[Illustration: From Strasburger's _Lehrbuch der Botanik_, by permission of Gustav Fischer.
FIG. 64.--Flower of _Paeonia peregrina_, in longitudinal section. k, Sepal; c, petal; a, stamens; g, pistil. (½ nat. size.)]
The stamens arise from the thalamus or torus within the petals, with which they generally alternate, forming one or more whorls, which collectively constitute the _androecium_. Their normal position is below the pistil, and when they are so placed (fig. 64, a) upon the thalamus they are hypogynous. Sometimes they become adherent to the petals, or are _epipetalous_, and the insertion of both is looked upon as similar, so that they are still hypogynous, provided they are independent of the calyx and the pistil. In other cases they are perigynous or epigynous (fig. 65). Numerous intermediate forms occur, especially amongst Saxifragaceae, where the parts are _half superior_ or _half inferior_. Where the stamens become adherent to the pistil so as to form a column, the flowers are said to be _gynandrous_, as in _Aristolochia_ (fig. 66). These arrangements of parts are of great importance in classification. The stamens vary in number from one to many hundreds. In acyclic flowers there is often a gradual transition from petals to stamens, as in the white water-lily (fig. 31). When flowers become double by cultivation, the stamens are converted into petals, as in the paeony, camellia, rose, &c. When there is only one whorl the stamens are usually equal in number to the sepals or petals, and are arranged opposite to the former, and alternate with the latter. The flower is then _isostemonous_. When the stamens are not equal in number to the sepals or petals, the flower is _anisostemonous_. When there is more than one whorl of stamens, then the parts of each successive whorl alternate with those of the whorl preceding it. The staminal row is more liable to multiplication of parts than the outer whorls. A flower with a single row of stamens is _haplostemonous_. If the stamens are double the sepals or petals as regards number, the flower is _diplostemonous_; if more than double, _polystemonous_. The additional rows of stamens may be developed in the usual centripetal (acropetal) order, as in Rhamnaceae; or they may be interposed between the pre-existing ones or be placed outside them, i.e. develop centrifugally (basipetally), as in geranium and oxalis, when the flower is said to be _obdiplostemonous_. When the stamens are fewer than twenty they are said to be _definite_; when above twenty they are _indefinite_, and are represented by the symbol [infinity]. The number of stamens is indicated by the Greek numerals prefixed to the term _androus_; thus a flower with one stamen is _monandrous_, with two, three, four, five, six or many stamens, di-, tri-, tetr-, pent-, hex- or polyandrous, respectively.