Stamens, Carpels and Placentation

The stamen is a pollen-producing floral organ

The androecium is the collective name for a flower's stamens. A typical stamen consists of a slender supporting filament and an anther. Many anthers are bilobed and each lobe commonly contains two pollen sacs, giving a total of four microsporangia in the familiar anther plan. Pollen grains develop in these sacs. A staminode is a sterile stamen; its existence shows why simply counting stamen-like structures need not reveal how many are functional pollen producers.

Filaments hold anthers at positions where pollen can be released or transferred. Stamens may stand free, in which case they are called polyandrous. They may instead join one another in one or more bundles. When all form one bundle the condition is monadelphous, as in china rose; two bundles are diadelphous, as in pea; more than two bundles are polyadelphous, as in citrus. The terms describe union among stamens, not whether anthers are joined to the corolla.

A separate relationship is adhesion across whorls. Stamens attached to petals are epipetalous, as in brinjal, while stamens attached to perianth segments are epiphyllous, as in lily. Some flowers also have stamens of unequal filament lengths; Salvia and mustard are familiar examples. For a floral description, record both the number and the way stamens are joined or attached. A flower may have five stamens that are all epipetalous without the stamens being fused into one another.

Carpels, ovules and the gynoecium

The gynoecium is the female reproductive region of the flower. It may consist of one or more carpels. In a typical carpel, a swollen ovary houses one or more ovules; above it a style connects to the receptive stigma. When pollen lands on a compatible stigma, pollen-tube growth can bring the male gametes towards the ovules. Following fertilisation, ovules develop into seeds while the ovary commonly develops into a fruit.

When several carpels remain separate, the gynoecium is apocarpous, as in lotus or rose. When carpels are fused into one compound gynoecium, it is syncarpous, as in mustard and tomato. This distinction refers to fusion of carpels, not to the presence or absence of a style. A syncarpous ovary can have one or several chambers, and it may contain numerous ovules.

Placentation: where the ovules attach

The placenta is the internal ovule-bearing tissue of the ovary. Placentation classifies the arrangement of ovules in relation to that tissue. The five principal patterns shown here differ in whether ovules attach at the ventral margin, on a central axis with septa, on the inner wall, on a free central axis without septa, or near the base. The word placenta is used in botany for ovule attachment tissue; it should not be confused with the temporary maternal-fetal organ in mammals.

In marginal placentation, ovules are borne along a ridge corresponding to the ventral suture of a single carpel, commonly in two rows, as in pea. In axile placentation, the placenta lies on a central axis and ovules occupy chambers separated by septa; tomato, china rose and lemon are standard examples. In parietal placentation, ovules develop on the inner ovary wall; mustard and Argemone provide examples, and a false septum may develop in some cases.

In free-central placentation, the ovules are attached to a central column but the ovary lacks internal septa dividing it into chambers, as in Dianthus and primrose. In basal placentation, a single ovule may arise from the base of the ovary, as in sunflower and marigold. The term basal is best judged from a longitudinal view; a single horizontal section can be misleading because the very bottom of the ovary may be missed.

How to identify the pattern rather than memorise a silhouette

Begin by locating the ovary wall and counting visible chambers. If ovules arise at the outer wall, think parietal; if they attach at a common central point and septa form separate chambers, think axile. A central column without septa indicates free-central. A single row or two rows along one carpel margin indicate marginal. If an ovule is anchored to the floor of the ovary in a longitudinal section, consider basal. A reliable identification must refer to attachment, not simply to where the ovule appears in one drawing.

Quick Quiz

Model Answers

1. A stamen has a filament supporting a usually lobed anther bearing pollen sacs. A carpel has stigma, style and ovary, with ovules attached inside the ovary.

2. Cohesion joins stamens to one another in bundles, described as mono-, di- or polyadelphous. Adhesion attaches stamens to petals (epipetalous) or to the perianth (epiphyllous).

3. Axile: central placenta with chambers and septa, tomato. Parietal: ovules at inner ovary wall, mustard. Free-central: central ovules without septa, Dianthus.

4. Apocarpous gynoecia retain free carpels (lotus, rose); syncarpous gynoecia have fused carpels (tomato, mustard).

5. A longitudinal ovary section shows the ovule arising from the floor or base; a transverse section taken higher up could omit that attachment.