Seed plant
| Seed Plants Temporal range: ???
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| Welwitschia mirabilis a member of the Gnetophyta | |
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The spermatophytes (from the Greek word "Σπερματόφυτα") (also known as phanerogams) comprise those plants that produce seeds. They are a subset of the embryophytes or land plants. The living spermatophytes form five groups:
- cycads, a subtropical and tropical group of plants with a large crown of compound leaves and a stout trunk,
- Ginkgo, a single species of tree,
- conifers, cone-bearing trees and shrubs,
- gnetae, woody plants in the genera Gnetum, Welwitschia, and Ephedra, and
- angiosperms, the flowering plants, a large group including many familiar plants in a wide variety of habitats.
Relationships and nomenclature
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 gymnosperms a paraphyletic group if it includes extinct taxa. Although not a monophyletic taxonomic unit, "gymnosperm" is still widely used to distinguish the four taxa of non-flowering, seed-bearing plants from the angiosperms.
Molecular phylogenies have conflicted with morphologically-based evidence as to whether extant gymnosperms comprise a monophyletic group. Some morphological data suggests that the Gnetophytes are the sister-group to angiosperms, but molecular phylogenies have generally shown a gymnosperm clade that includes the Gnetophytes as sister-group to the conifers.
A traditional classification grouped put all the seed plants in a single division, with classes for our five groups:
- Division Spermatophyta
- Cycadopsida, the cycads
- Ginkgoopsida, the ginkgo
- Pinopsida, the conifers, ("Coniferopsida")
- Gnetopsida, the gnetae
- Magnoliopsida, the flowering plants, or Angiospermopsida
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 tree genus in the ancient southern supercontinent of Gondwana during the Permian period. By the Triassic period, seed ferns had declined in ecological importance, and representatives of modern gymnosperm groups were abundant and dominant through the end of the Cretaceous, when angiosperms radiated. Another Late Paleozoic group of probable spermatophytes were the gigantopterids.
A more modern classification ranks these groups as separate divisions (sometimes under the Superdivision Spermatophyta):
- Cycadophyta, the cycads
- Ginkgophyta, the ginkgo
- Pinophyta, the conifers
- Gnetophyta, the gnetae
- Magnoliophyta, the flowering plants
References
- Bowe, L. Michelle, Gwénaële Coat, and Claude W. dePamphilis. 2000. Phylogeny of seed plants based on all three genomic compartments: Extant gymnosperms are monophyletic and Gnetales' closest relatives are conifers. Proceedings of the National Academy of Sciences 97: 4092-4097.
- Soltis, Douglas E., Pamela S. Soltis and Michael J. Zanis. 2002. Phylogeny of seed plants based on evidence from eight genes. American Journal of Botany 89: 1670-1681 (abstract here).
- Chaw, Shu-Miaw, Christopher L. Parkinson, Yuchang Cheng, Thomas M. Vincent, and Jeffrey D. Palmer. 2000. Seed plant phylogeny inferred from all three plant genomes: Monophyly of extant gymnosperms and origin of Gnetales from conifers. Proceedings of the National Academy of Sciences 97: 4086-4091 (abstract here).