Flower Structure Stamen

Introduction: The Continuity of Life

Biology is, at heart, the story of life on Earth — and no property of living systems is more essential than the ability to make more of themselves. Sexual reproduction, with its elegant shuffling of genes between two parents, is how most complex organisms keep their species going across generations. In flowering plants (angiosperms), this entire drama unfolds inside a single, astonishingly compact structure: the flower.

A flower is not just a pretty thing pinned in a lapel. It is a highly modified shoot whose leaf-like parts have been repurposed into the reproductive organs of the plant. Every rose, every dandelion, every stalk of wheat is performing the same fundamental task — produce gametes, bring them together, and package the result as a seed.

Pre-Fertilisation: Structures and Events

Before fertilisation can occur, the plant must build its reproductive parts and produce the gametes. These events — flower differentiation, formation of pollen, development of the embryo sac, pollination and pollen-pistil interaction — are grouped together as the pre-fertilisation phase.

1.1 The Flower — a Fascinating Organ

A typical angiosperm flower is arranged in four concentric whorls borne on the swollen tip of a stalk called the thalamus (or receptacle). From the outside inward these whorls are:

• Calyx — the outermost whorl of sepals — protects the bud.
• Corolla — the whorl of petals — usually bright and fragrant, attracts pollinators.
• Androecium — the male whorl made up of stamens.
• Gynoecium — the innermost female whorl made up of one or more carpels (also called pistils).

The calyx and corolla are accessory whorls; the androecium and gynoecium are the truly reproductive whorls — the stars of the show.

Bisexual and Unisexual Flowers

When a flower carries both stamens and carpels it is bisexual (e.g., hibiscus, mustard). If only one sex is present, it is unisexual — specifically staminate (only male parts, e.g., male flowers of cucurbits) or pistillate (only female parts, e.g., female flowers of maize where the "silks" are stigmas).

1.2 Pre-Fertilisation: Structures and Events

We now zoom into the two reproductive whorls and follow how the male and female gametes are made. This section deals with stamen and pollen; Part 2 continues with the pistil and embryo sac.

1.2.1 Stamen, Microsporangium and Pollen Grain

A stamen has two parts: a long, slender filament and a terminal, bilobed anther. The filament is attached to the thalamus (or sometimes to a petal, in which case the stamen is called epipetalous, as in brinjal).

Structure of the Anther

A typical angiosperm anther is dithecous — it has two theca (lobes) separated by a longitudinal groove. Each theca houses two microsporangia, so a single anther has four microsporangia arranged at its corners. An anther is therefore described as tetralocular.

Wall Layers of a Microsporangium

In cross-section, a young microsporangium shows four protective wall layers surrounding a central mass of sporogenous tissue:

1. Epidermis — the outermost layer.
2. Endothecium — a layer of cells with unevenly thickened walls that help the mature anther split open (dehiscence).
3. Middle layers — usually one to three layers of thin-walled cells.
4. Tapetum — the innermost layer; its cells have dense cytoplasm and often more than one nucleus. The tapetum nourishes the developing pollen.

Microsporogenesis

As the anther matures, the diploid cells of the sporogenous tissue become microspore mother cells (MMCs), also known as pollen mother cells. Each MMC undergoes meiosis — the reduction division — and produces a cluster of four haploid cells, the microspore tetrad. This process of producing microspores from an MMC by meiosis is called microsporogenesis.

As the anther dries and matures, the microspores dissociate and develop into pollen grains. A mature anther carries thousands of pollen grains; when it splits open they are released into the air.

Structure of a Pollen Grain

Pollen grains are typically spherical and about 25–50 µm across. Their wall has two distinct layers:

• Exine — the outer, tough layer made of sporopollenin, one of the most resistant organic materials known. Germ pores are small thin areas where sporopollenin is absent — the pollen tube emerges through these.
• Intine — the inner, thin and continuous layer of cellulose and pectin.

Inside, the cytoplasm is surrounded by a plasma membrane. At maturity the pollen is usually 2-celled, containing:

• A large vegetative cell with abundant reserve food and a big, irregular nucleus.
• A small generative cell that floats within the vegetative cell's cytoplasm. It is spindle-shaped and carries a dense nucleus.

In roughly 60% of angiosperms, pollen is shed at this 2-celled stage. In the remaining 40% (including members of the grass family), the generative cell divides mitotically before shedding, producing two male gametes — so the pollen is 3-celled when released. In 2-celled pollens this division happens later, inside the pollen tube.

Stages of Pollen Development

Pollen, People and Products

Pollen allergy: Airborne pollen can trigger severe allergies and respiratory problems. A notorious culprit in India is Parthenium (carrot grass) — an invasive weed whose pollen causes hay fever, dermatitis and asthma in many people.

Pollen as food: Pollen grains are rich in nutrients — they contain protein, starch, vitamins and minerals. Bee pollen (pellets collected by honeybees) and pollen tablets are sold worldwide as health supplements. Some athletes use pollen supplements to boost performance.

Pollen banks: Like seed banks for zygotes, pollen banks store pollen under cryopreservation (liquid nitrogen, –196 °C). This keeps crop pollen viable for years and is invaluable for plant breeders who want to cross varieties flowering in different seasons or continents.