Stamen
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Insects, while collecting pollen, accidentally transfer it from one flower to another, bringing about pollination |
The
stamen is the male
organ of a
flower. Each stamen generally has a stalk called the
filament, and, on top of the filament, an
anther.
The anther is usually composed of four pollen sacs, which are called
microsporangia. The development of the
microsporangia and the contained
haploid spores (called
pollen-grains) is closely comparable with that of the microsporangia in
gymnosperms or heterosporous
ferns. The pollen is set free by the opening (
dehiscence) of the anther, generally by means of longitudinal slits, but sometimes by pores, as in the
heath family (
Ericaceae), or by valves, as in the
barberry family (
Berberidaceae). It is then dropped, or carried by some external agent — wind, water or some member of the
animal kingdom — onto the receptive surface of the
carpel of the same or another flower, which is thus
pollinated.
Typical flowers have six stamens inside a
perianth (the
petals and
sepals together), arranged in a whorl around the carpel (pistil). But in some species there are many more than six present in a flower (see, for example, the spider tree flower, below). Collectively, the stamens are called an
androecium (from
Greek andros oikia: man's house). They are positioned just below the
gynoecium. The anthers are bilocular, i.e. they have two locules. Each locule contains a microsporangium. The tissue between the locules and the cells is called the
connective.
In an immature, unopened flower
bud, the filaments are still short. Their function is then to transport
nutrients to the developing pollen. They start to lengthen once the bud opens. The anther can be attached to the filament in two ways:
*basifixed : attached at its base to the filament; this gives rise to a
longitudinal dehiscence (opening along its length to release pollen)
*versatile : attached at its center to the filament; pollen is then released through pores (
poricidal dehiscence).
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Scanning electron microscope image of Penta lanceolata anthers, with pollen grains on surface |
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
*
synantherous : 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*
didynamous : occurring in two pairs of different length
*
tetradynamos : occurring as a set of six filaments with two shorter ones
*
exserted : extending beyond the corolla
*
included : not extending from the corolla.
Main article:
Plant sexualityIn the typical flower (that is, the majority of flowering plant species) each flower has both a
pistil and
stamens. Bisexual plants are named
hermaphrodites or perfect flowers.
In some species, however, the flowers are unisexual with only either male or female parts (
monoecious = on the same plant;
dioecious = on different plants). A flower with only male reproductive parts is called
androecious. A flower with only female reproductive parts is called
gynoecious.
A flower having only functional stamens is called a
staminate flower.
An abortive or rudimentary stamen is called a
staminodium, such as in
Scrophularia nodosa.
The pistil and the stamens of
orchids are fused into a
column. The top part of the column is formed by the anther. This is covered by an
anther cap.
Image:Crateva_religiosa.jpg|Flower of the spider tree (Crateva religiosa) with its numerous conspicuous stamensImage:WheatFlower3.JPG|Flowers of wheat at anthesis showing stamens. Like all grasses (Poaceae), wheat is wind-pollinatedImage:Daylily Stamens dry 01v2.jpg|Stamens of a daylily (Hemerocallis), thickly covered in pollenImage:Albizia_julibrissin_'Rosea'_flower_detail.jpg|Flowers of the "silk tree" (Albizia julibrissin) have many long thread-like stamensImage:Lilydetail.jpg|Detail of the stamen of a lily.
