An essential characteristic of a seed plant is that it begins its life as an embryonic plant inside the protective covering of a seed. When conditions are right—for example, when it has been exposed to water for a period of time—the seed germinates. As the young plant develops, stems and roots grow. The plant reaches maturity when it is able to reproduce by creating new seeds.

Seed plants that reproduce through the pollination of flowers are called angiosperms. Angiosperms have male and female parts that work together to create a new plant. The male parts are the stamens—a filament and anther (where the pollen is produced). The female part is the pistil—the stigma, style, and ovary (where the eggs are produced in ovules). When a flower is pollinated, often with the help of birds or insects, pollen lands on the sticky surface of the stigma. It then travels down the style, which serves as a pathway to the ovary. An egg fertilized by pollen becomes a zygote; as the zygote develops, it becomes an embryonic plant. The embryo, along with the endosperm that nourishes the embryo, is enclosed within the protective shell of a seed. Meanwhile, the surrounding ovary also grows and becomes the fruit that contains the seeds.

Seed plants vary greatly in the time that they take to cycle through their developmental stages. For example, some plants may take just a few weeks to reach maturity while others take years before they are able to produce seeds. Likewise, some flowers last just a few hours while others can last for weeks.

However, not all seed plants are flowering plants. Seed plants that do not have flowers—such as cycads, ginkgo, and conifers—are called gymnosperms. Conifers are common gymnosperms; instead of flowers, conifers have cones that produce pollen or eggs. Male cones are smaller and soft, and female cones are large and hard. Wind carries pollen from the male cone to the female cone. As the eggs are pollinated and seeds develop, the scales of the cone open up to release the seeds.

Furthermore, not all plants are seed plants. Some plants, such as ferns and mosses, reproduce with spores instead of seeds. Spores, like seeds, can survive harsh conditions and develop into new plants; however, unlike seeds, spores are produced without fertilization and contain neither a plant embryo nor endosperm. Some plants can reproduce without spores or seeds through vegetative reproduction, in which a part of the stem or root gives rise to a new plant.

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