Stamens, forming the male part of the Flower (q.v.), comprise a filament and an anther, and are inside flower leaves, petals, and sepals, and outside the fruit leaves, the carpels. But their position on the axis, whether below the carpels, as is most commonly the case, or on a level with them, or above them, varies, and is used as a means of classification. When they are inserted on the thalamus (flower receptacle) below the carpels the flower is said to be hypogynous; when, carried upwards by the continued growth of the outer part of the flower receptacle, they are inserted on the same level as the carpels then the flower is perigynous; when, carried up still farther, they are inserted above the carpels the flower is epigynous. If the stamens are adherent to the petals they are epipetalous, if adherent to the pistil (united carpels) they are gynandrous. If the filaments only are more or less coherent (Mallow) the stamens are monadelphous; if united into two bundles (pea) they are diadelphous; if only the anthers cohere then the stamen is synanthrous. The stamens vary in number from a single one in a flower to as many as several hundreds. These variations also are used in classification. When the stamens are definite, few (five to ten) in number, the individuals are inserted one opposite each of the petals, or each of the sepals, or one opposite all of them. If they are less in number than the petals or sepals of the flower, then their position varies. When there is more than one whorl of stamens then the individuals of each whorl alternate with the individuals of the next whorl below it. Often the full number of stamens, that comparison with other flowers of the same order would lead us to expect, is not present; but usually aborted traces of them may be seen. Thus the Scrophulariæ are peculiar in having only four stamens, but the fifth is represented by a minute scale. Remnants of this description are called staminodes.
The stamens are commonly said to be metamorphosed leaves. But, since a leaf is defined as an appendage of the axis or stem, this statement is only an assertion of the general homology, or similarity of origin, of the two kinds of appendages. Often, too, stamens are said to be altered flower-leaves—i.e. petals. But this is a case of putting the cart before the horse. Stamens very commonly become petaloid, as for instance when a flower becomes 'double' under cultivation, and in a few cases in the natural state, the white water-lily for instance. In all such cases there may be seen in the same blossom a complete series of transition forms between stamens and petals. This indicates the homology of the two kinds of appendages, but of itself gives no evidence as to which form is the precursor of the other. But the fact that the Gymnospermus (q. v.), conifers, for example, which are older forms and are less highly developed than the true flowering plants, have stamens but not petals, shows that the petals of the Phanerogams are derived from stamens, and not the stamens from the petals. The further facts that the characteristic colour of stamens is yellow, and that the simplest, lowest flowering plants have yellow flowers, is another piece of evidence that leads to the same conclusion.
The anther which contains the fertilising pollen is the essential part of a stamen. The pollen is set free by the splitting—dehiscence—of the anthers. The mode of dehiscence is sometimes characteristic of the plant. Thus the anthers of the Rhododendra open and shed their pollen through a small circular pore at the upper end of each lobe; and in a few cases the dehiscence is transverse, or across the anther. But the most usual mode of dehiscence is by a longitudinal slit in each lobe, either on the inner or outer face of the anther.
The time at which the stamens dehisce relatively to the ripening of the stigma is important.
For if the anther delisces when the stigma is ready to receive pollen then the flower may be self-fertilised; but if it does not open at that time then self-fertilisation will be impossible, or nearly so, and that flower must be cross-fertilised. Stamens present numerous modifications of form that are apparently adaptations to the process of fertilisation.
See Sir John Lubbock's Flowers in their Relation to Insects ('Nature' series, 1875); Kerner's Pflanzenleben (vol. ii. 1891).