Stamen

Stamen, the male sporophyll of a flowering plant, consisting typically of the usually thread-like filament surmounted by the anther (q.v.), which contains the pollen (q.v.). In its earliest stages, in almost all cases, a stamen closely resembles a foliage leaf. As it develops there is generally a central bundle of spiral vessels, or midrib, and certain hypodermal cells (archesporium) give rise to the pollen-sacs (sporangia), which are generally four, though afterwards merged into two chambers (loculi). In them originate the pollen-mother-cells. The two outer layers of cells in the anther become specially modified, the outer into a slightly cuticularised epidermis or exothecium, the inner, or endothecium, into a layer of spirally-thickened cells interrupted in the region at which the anther splits when ripe. The central portion of the stamen between the pollen-sacs is termed a connective. It is usually small, but in the violet it is produced into a triangular buff-tip, and in two of the stamens is also appendiculate, being furnished with a tail-like nectariferous appendage at the base of each, which is enclosed in the speer of the corolla. In heaths there are two similar processes, non-nectariferous, at the base of each anther. In the hornbeam the connective bifurcates, each branch bearing an anther-lobe or dimidiate (i.e. halved) anther, whilst in the sage (Salvia) the connective is a long, unequal-armed lever, with an anther-lobe at each end, the lower one abortive. When the connective is thus enlarged the anther is termed distractile. If the filament be absent, the anther is sesile; whilst if the more essential anther be absent, the stamen or filament is abortive or sterile, and is commonly termed a staminode.

The stamens may be described with reference to their (1) number, (2) relative length, (3) union or cohesion, (4) insertion or adhesion, (5) form of filament and anther and the mode of insertion of the latter on the former, and (6) mode of dehiscence of anther.

The numbe of stamens in a flower may vary from one to twelve, twenty, or more, and the first eleven classes of the artificial system of classification proposed by Linnaeus are named according to this character. The stamens are commonly equal in length, but sometimes of various lengths, according to their order of development; and if they are in more than one whorl, those of one whorl are often longer than those of another, as in the purple loosestrife. In special cases of four stamens, two locng and two short, characteristic of most Labiatae (q.v.) and Scrophulariaceae (q.v.), and of six stamens, four long and two short, as in Cruciferae, they are known as didynamous and tetradynamous respectively.

The stamens may either be free, as in all the Linnaean classes as yet referred to, or they may be united by their filaments, by their anthers, or by both. Some of the cases of apparent union by the filaments are truly due to branching (chorisis). Intercalary growth of a zone of tissue below all the stamens carrying them up on a tube, as if all united by the lower part of their filaments, as in Malvaceae, geranium, furze, broom, etc., produces what is called the monadelphous condition. Most Leguminosae (q.v.) have ten stamens, nine united and one free. This is termed diadelphous, whilst the branching of three or more stamens produces the condition known as polyadelphous. The Compositae (q.v.) are the most important case of the union of stamens by their anthers, which Linnaeus styled syngenesious, and the Cucurbitaceae illustrate union by both filaments and anthers. The insertion or adhesion of the stamens can usually de described by the same terms as that of the corolla - vis. hypogynous, perigynous, or epigynous; but in gamopetalous (or gamophyllous) flouwers, owing to intercalary growth beneath both the corolline and the staminal whorl, they often appear to spring from the petals, corolla-tube (or perianth), and then are termed, in addition to being hypogynous or epigynous, as the corolla may happen to be, epipetalous (or epiphyllous), as in the primroses, lilac, etc. In orchids and a few other plants the stamens are adherent to the gynaeceum, forming a column or gynostemium, and the flower is then termed gynandrous.

Though commonly thread-like or filiform, the filament is sometimes, as in grasses, so slender, hair-like, or capillary as to bend under the weight of the anther. In other cses it is broader at the base, tapering like an awl or subulate, or it may be broad and petaloid.

The anther (q.v.) is sometimes attached to the filament or to its direct continuation, the connective, throughout its whole length, as in water-lilies, violets, etc., when it is termed dorsifixed or adnate. In other cases it is articulated at its base to the apex of the filament, and is called basifixed or innate, as in sedges (Carex); or, again, it may be only attached by a point about the middle of its back so that it can turn freely as on a ball-and-socket joint, and is therefore called versatile, as in grasses and lilies. In Salvia the long connective is attached in this way to a short stout filament, on which it swings like the ancient quintain.

To discharge its pollen when ripe, the anther generally splits or dehisces longitudinally, by a slit down the face of each lobe, as in lilies, grasses, violets, etc. When short and rounded, it sometimes dehisces transversely by a horizontal split, as in Alchemilla. In the heath family (Ericaceae) dehisence is porous, by a hole at the top of each lobe, the lobes in some genera, such as the cranberries, being produced upward into tubular processes. In the barberry and in the bay-tree dehiscence is opercular or valvular, two parallel splits and one transverse one on the face of each lobe forming a little door or operculum, which folds back in an upward direction. Dehiscence is often an important classificatory character, and from this point of view we must observe not only the mode, but also the direction in which it takes place. In Compositae, Amaryllidaceae, and Liliaceae the anthers burst towards the centre of the flower, and are termed introrse; in Berberis, Iridaceae, and Colchicaceae they burst outwards - i.e. towards the perianth - and are called extrorse. [ANTHER, POLLEN.]