Scientists divide seed plants into two basic groups: gymnosperms, which do not produce flowers; and angiosperms, which do produce flowers. Unlike non-seed plants -- such as mosses, which reproduce by releasing spores -- seed plants utilize seeds for reproduction. Each seed consists of a plant embryo; food for that embryo and a protective casing, known as a coat.
According to Maricopa Community Colleges, gymnosperm-type plants evolved during the Paleozoic Era, which lasted from approximately 570 million to 245 million years ago; and became the dominant seed plant on the planet during the Mesozoic Era, which lasted from approximately 245 million to 65 million years ago. While many gymnosperms have gone extinct, about 700 species are still living. Scientists divide these gymnosperm species into four groups: gnetophytes, such as Ephedra; ginkgo, such as Ginkgo biloba; cycads, such as the sago palm tree; and conifers, such as spruce, redwood, fir and pine trees. As the University of Minnesota notes, all gymnosperms have "naked seeds," which means their seeds develop externally, on the surface of leaves known as sporophylls. Unlike angiosperms seeds, gymnosperm seeds do not have enclosed ovaries. In addition, the reproductive organs of gymnosperm seed plants are small and inconspicuous. These organs, known as sporangium, are the megasporangium, which produces eggs; and the microsporangium, which produces pollen.
Angiosperm seed plants evolved from their gymnosperm predecessors during the Cretaceous Era, which began about 140 million years ago, as Maricopa Community Colleges notes. All angiosperm plants produce flowers, which are groupings of modified leaves that in turn produce seeds and fruits. These flowers consist of stamens, which are the male reproductive organs that release pollen; carpels, which are the female reproductive organs that generate eggs; corollas, which are the typically colorful petals; and calyxes, which are the sepals or specialized leaves surrounding and supporting the petals. Unlike gymnosperm seeds, angiosperm seeds have enclosed ovaries, which develop into fruits. According to the University of Minnesota, there are approximately 250,000 living angiosperm plant species. Scientists divide these species into two groups: monocots, such as orchids, grasses and lilies; and dicots, such as peas, sunflowers and roses. While each monocot plant seed has one leaf, or cotyledon, each dicot seed has two leaves.
According to the University of California Museum of Paleontology, the division of seed plants into the basic gymnosperm and angiosperm categories is arbitrary. This is because scientists define gymnosperms by the absence of a particular feature -- flowers -- as opposed to defining them by shared characteristics.