscus flower onto a drop of water on a glass slide, a microscope reveals an astonishing variety of shapes, sizes, colours, and surface patterns across different species.

Fig 1.4: Scanning electron micrographs of a few pollen grains

Size and shape: Most pollen grains are roughly spherical, measuring about 25 to 50 micrometres in diameter.

The two-layered wall:

A pollen grain has a prominent wall with two distinct layers:

Exine (outer wall) — made of sporopollenin, one of the toughest organic substances found in nature. It can survive high temperatures, concentrated acids, and strong alkalis. No enzyme discovered so far is able to break it down. This extraordinary toughness is the reason pollen grains survive as fossils for millions of years. The exine surface displays intricate patterns and ridges that are unique to each species. At certain spots on the exine, sporopollenin is absent, creating thin areas called germ pores (the openings through which the pollen tube will later emerge during germination).
Intine (inner wall) — a thinner, continuous layer composed of cellulose (a structural carbohydrate) and pectin (a gel-like polysaccharide). Unlike the exine, the intine has no gaps or pores.

Cells inside a mature pollen grain:

Fig 1.5: (a) Enlarged view of a pollen grain tetrad; (b) Stages of a microspore maturing into a pollen grain

Beneath the plasma membrane, the pollen grain’s cytoplasm contains two cells at maturity:

Vegetative cell — the larger of the two, with abundant food reserves and a large, irregularly shaped nucleus. This cell will later grow the pollen tube.
Generative cell — much smaller, spindle-shaped, with dense cytoplasm and its own nucleus. It floats within the cytoplasm of the vegetative cell. This cell will eventually divide to produce the two male gametes (sperm cells).

In more than 60% of angiosperm species, pollen grains are released from the anther at the 2-celled stage (one vegetative cell + one generative cell). In the remaining species, the generative cell divides by mitosis before the pollen is shed, so these grains leave the anther already at the 3-celled stage (one vegetative cell + two male gametes).

Pollen Allergies, Nutrition, and Storage

Allergies: Pollen grains of many species trigger severe allergic reactions in sensitive people, leading to conditions such as asthma and bronchitis. A well-known example is Parthenium hysterophorus (carrot grass), an invasive weed that arrived in India as a contaminant in imported wheat and has now spread almost everywhere. Its pollen is a major cause of pollen allergy.

Fig 1.6: Pollen products

Nutritional value: Pollen grains are rich in nutrients. In western countries, pollen-based food supplements (tablets, syrups, shakes) are widely marketed. Some claims suggest that pollen consumption can boost the performance of athletes and even race horses, although scientific evidence for these claims varies.

Viability and storage: Once pollen grains are shed from the anther, they must reach a compatible stigma while still alive. How long they remain viable depends heavily on temperature and humidity. In cereals like rice and wheat, viability lasts only about 30 minutes after release. By contrast, some members of the Rosaceae, Leguminosae, and Solanaceae families maintain pollen viability for months. For long-term preservation, pollen grains of many species can be stored in liquid nitrogen (at minus 196 degrees Celsius) for years. These collections, called pollen banks, serve the same purpose as seed banks and are valuable resources for crop breeding programmes.

The Female Side: Pistil, Ovule, and Embryo Sac

Pistil Structure

The gynoecium (the female reproductive whorl) is made up of one or more pistils (also called carpels, the individual leaf-like units that fold and fuse to form the female structure). The gynoecium can be:

Monocarpellary — consisting of a single pistil (one carpel)
Multicarpellary — consisting of more than one pistil. When the multiple carpels are fused together, the condition is syncarpous. When they remain separate and free from one another, it is apocarpous.

Fig 1.7: (a) Pistil of Hibiscus; (b) Syncarpous pistil of Papaver; (c) Apocarpous gynoecium of Michelia; (d) A typical anatropous ovule

Each pistil has three distinct regions:

Stigma — the topmost part, serving as a landing platform for pollen grains. It is often sticky or feathery to help trap pollen.
Style — the elongated, slender middle section connecting the stigma to the ovary
Ovary — the swollen basal part that encloses a hollow space called the ovarian cavity (or locule). Within this cavity, the placenta (the tissue from which ovules arise) is located. Attached to the placenta are the megasporangia, commonly known as ovules.

The number of ovules per ovary varies enormously: wheat, paddy, and mango have just one ovule per ovary, while papaya, watermelon, and orchids can have many.

Ovule (Megasporangium) Structure

Each ovule is a small but intricately built structure:

Funicle — a stalk that connects the ovule to the placenta
Hilum — the point where the body of the ovule merges with the funicle (essentially the “junction” between the two)
Integuments — one or two protective layers that wrap around the ovule, covering everything except a small opening at the tip called the micropyle (the gateway through which the pollen tube will later enter)
Chalaza — the region at the opposite end from the micropyle, representing the base of the ovule
Nucellus — a mass of nutritive cells enclosed within the integuments, rich in reserve food materials
Embryo sac (or female gametophyte) — located within the nucellus. An ovule usually contains a single embryo sac, which develops from a megaspore.

