The stamen (plural stamina or stamens, from Latin stamen meaning "thread of the warp") is the pollen-producing reproductive organ of a flower. Stamens typically consist of a stalk called the filament (from Latin filum, meaning "thread"), and an anther (from Ancient Greek anthera, feminine of antheros "flowery," from anthos "flower"), which contains microsporangia. Anthers are most commonly two-lobed and are attached to the filament either at the base or in the middle portion. The sterile tissue between the lobes is called the connective.
A typical anther contains four microsporangia. The microsporangia form sacs or pockets (locules) in the anther. The two separate locules on each side of an anther may fuse into a single locule. Each microsporangium is lined with a nutritive tissue layer called the tapetum and initially contains diploid pollen mother cells. These undergo meiosis to form haploid spores. The spores may remain attached to each other in a tetrad or separate after meiosis. Each microspore then divides mitotically to form an immature microgametophyte called a pollen grain.
The pollen is eventually released when the anther forms openings (dehiscence). These may consist of longitudinal slits, pores, as in the heath family (Ericaceae), or by valves, as in the barberry family (Berberidaceae). In some plants, notably members of Orchidaceae and Asclepiadoideae, the pollen remains in masses called pollinia, which are adapted to attach to particular pollinating agents such as birds or insects. More commonly, mature pollen grains separate and are dispensed by wind or water, pollinating insects, birds or other pollination vectors.
Pollen of angiosperms must be transported to the stigma, the receptive surface of the carpel, of a compatible flower, for successful pollination to occur. After arriving, the pollen grain (an immature microgametophyte) typically completes its development. It may grow a pollen tube and undergoing mitosis to produce two sperm nuclei.
The stamens in a flower are collectively called the androecium (from Greek andros oikia: man's house). The androecium forms a whorl surrounding the gynoecium (carpels) and inside the perianth (the petals and sepals together) if there is one. (The one exception is a few members of the family Triuridaceae, particularly Lacandonia schismatica, in which the gynoecium surrounds the androecium.)
Variation in stamen morphology
Stamens can be free or fused to one another in many different ways, including fusion of some but not all stamens. The filaments may be fused and the anthers free, or the filaments free and the anthers fused. Rather than including two locules, one of the locules may fail to develop, or alternatively the two locules may merge late in development to give a single locule. Extreme cases of stamen fusion occur in some species of Cyclanthera (of family Cucurbitaceae) and in section Cyclanthera of genus Phyllanthus (family Euphorbiaceae) where the stamens form a ring around the gynoecium, with a single locule.
- A column formed from the fusion of multiple filaments is known as an androphore.
The anther can be attached to the filament in two ways:
- basifixed: attached at its base to the filament
- dorsifixed: attached at its center to the filament, usually versatile (able to move)
Stamens can be connate (fused or joined in the same whorl):
- monadelphous: fused into a single, compound structure
- diadelphous: joined partially into two androecial structures
- pentadelphous: joined partially into five androecial structures
- synandrous: only the anthers are connate (such as in the Asteraceae)
Stamens can also be adnate (fused or joined from more than one whorl):
- epipetalous: adnate to the corolla
They can be different lengths:
- didynamous: occurring in two pairs of different length
- tetradynamous: occurring as a set of six filaments with two shorter ones
- exserted: extending beyond the corolla
- included: not extending beyond the corolla
Sexual reproduction in plants
In the typical flower (that is, in the majority of flowering plant species) each flower has both carpels and stamens. In some species, however, the flowers are unisexual with only carpels or stamens. (monoecious = both types of flowers found on the same plant; dioecious = the two types of flower found only on different plants). A flower with only stamens is called androecious. A flower with only carpels is called gynoecious.
A flower having only functional stamens and lacking functional carpels is called a staminate flower, or (inaccurately) male. A plant with only functional carpels is called pistillate, or (inaccurately) female.
An abortive or rudimentary stamen is called a staminodium or staminode, such as in Scrophularia nodosa.
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Insects collecting nectar unintentionally transfer pollen to other flowers, causing pollination
Flower of the spider tree (Crateva religiosa) with its numerous conspicuous stamens
Flowers of wheat at anthesis showing stamens.
Stamens of a daylily (Hemerocallis), thickly covered in pollen
Flowers of the "silk tree" (Albizia julibrissin) have many long thread-like stamens
Stamens of a passion flower (Passiflora caerulea) showing interesting symmetry
Hylocereus undatus showing the style and stigma, and stamens
These Solanum anthers release pollen through a pore at their tip
Blue and red waterlily stamens deploying in the morning sun (Nymphaea caerulea)