Seed-bearing plants were traditionally divided into angiosperms, or flowering plants, and gymnosperms, which includes the gnetae, cycads, ginkgo, and conifers. Angiosperms are now thought to have evolved from a gymnosperm ancestor, which would make the gymnosperm taxon paraphyletic. Modern cladistics attempts to define taxa that are monophyletic, traceable to a common ancestor and inclusive therefore of all descendants of that common ancestor. Although not a monophyletic taxon, gymnosperm is still widely used to distinguish the four taxa of non-flowering, seed-bearing plants from the angiosperms.
A traditional classification grouped all the seed plants together as follows:
In addition to the taxa listed above, the fossil record contains evidence of many extinct taxa of seed plants. The so-called "seed ferns" (Pteridospermae) were one of the earliest successful groups of land plants, and forests dominated by seed ferns were prevalent in the late Paleozoic. Glossopteris was the most prominent treegenus in the ancient southern supercontinent of Gondwana during the Permian period. By the Triassic period, seed ferns had declined in importance, and gymnosperms were predominant until the Cretaceous, when the Angiosperms became predominant.
Phylogeny of the Spermatophyta
A more modern classification splits these groups into separate divisions (sometimes under the Superdivision Spermatophyta):
Seeds of the angiosperm, or flowering plant, differ from those of the gymnosperm, or conifer and related plants, in being enclosed in the ovary that later forms a fruit; gymnospermseeds lie exposed on the scales of the cones.
In a few plants, such as the orchids, the embryo is a small, undifferentiated mass of cells until after the seed has parted from the parent plant; during the period between separation from the parent plant and eventual germination, the undifferentiated cells develop into an embryonic root, bud, stalk, and leaf.
In most other plants this development occurs prior to seed dispersal: the embryonic root, or radicle, usually grows toward the micropyle; the embryonic bud, called plumule, or epicotyl, is at the end of the embryo opposite to the radicle; the embryonic stem, or hypocotyl, connects the radicle with the seed leaves, or cotyledons.
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