Megasporogenesis: Forming the Megaspores

The process by which megaspores are produced from a megaspore mother cell (MMC) is called megasporogenesis (mega = large; sporo = spore; genesis = origin).

In the micropylar region of the nucellus, a single, large cell with dense cytoplasm and a prominent nucleus differentiates as the MMC. This cell is diploid ( $2n$ ). The MMC undergoes meiosis, producing four haploid ( $n$ ) megaspores arranged in a row.

Fig 1.8: (a) Megaspore mother cell, dyad, and tetrad of megaspores; (b) 2, 4, and 8-nucleate stages of embryo sac development; (c) Mature embryo sac

Why does the MMC undergo meiosis rather than mitosis? Because meiosis reduces the chromosome number from diploid ( $2n$ ) to haploid ( $n$ ). This ensures that when a haploid male gamete eventually fuses with the haploid egg cell during fertilisation, the resulting zygote will be diploid ( $2n$ ), restoring the original chromosome number of the species.

Female Gametophyte (Embryo Sac) Development

Of the four megaspores produced, typically only one survives. The other three degenerate (break down and disappear). The single surviving cell is called the functional megaspore, and it goes on to develop into the female gametophyte (the embryo sac). Because the embryo sac forms from just one megaspore, this pattern is called monosporic development (mono = single; sporic = from a spore).

What is the ploidy at each stage? The nucellus cells and the MMC are diploid ( $2n$ ). After meiosis, each megaspore and the resulting female gametophyte are haploid ( $n$ ).

The functional megaspore builds the embryo sac through three successive rounds of mitosis (nuclear division without chromosome reduction):

First mitosis — the megaspore nucleus divides into two nuclei, which migrate to opposite ends of the cell. This is the 2-nucleate stage.
Second mitosis — each of those two nuclei divides again, giving four nuclei total: two at each pole. This is the 4-nucleate stage.
Third mitosis — all four nuclei divide, producing eight nuclei: four at each pole. This is the 8-nucleate stage.

A crucial detail: these divisions are free nuclear (the nuclei divide, but cell walls do not form immediately after each division). Only after all eight nuclei are present do cell walls finally get laid down, organising the embryo sac into its definitive structure.

Organisation of the Mature Embryo Sac

Once cell walls form, the eight nuclei are distributed into a precise arrangement:

At the micropylar end — the Egg Apparatus (3 cells):

Two synergids — helper cells that sit on either side of the egg cell. At their micropylar tips, they have distinctive thickenings called the filiform apparatus (finger-like projections of cell wall material). The filiform apparatus plays a critical role: it secretes chemical signals that guide the pollen tube toward the correct entry point.
One egg cell — the female gamete, positioned between the two synergids

At the chalazal end (3 cells):

Three antipodals — cells whose exact function is not entirely clear, though they may assist in nourishing the embryo sac

In the middle — the Central Cell (1 large cell):

This single, large cell contains the two polar nuclei (one contributed from each pole during the 8-nucleate stage). The polar nuclei sit in the centre of this cell, ready to participate in a second fusion event during fertilisation.

So the final count: six cells with their own walls (2 synergids + 1 egg + 3 antipodals) plus one central cell containing two free polar nuclei = 7 cells, 8 nuclei. This is the hallmark description of a typical angiosperm embryo sac: 8-nucleate but 7-celled.

Pollination: Bringing Pollen to the Stigma

Why Pollination is Necessary

Both male and female gametes in flowering plants are non-motile (they cannot swim or move on their own). Unlike the sperm cells of mosses or ferns, which swim through water to reach the egg, angiosperm gametes need an outside agent to bring them together. The process that achieves this first critical step, getting pollen grains from the anther to a receptive stigma, is called pollination.

Three Types of Pollination

Depending on where the pollen comes from, pollination falls into three categories:

Fig 1.9: (a) Self-pollinated flowers; (b) Cross-pollinated flowers; (c) Chasmogamous and cleistogamous flowers

1. Autogamy (self-pollination within the same flower)

Pollen moves from the anther to the stigma of the very same flower. For this to work in an open flower, two things must coincide: the pollen must be released at the exact time the stigma is receptive, and the anther must be positioned close enough to the stigma for transfer to happen. True autogamy in open flowers is therefore relatively rare.

Some plants have evolved a clever workaround. Species such as Viola (common pansy), Oxalis, and Commelina produce two kinds of flowers on the same plant:

Chasmogamous flowers (chasmo = opening; gamous = marriage) — normal, open flowers with exposed anthers and stigma, capable of cross-pollination
Cleistogamous flowers (cleisto = closed) — flowers that never open. The anthers and stigma lie in direct contact within the closed bud. When the anthers dehisce, pollen lands directly on the stigma, guaranteeing self-pollination every single time with no possibility of foreign pollen arriving.

