The most familiar flowers of the angiosperms (flowering plants) are colorful, with the colors serving to attract insect or other animal pollinators. However, many plants, including grasses and many broad-leaved trees of the Northern Temperate Zone, have inconspicuous flowers. These are pollinated by wind, so colorful structures are not needed and are not produced. The essential features that define a flower are the reproductive structures at the center of the flower, not colorful petals. Stamens are the male structures of flowers. Stamens surround the female structures, the carpels. Stamens and carpels have many technical features that distinguish them from the reproductive organs of nonflowering plants. The nonflowering plants include the gymnosperms, which make seeds but do not make flowers, and the ferns, lycopods, mosses, and liverworts, which reproduce by means of tiny spores rather than by seeds.