Botanists divide plants into two large categories: plants that create seeds, and plants that do not. Seed-producing plants are then divided into two more categories. These plants are classified as either gymnosperms, which produce seeds in cones, and angiosperms, which produce seeds in flowers. These two classifications of plants produce the two main types of seeds: naked and enclosed. There are other, less common types of seeds as well.
Angiosperms (flowering plants) produce enclosed seeds, which are enclosed by an ovary. In most cases, the ovary becomes the fruit of the plant as the seed ripens. The fruit is meant to protect the seed. An apple seed is an enclosed seed, as are peas, (the ovary develops into the pod) and poppies, which also produce a large pod around the seeds.
Gymnosperms produce naked seeds. Gymnosperms are more commonly called conifers because the plants produce cones. The seeds are created on the upper surface of the cones, which fold up to form a hard, protective covering as the seeds ripen. When they are ripe, the cones open, and the seeds may scatter on the wind, or drop to the ground with the cones, or even attach themselves to the bodies of animals such as birds and squirrels.
Other Types of Seeds
Some seeds are not protected by a fleshy fruit, pod or other covering, and are not produced "naked" in a cone. Grain plants are unique in that the ovary and seed actually merge together, creating a kernel. When you eat corn, you are actually eating the seed of the plant. Other "seeds," such as sunflower seeds, are not really seeds at all, but the hard fruit of the plant. The actual seed of a sunflower seed is obtained after the hard shell of the fruit is cracked open. Many types of nuts, such as an acorn, are also hard fruits; the nut is actually the seed of the plant.