Seed-bearing plants, the spermatophytes, are the best-known group of plants on earth, and probably the most important to humans. Conifers, cycads, angiosperms and ginkgos all bear seeds. The angiosperms, or flowering plants, are the focus of most gardeners. There are nearly 400,000 species in this group, and all of them produce seeds.
The embryo of the new plant is the essential element of a seed. The seed also contains food supplies to support the embryo. The seed coat protects the seed. Some seeds also have specialized structures to aid in their dispersal.
Germination and Growth
All seeds have an inert period, allowing time for them to be dispersed away from the parent plant. When conditions are right, germination occurs. The embryo begins to grow, bursting through the seed coat. The seedling first displays seeds leaves, called cotyledons. A little later, true leaves appear, and photosynthesis can begin. The young plant can nourish itself, and its energy is focused on further growth.
When the plant reaches maturity, it develops buds, which open into flowers. These are the reproductive organs of plants. Most flowers contain both male and female parts. The stamens produce pollen grains that hold the male reproductive cells. The pistil is the outermost female part, which conducts pollen to the ovule, where the female reproductive parts are located. These essential structures are surrounded by petals, sepals or tepals, which attract pollinators and protect the reproductive organs.
Flowers have various mechanisms to prevent self-fertilization, even though they have both male and female parts. The movement of pollinators carries the pollen from one plant to another. When that pollen is deposited on the pistil, a pollen tube is formed. The pollen grains travel down the tube, and when they reach the ovule, male cells are released. These fertilize the egg in the ovum. The flower, having served its purpose, withers. The seed develops and, when it is mature, falls from the parent plant.
Although some seeds have no further structures, many are enclosed in fruits. In some cases the fruit is fused to the seed coat, while in others the two structures remain distinct. Sometimes many seeds are contained in one fruit. The fruit helps to protect the seeds and to encourage their dispersal.
Sometimes seeds are dropped at the base of the parent plant, where they grow into new plants. However, it is an advantage to have the seed further away, so that all the new seedlings are not competing for the same resources. Seeds are dispersed in various ways--by wind or water, in the digestive tracts of birds and animals, carried on the fur or skin of passing creatures. Often the shape of the seed coat or the type of fruit lends itself to a specific dispersal method.