Male Reproductive Part - Androecium

Each stamen of the androecium is made up of an anther and a filament; the anther is the business end, since it bears the pollen grains that represent the male gametophyte. A very young anther starts out as a homogeneous mass of cells enclosed by a single epidermis; as it develops it assumes a four-lobed shape, and in each lobe a row (or a few rows) of hypodermal cells enlarges and becomes conspicuously nucleated to form the archesporium. These archesporial cells divide periclinally (in planes parallel to the anther surface) to give primary parietal cells towards the epidermis and primary sporogenous cells towards the anther's interior; the primary parietal cells then divide further, both periclinally and anticlinally, to build up two to five layers of anther wall - the endothecium, one or more middle layers, and the tapetum, arranged from the periphery inward towards the centre. Microsporogenesis is the specific process, occurring within this developing anther, by which haploid microspores are formed from a diploid microspore mother cell through meiosis. The primary sporogenous cells either directly, or after a few mitotic divisions, form a body of sporogenous tissue whose cells act as microspore mother cells; each microspore mother cell undergoes meiosis to produce a tetrad of four haploid microspores, arranged tetrahedrally, decussately, linearly, T-shaped or isobilaterally depending on the species. The microspores usually separate and lie free within the anther locule as they mature into pollen grains, though in a few plants (e.g. Calotropis) all the microspores of one microsporangium stay bound together as a single mass called a pollinium, and in others (e.g. Drosera, Drymis) several pollen grains remain fused as compound pollen. The transverse section (T.S.) of a mature anther shows a bilobed anther, each lobe containing two thecae (making the anther dithecous), so that the anther as a whole is tetrasporangiate (has four pollen sacs). Its wall, from outside in, consists of: (a) the epidermis, a single protective layer that divides anticlinally to keep pace with the rapidly enlarging tissue beneath it; (b) the endothecium, a single layer of radially elongated, hygroscopic cells lying just below the epidermis, whose inner wall develops bands of thickening (usually of alpha-cellulose) except along a thin-walled strip called the stomium, and it is this hygroscopic behaviour combined with the weak stomium that causes the mature anther to split open (dehisce) - endothecial differentiation is notably absent in aquatic plants, saprophytes, cleistogamous flowers and extreme parasites; (c) the middle layers, two to three short-lived (ephemeral) layers that disintegrate or get crushed as the anther matures; and (d) the tapetum, the innermost and nutritionally most important layer, which is dual in origin (derived partly from the peripheral wall layer and partly from the anther's connective tissue), reaches its peak development at the microspore tetrad stage, and nourishes the developing sporogenous tissue, microspore mother cells and microspores while also contributing sporopollenin, pollenkitt, tryphine and various incompatibility-controlling proteins to the pollen wall; tapetal cells may be uninucleate, multinucleate or polyploid, and tapetum behaviour comes in two recognised forms - secretory (parietal/glandular/cellular) tapetum, which keeps its original position and cellular integrity while nourishing the microspores, and invasive (periplasmodial) tapetum, whose cells lose their inner tangential and radial walls so that all the tapetal protoplasts fuse into one periplasmodium (a third, amoeboid, form is sometimes also described, in which the cell wall is retained but the cells protrude into the anther cavity, often linked with male sterility). Within this wall sits the anther cavity, filled with microspores in young stages and with mature pollen grains later, while the connective is the sterile, vascularised column of tissue that runs through the anther lobe and also contributes to the inner tapetum. A microspore is the direct product of meiosis and still has an incompletely developed wall, whereas the pollen grain, derived from it, has dense cytoplasm with a central nucleus and a fully differentiated two-layered wall - a thin, uniform inner intine made of pectin, hemicellulose, cellulose, callose and proteins, and a thick, patterned outer exine made of cellulose, sporopollenin and pollenkitt; thin, rounded gaps in the exine (lacking sporopollenin) are germ pores, and elongated ones are furrows, both associated with pollen-tube germination, while the exine's surface sculpturing (smooth, rod-like, grooved, warty or punctate) is distinctive enough to be used in plant identification (the study of pollen is called palynology). Pollen size varies enormously between species, from about 10 micrometres in Myosotis to about 200 micrometres in the Cucurbitaceae and Nyctaginaceae, and shape may be globose, ellipsoid, fusiform, lobed, angular or crescent-shaped. Sporopollenin - contributed jointly by the pollen cytoplasm and the tapetum, and derived from carotenoids - is exceptionally resistant to heat, strong acids, alkalis and enzymatic breakdown, which is why pollen survives for very long periods in fossil deposits and also why it can safely make the journey from anther to stigma; pollenkitt, contributed only by the tapetum and coloured yellow or orange from carotenoids or flavonoids, is instead an oily visco …