The outermost series of the essential organs, collectively termed the androecium, is composed of the microsporophylls known as the staminal leaves or stamens.
Apart from this, botanists are generally agreed that the concrescence of parts of the flower-whorls - in the gynaeceum as the seed-covering, and in the corolla as the seat of attraction, more than in the androecium and the calyx - is an indication of advance, as is also the concrescence that gives the condition of epigyny.
22, we recognize four distinct whorls of leaves: an outer whorl, the calyx of sepals; within it, another whorl, the parts alternating with those of the outer whorl, the corolla of petals; next a whorl of parts alternating with the parts of the corolla, the androecium of stamens; and in the centre the gynoecium of carpels.
The androecium and gynoecium are not present in all flowers.
A flower then normally consists of the four series of leaves - calyx, corolla, androecium and gynoecium - and when these are all present the flower is complete.
The symmetry in the flower is evidently dimerous, and the abnormality in the androecium, where the four long stamens are opposite the posterior sepals, takes place by a splitting, at a very early stage of development, of a single outgrowth into two.
Changes are produced in the whorl of stamens by cohesion of the filaments to a greater or less extent, while the anthers remain free; thus, all the filaments of the androecium may unite, forming a tube round the pistil, or a central bundle when the pistil is abortive, the stamens becoming monadelphous, as occurs in plants of the Mallow tribe; or they may be arranged in two bundles, the stamens being diadelphous, as in Polygala, Fumaria and Pea; in this case the bundles may be equal or unequal.