Cleistogamous flowers provide assured seed set regardless of whether pollinators are available. However, there is a trade-off: because only self-pollen is used, these flowers produce no genetic variation. Over many generations, this can reduce the adaptability of the population.

2. Geitonogamy (pollination between flowers on the same plant)

Pollen travels from the anther of one flower to the stigma of a different flower, but both flowers are on the same individual plant. This looks like cross-pollination from the outside (a pollinator carries pollen between flowers), but genetically it is the same as self-pollination. The pollen still carries the same genetic makeup as the plant receiving it. So geitonogamy does not introduce any new genetic variation.

3. Xenogamy (true cross-pollination)

Pollen moves from the anther of a flower on one plant to the stigma of a flower on a completely different plant. This is the only form of pollination that brings together genetically different pollen and egg, creating the variation that drives evolution and adaptation.

Agents That Carry Out Pollination

Flowering plants rely on external agents to move pollen. Since pollen landing on a stigma is partly a matter of chance (especially with non-living agents), many plants compensate by producing vastly more pollen than there are ovules waiting to be fertilised.

The agents fall into two broad groups:

Abiotic agents (non-living):

Wind and water. Only a small proportion of flowering plants use these.

Biotic agents (living):

Animals, especially insects. The vast majority of flowering plants depend on animal pollinators.

Wind Pollination

Fig 1.10: A wind-pollinated plant showing compact inflorescence and well-exposed stamens

Wind pollination is the most common form of abiotic pollination. Plants adapted to it share a set of recognisable features:

Light, non-sticky pollen that floats easily on air currents
Well-exposed stamens that dangle freely so the wind can pick up pollen
Large, feathery stigmas that act like nets to trap airborne pollen grains
Abundant pollen production to compensate for the low probability of any single grain hitting a stigma
Simple flowers that are often small and lack bright colours, fragrance, or nectar (since there is no need to attract animals)
Flowers are often packed into compact inflorescences, and each ovary typically contains just a single ovule

A familiar example is the corn cob: the long, silk-like threads hanging from the top of each ear are actually the stigma and style of individual flowers, waving in the wind to catch pollen. Grasses in general are predominantly wind-pollinated.

Water Pollination

Fig 1.11: (a) Pollination in Vallisneria; (b) Pollination by a bee

Water pollination is quite rare among flowering plants, found in only about 30 genera, most of them monocots. This is noteworthy because in lower plant groups (algae, bryophytes, pteridophytes), water is the standard way male gametes reach the egg. In fact, the dependence on water for gamete transport is thought to be one reason these groups have limited geographical distributions.

Important clarification: not all aquatic plants use water for pollination. Many aquatic species, such as water hyacinth and water lily, push their flowers above the water surface and rely on insects or wind, just like land plants.

Two patterns of water pollination:

Surface pollination (e.g., Vallisneria): The female flower is pushed to the water surface on a long, coiled stalk. Male flowers (or free pollen grains) are released onto the surface and drift passively with the current until some reach the female flower.
Submerged pollination (e.g., seagrasses like Zostera): The female flowers stay underwater. Pollen grains, which are often long and ribbon-shaped, are released directly into the water and float passively until they encounter a stigma.

In many water-pollinated species, pollen grains are coated with a mucilaginous (slimy, gel-like) covering that protects them from getting waterlogged.

Like wind-pollinated flowers, water-pollinated flowers tend to be plain in appearance and do not produce nectar. The reason is the same: colour, fragrance, and nectar exist to attract animal pollinators, and since these flowers rely on abiotic agents, there is no selective pressure to develop those features.

Animal Pollination

The majority of flowering plants rely on animals to carry their pollen. The most common animal pollinators include bees, butterflies, flies, beetles, wasps, ants, moths, birds (particularly sunbirds and hummingbirds), and bats. Among all of these, insects, especially bees, are the most important and widespread group of pollinators.

Even some larger animals have been observed acting as pollinators in certain species: primates such as lemurs, tree-dwelling rodents, and reptiles like gecko lizards and garden lizards.

How animal-pollinated flowers attract visitors:

Large, colourful petals that catch the eye (or, when individual flowers are small, they cluster into showy inflorescences)
Fragrance that guides animals from a distance
Nectar (a sugary liquid) and pollen grains themselves serve as the main rewards that keep animals coming back

Some flowers attract flies and beetles specifically by producing foul odours that mimic rotting material.

How pollination happens: When an animal visits a flower to collect nectar or pollen, its body brushes against the anthers and picks up sticky pollen grains. When the same animal visits the next flower, some of those grains rub off onto the stigma, achieving pollination.

