Many plants produce seeds. All conifers produce seeds, as do ginkgos, cycads, and most woody plants. Angiosperms, the flowering plants, are usually of the greatest interest to gardeners. There are 250,000 to 400,000 species of angiosperms, and many more varietals and cultivars. Seeds are the primary means of reproduction for most angiosperms, although many can also reproduce by root division, rhizomes, or bulb division.
The seed contains the embryo of a new plant. Typically, it also contains some form of nourishment for the embryo to consume when it begins to develop. In flowering plants, this is the endosperm. The embryo and endosperm are enclosed in and protected by the seed coat. The seeds of most flowering plants remain dormant for a period before beginning to grow.
Germination occurs when conditions are favorable for the embryo to develop into a new plant, typically when soil temperature and the presence of moisture are correct for the species. Some seeds require additional conditions, such as the presence or absence of light. Some seeds require external abrasion of the seed coat, such as might occur by passing through the digestive system of a bird or being exposed to freezes or fires.
Seeds grow from two points. Typically, the seed begins growing with radicle, which will develop into roots. Very soon, the seed also produces a single shoot, the hypocotyl. Soon, the shoot displays cotyledons, or seed leaves. At this point, the seedling is still nourished by the contents of the seed. The cotyledons open when they sense light, exposing the shoot apical meristem, which grows true leaves. Then photosynthesis begins and the seedling can nourish itself.
At first, the young plant's growth is concentrated on the development of roots, stems and leaves. As the plant matures, flower buds begin to form. When these buds are ready to open, the plant is sexually mature. Annual plants go through this growth stage only once, while perennial plants experience it each year.
Flowers are the reproductive organs of plants, and they grow in an amazing variety of shapes, colors and sizes. Most flowers contain both male and female reproductive organs. The stamens bear pollen grains, which hold the male reproductive cells. The pistil is the structure that receives the pollen and conducts the male cells to the ovule. The ovule, which contains the female reproductive cells, is enclosed by the carpel. These are the essential elements of flowers. Most flowers also have petals, sepals or tepals, which serve to protect the reproductive structures and to lure pollinators. Although most flowers contain both male and female parts, many methods exist to avoid self-fertilization.
Fertilization begins when a pollen grain is deposited on the pistil. A pollen tube forms, reaching down through the carpel. When it reaches the ovule, male cells are released from the pollen and can fuse with the female cells in the ovule. When fertilization is complete, a zygote has formed and will begin to develop into an embryo.
The fertilized ovule develops into a seed, while the rest of the ovary develops into fruit. The form of the fruit is closely linked to the form of the seed coat. Together, they protect the embryo and aid in the dispersal of the seed, so that the new plant will have the best chance for germination and growth.