Sexual Reproduction

Sexual reproduction is built around two linked processes: gametogenesis, the production of male and female gametes, and fertilization, their fusion. In lower plants such as algae and bryophytes, the gametes produced may be motile or non-motile depending on the species, and their fusion follows one of three patterns - isogamy (fusion of similar gametes), anisogamy (fusion of dissimilar motile gametes) or oogamy (fusion of a large non-motile egg with a small motile sperm) - with fertilization taking place externally, outside the parent's body. In higher (flowering) plants, by contrast, fertilization is internal. The flower is the structure built specifically for this internal sexual reproduction; to a morphologist it is simply a highly condensed shoot meant for reproduction, though culturally it carries far richer meaning - as inspiration for poets, as decoration for celebrations, as a symbol on national flags, and as the raw material for perfumes; in Tamil literature, for instance, the five traditional landscapes (thinai) are each represented by a characteristic flower. A flower is built from four whorls arranged concentrically around the floral axis - the calyx (sepals), corolla (petals), androecium (stamens) and gynoecium (carpels) - of which the androecium and gynoecium are the essential (reproductive) organs, while the calyx and corolla are chiefly protective and attractive respectively. The whole process of sexual reproduction in higher plants unfolds across three sequential stages: pre-fertilization structure and events (the differentiation and maturation of the male and female re …