Flowering plants first appeared during the late Jurassic period and were probably evergreens with large magnolia-like flowers. They were the last seed producing plants to evolve and were named angiosperms by Charles Darwin. Flowers produce seeds through sexual reproduction; cell nuclei from male and female organs combine. Perfect flowers are both male and female, imperfect flowers are only male or female.
The roots, leaves and leaf supporting structures of an angiosperm have one purpose--to provide support and energy to the plant's reproductive system. The roots take up water and minerals from the soil and pass it to the leaves. Photosynthesis within the leaves converts carbon dioxide, water and minerals into food. In some perennial plants, food is stored in tubers or bulbs beneath the soil. Annual plants supply more food energy for the development of the flower portion of the plant than any other.
The stem of the plant lifts leaves and flowers above the ground. The leaves receive more sunlight so are able to produce more energy. The flowers positioned where they can be pollinated and produce seeds. The stems of woody plants, such as trees, may reach great heights, often more then 100 ft. Some plants reach full height in a few weeks or months, others require many years of growth. Certain species place only the flowers on stems; the leaves, stem and roots grow from a single point.
Sterile Flower Parts
Portions of the flower have nothing to do with the actual seed formation. The petals and the sepals have different functions. The sepal, at the base of the flower, protects the inner seed-producing portions of the flower and holds the petals in place. The petals produce fragrant oils and colors that attract pollinators to the flowers. Many flowers have green sepals that looks similar to leaves. Sepals and petals of the same color are called tepals.
Male Flower Parts
The stamen, or androecium, is made up of two parts--the anther and the filament. A flower usually has a number of stamens arranged in a circle. If the flower has both male and female portions, the stamens are arranged around a pistol, or female part of the flower. The anther, or microsporangium, produces the male gametes--called pollen--through meiosis. After the pollen is transferred to the pistol pollen tube, it germinates sperm nuclei.
Female Flower Parts
The pistil, or gynoecium, is the female part of the flower. It has a stigma that is supported by the style above the ovary. Inside the ovary, one or more ovules produce female gametes, or eggs, via meiosis. The stigma receives the pollen from a pollinator or, in some cases, directly from the anther. Upon reaching the stigma, pollen germinates sperm nuclei and grows a tube down the pistol pollen tube to the ovary, where the sperm nuclei combine with the female gamete. The fertilized female gamete develops into a seed within the embryo sac inside the ovule.