Flowering species-including the trees-represent the top of the evolutionary line for plants, with a reproductive process that generates seeds to carry on the species.
In the progress from single-celled algae to spore-bearing plants such as ferns, mosses, lichens, and fungi, the complexity of the reproductive process is greatly increased. The flowering plants go a step further by distancing the male and female organs to facilitate cross-pollination, ensuring recombination of genes and more possibilities for natural selection and evolution. The seed has the potential to carry the new genetic combination far from the parent plant and create a new community of plants.
Germination of seeds
The seeds contained in fruits or pods all summer long, or attached to plant skeletons in autumn, are destined to wander through the world until some of them are planted in favorable locations. (See the article Seed Dispersal.) Under the right conditions of temperature, shade, oxygen, and moisture, a seed will germinate. The seed contains the plant embryo (miniature versions of plant parts) and endosperm, (nourishing material for the baby plant), surrounded by a protective seed coat.
In germination, a radicle, or embryonic root, elongates, breaks through the water-softened seed coat, and grows downward into the soil. Nourished by material within the seed, it grows root hairs. Another section of the embryo develops into a stem that grows upward and sprouts leaves. By the time the endosperm is consumed, the plant has leaves and can create its own nourishment to produce more leaves and, eventually, flowers.
Fertilization of flowers
During spring and summer, we are surrounded by plants having sex in a fashion that is, at the cellular level, remarkably like human sex, involving passage of sperm through a narrow channel and penetration of an egg cell.
In flowers, the stamens (male parts) produce pollen grains. Pollen is carried by wind or an insect to the stigma (tip) of the pistil (female part), most often the pistil of a different plant from the one supplying the pollen. The male and female parts of a given plant often mature at different times, to prevent self-pollination.
There the pollen germinates, sending a long pollen tube down the style (the stem of the pistil) and into the ovule (the chamber containing the egg cell). This is a slow process. Two tailless sperm remain inside the pollen tube until the tube reaches the micropyle, the open end of the embryo sac, with its egg and seven other nuclei.
The two sperms enter the embryo sac and carry out two fertilizations: one sperm fuses with the egg and forms an embryo. The other sperm fuses with the two polar nuclei, creating an endosperm (the nutritive tissue of the future seed), while tissues of the ovule develop into the seed coat. When mature, the new seed is ready to separate from the plant and start the next generation.
