|1. Flowers, the characteristic reproductive structures of angiosperms, are composed of sepals, petals, stamens, and carpels. Modifications of the basic floral organs are common, often resulting in incomplete and imperfect flowers.
2. Meiosis is a form of cell division that reduces the number of chromosomes from diploid to haploid. The process consists of two consecutive divisions, with the reduction in chromosome number occurring in the first division. The most significant events of meiosis occur in prophase I, when synapsis occurs, and anaphase I, when the homologous chromosome pairs separate.
3. In angiosperms, meiosis occurs before the formation of male and female gametophytes, which are small and relatively short lived. Ovules, which include the female gametophytes, develop within the carpels; the pollen grains, or male gametophytes, develop in the stamen.
4. Pollen is transferred passively by animals or wind from stamen to stigma. Insect-pollinated flowers typically have bright, showy petals and fragrant aromas and are rich in nectar. Pollen in these flowers is often sticky, adhering to the insect body. Wind-pollinated flowers are usually small and inconspicuous but produce copious amounts of dry, lightweight pollen. Only a small amount of pollen from wind-pollinated plants reaches the female organ. Most pollen grains settle to the ground, where they can leave a lasting record in the sediment.
5. Before fertilization, the pollen tube grows down the style into the ovary and ovule. The generative nucleus gives rise to two sperm. Within the ovule, double fertilization occurs as one sperm fertilizes the egg, producing the zygote, while the second sperm fuses with the polar nuclei, giving rise to the primary endosperm nucleus. After fertilization, the ovary becomes a fruit, and each ovule becomes a seed.