Sexual Reproduction in Flowering Plants

A typical angiosperm flower consists of four whorls: calyx (sepals), corolla (petals), androecium (male), and gynoecium (female). The androecium comprises stamens, each having a filament and an anther. The anther contains microsporangia, where microsporogenesis occurs: pollen mother cells (2n) undergo meiosis to produce microspore tetrads, each developing into a pollen grain. The mature pollen grain is a two-celled structure containing a larger vegetative cell (with a tube nucleus) and a smaller generative cell. The pollen wall has two layers: the outer exine, composed of sporopollenin -- the most resistant biological material known, resistant to high temperatures, strong acids, and enzymatic degradation -- and the inner intine, made of cellulose and pectin. Germ pores are thin regions in the exine through which the pollen tube emerges during germination.

The gynoecium consists of one or more pistils, each with a stigma (receives pollen), style (connecting tube), and ovary (contains ovules). Inside each ovule, megasporogenesis occurs: the megaspore mother cell (2n) undergoes meiosis to produce four megaspores, of which three degenerate and one functional megaspore survives. This functional megaspore undergoes three rounds of free nuclear mitotic divisions to form the embryo sac, which is a 7-celled, 8-nucleate structure. The egg apparatus sits at the micropylar end and comprises the egg cell flanked by two synergids (with filiform apparatus that guides the pollen tube). Three antipodal cells occupy the chalazal end. The large central cell contains two polar nuclei that later fuse to form the secondary nucleus.

Pollination is the transfer of pollen from anther to stigma. Autogamy (self-pollination within the same flower) ensures seed set but reduces genetic variation. Geitonogamy is transfer between different flowers on the same plant -- ecologically it appears as cross-pollination, but genetically it is equivalent to self-pollination since the pollen comes from the same genotype. Xenogamy (cross-pollination between different plants) introduces genetic diversity and is favoured evolutionarily. Plants promote xenogamy through outbreeding devices: self-incompatibility (pollen rejection by pistil), dicliny (unisexual flowers or dioecious plants), and dichogamy (maturation of stamens and pistils at different times).

Double fertilization is unique to angiosperms. After pollen tube growth through the style, it enters the embryo sac through the micropyle. One sperm fuses with the egg cell (syngamy) to form the zygote (2n). The other sperm fuses with the two polar nuclei (triple fusion) to form the primary endosperm nucleus (PEN, 3n). This simultaneous occurrence of syngamy and triple fusion is called double fertilization.

Following fertilization, the zygote develops into the embryo through sequential stages (globular, heart, torpedo, mature), the PEN develops into the triploid endosperm (nutrition for the developing embryo), the ovule transforms into the seed (with seed coat from integuments), and the ovary develops into the fruit. In special reproductive modes, apomixis produces seeds without fertilization (asexual seed formation), polyembryony involves multiple embryos in a single seed, and parthenocarpy produces seedless fruits without fertilization. The key testable concept is that double fertilization involves two fusion events (syngamy producing a diploid zygote and triple fusion producing a triploid endosperm), and that geitonogamy is genetically self-pollination despite being ecologically cross-pollination.