Microsporogenesis is the meiotic process by which haploid microspores form from a diploid microspore mother cell inside the anther, the pollen-producing organ of the androecium (each stamen has an anther plus a filament). A very young anther is a homogeneous cell mass under one epidermis; it becomes four-lobed as hypodermal cells in each lobe enlarge into an archesporium, whose periclinal divisions give primary parietal cells (facing the epidermis) and primary sporogenous cells (facing the anther's interior). The parietal cells build up the 2-5 layers of the anther wall - epidermis, endothecium, middle layers and tapetum, from periphery inward - while the sporogenous cells (directly, or after a few mitoses) form sporogenous tissue whose cells act as microspore mother cells; each undergoes meiosis to give a tetrad of four haploid microspores (tetrahedral, decussate, linear, T-shaped or isobilateral), which normally separate into free pollen grains but stay fused as a pollinium in Calotropis or as compound pollen in Drosera/Drymis. A mature anther, seen in T.S., is bilobed with two thecae per lobe (dithecous) and four pollen sacs overall (tetrasporangiate); its wall, from outside in, is the protective, anticlinally-dividing epidermis; the hygroscopic, radially-elongated endothecium, whose thickened walls (interrupted at the thin stomium) drive anther dehiscence and which is absent in aquatic, saprophytic, cleistogamous or parasitic anthers; the short-lived, ephemeral middle layers; and the innermost tapetum, dual in origin (peripheral wall layer plus connective tissue), peaking at the tetrad stage, that nourishes the sporogenous tissue and microspores and contributes sporopollenin, pollenkitt, tryphine and incompatibility-controlling proteins - existing as secretory (parietal/glandular/cellular) tapetum, which keeps its cellular integrity, or invasive (periplasmodial) tapetum, whose protoplasts fuse into one periplasmodium (a third, amoeb …