Specialised relationships:

Some plant-pollinator partnerships have become so intertwined that neither partner can survive without the other:

Amorphophallus produces one of the tallest flowers in the world (up to 6 feet). A moth lays its eggs inside the ovary of the flower, and while doing so, it pollinates the plant.
The Yucca plant and its moth pollinator represent one of the best-known examples of obligate mutualism. The moth can only lay its eggs inside the Yucca flower, and the Yucca can only be pollinated by that specific moth. The moth larvae hatch and feed on some of the developing seeds, but enough seeds survive to ensure the plant reproduces.

Outbreeding Devices: How Plants Avoid Too Much Self-Pollination

Most flowering plants produce hermaphrodite (bisexual) flowers, meaning pollen and stigma are present in the same flower. Without any safeguards, pollen would routinely land on its own stigma, leading to continuous self-fertilisation. Over generations, this causes inbreeding depression (a decline in the health and vigour of offspring due to reduced genetic diversity).

To counter this, plants have evolved several outbreeding devices:

Timing mismatch (dichogamy) — In some species, pollen is released and stigma becomes receptive at different times. Either the pollen is shed before the stigma is ready, or the stigma matures and loses receptivity before the anthers open. This prevents self-pollination within the same flower.
Spatial separation (herkogamy) — The anther and stigma are positioned at different heights or angles within the flower, making it physically difficult for pollen to reach its own stigma.
Self-incompatibility — A powerful genetic mechanism that blocks self-pollen even if it does reach the stigma. The pistil recognises pollen from the same plant (or the same flower) and actively prevents it from germinating or growing a tube through the style. This stops self-fertilisation at the molecular level.
Unisexual flowers — Some species produce separate male and female flowers. If both appear on the same plant (as in castor and maize), the plant is monoecious (mono = one; oecious = house). This prevents autogamy but not geitonogamy. If male and female flowers are on entirely different plants (as in papaya), each plant is either male or female. This condition is called dioecy (di = two; oecious = house), and it prevents both autogamy and geitonogamy.

Pollen-Pistil Interaction: The Chemical Conversation

Pollination does not guarantee success. Pollen of the wrong species, or self-pollen in a self-incompatible plant, can land on the stigma alongside compatible pollen. The pistil has a built-in ability to distinguish compatible pollen from incompatible pollen through a chemical dialogue.

Fig 1.12: (a) Pollen germinating on stigma; (b) Pollen tubes in the style; (c) Path of pollen tube growth; (d) Pollen tube entering a synergid; (e) Discharge of male gametes

If the pollen is compatible, the pistil promotes a chain of events:

The pollen grain germinates on the stigma, sending out a pollen tube through one of the germ pores.
The contents of the pollen grain (including the vegetative nucleus and either the generative cell or the two pre-formed male gametes) move into the growing tube.
The pollen tube pushes its way through the tissues of the stigma and style, eventually reaching the ovary.
In species where pollen was shed at the 2-celled stage, the generative cell divides inside the tube during this journey, producing two male gametes. In species that shed 3-celled pollen, the two gametes are already present from the start.
The pollen tube enters the ovule through the micropyle, then penetrates one of the synergids via the filiform apparatus.

If the pollen is incompatible, the pistil blocks progress. It may prevent the pollen from germinating on the stigma surface, or it may stop the pollen tube from growing through the style. This rejection is mediated by specific chemical components on the pollen interacting with corresponding molecules on the pistil surface and style tissues.

All of these events, from pollen landing on the stigma through to the pollen tube entering the ovule, are collectively called pollen-pistil interaction. Understanding these chemical recognition systems is of practical importance: plant breeders can sometimes manipulate pollen-pistil interactions to achieve hybridisation even between species that would normally be incompatible.

Artificial Hybridisation: Emasculation and Bagging

Artificial hybridisation is a central technique in crop improvement. The goal is to cross two selected parent plants, combining desirable traits from each into the offspring. For this to work, the breeder must ensure that only the intended pollen reaches the stigma, with no contamination from unwanted pollen.

The technique involves two key steps:

Emasculation — If the female parent has bisexual flowers, the anthers must be removed before they open and release pollen. This is done using fine forceps while the flower is still in the bud stage. Removing the anthers eliminates any chance of self-pollination.
Bagging — The emasculated flower is covered with a bag (usually made of butter paper) to shield the stigma from stray pollen carried by wind or insects. When the stigma becomes receptive, the breeder carefully opens the bag, dusts pollen collected from the chosen male parent onto the stigma, then re-bags the flower. The fruit is then allowed to develop.

If the female parent naturally bears unisexual flowers (only female flowers), emasculation is unnecessary since there are no anthers to remove. In this case, the female flower buds are simply bagged before they open, pollinated at the right time with the desired pollen, and re-bagged.