Flowers bring vibrancy and color to your garden but, to the plant, they represent the pinnacle of plant evolution, a structure evolved to almost guarantee successful fertilization. Like all higher-order organisms, flowering plants prefer to reproduce sexually, combining their genetic material with that of another plant to increase diversity. Unlike animals, however, plants are unable to actively seek mates, relying instead on other methods to accomplish fertilization.
The lovely flowers gracing your garden serve an essential reproductive function for their plants. In flowering plants, reproductive function is confined inside of the flower, where sex cells are produced, fertilization occurs and seeds eventually develop. The showy coloration and enticing scents of the flower further aid in the process of fertilization by attracting pollinators.
The flower's anthers produce pollen, which contains the male sex cells, while the pistil leads into the ovary, where the female sex cells reside. Looking at the average flower, the pistil arises from the center, and the anthers surround it as small filaments with club-shaped tops. The yellow powder upon them is pollen. Inside of the ovary are one or more ovules, each containing an egg cell. Each ovule, if fertilized, will eventually develop into a seed.
Pollen grains contain two cells, the germ cell and the tube cell. When a pollen grain lands on the pistil, the tube cell begins growing, extending down into the flower and entering the ovary. The germ cell divides just once, forming two sperm cells. Once the tube cell reaches the egg inside the ovule, sperm exits the tube. One fertilizes the egg, and the other fuses with other female cells to produce endosperm, a nutritive substance that fills the seed and nourishes the seedling. This process, called double fertilization, is unique to flowering plants.
All aspects of the flower have evolved with fertilization and successful germination in mind. The color of the flower determines what kinds of animals are attracted to it, and they carry pollen from plant to plant, ensuring genetic variety. Alone of the plants, flowers contain ovaries, which necessitate the development of the pollen tube but also protect the developing seeds from harm. Finally, through the process of double fertilization, seedlings have available the nutrition that they need to begin germination.
The seemingly complex process of fertilization in flowers in fact accounts for the genus's success. Flowering plants are the most common plants on earth, attesting to the effectiveness of their adaptations. Non-flowering seed plants rely primarily on the wind to carry pollen to uncovered seeds attached to cones. Flowers make fertilization deliberate where it was often left to chance, and afford extra protection and support to their developing seeds.