Seed plants are divided into two main groups: angiosperms (flowering plants) and gymnosperms (coniferous trees and shrubs). Angiosperms ovule is located in the carpels of flowers. After fertilization by a pollen grain, the ovule turns into a seed inside the fruit. In gymnosperms the bare ovule is sheltered by scales grouped in a cone. It turns into a seed after being directly fertilized by pollen grains contained in the male cones.
Whatever its origin, the seed houses the embryo of the future plant and nutrient reserves necessary for its development, the unit is protected by a more or less rigid shell called the integument.