An essential characteristic of a seed plant is that it begins its life as an embryonic plant inside the protective covering of a seed. When conditions are right—for example, when it has been exposed to water for a period of time—the seed germinates. As the young plant develops, stems and roots grow. The plant reaches maturity when it is able to reproduce by creating new seeds.
Seed plants that reproduce through the pollination of flowers are called angiosperms. Angiosperms have male and female parts that work together to create a new plant. The male parts are the stamens—a filament and anther (where the pollen is produced). The female part is the pistil—the stigma, style, and ovary (where the eggs are produced in ovules). When a flower is pollinated, often with the help of birds or insects, pollen lands on the sticky surface of the stigma. It then travels down the style, which serves as a pathway to the ovary. An egg fertilized by pollen becomes a zygote; as the zygote develops, it becomes an embryonic plant. The embryo, along with the endosperm that nourishes the embryo, is enclosed within the protective shell of a seed. Meanwhile, the surrounding ovary also grows and becomes the fruit that contains the seeds.
Seed plants vary greatly in the time that they take to cycle through their developmental stages. For example, some plants may take just a few weeks to reach maturity while others take years before they are able to produce seeds. Likewise, some flowers last just a few hours while others can last for weeks.
However, not all seed plants are flowering plants. Seed plants that do not have flowers—such as cycads, ginkgo, and conifers—are called gymnosperms. Conifers are common gymnosperms; instead of flowers, conifers have cones that produce pollen or eggs. Male cones are smaller and soft, and female cones are large and hard. Wind carries pollen from the male cone to the female cone. As the eggs are pollinated and seeds develop, the scales of the cone open up to release the seeds.
Furthermore, not all plants are seed plants. Some plants, such as ferns and mosses, reproduce with spores instead of seeds. Spores, like seeds, can survive harsh conditions and develop into new plants; however, unlike seeds, spores are produced without fertilization and contain neither a plant embryo nor endosperm. Some plants can reproduce without spores or seeds through vegetative reproduction, in which a part of the stem or root gives rise to a new plant.
The spermatophytes, also known as phanerogams or phenogamae, comprise those plants that produce seeds, hence the alternative name seed plants. They are a subset of the embryophytes or land plants. The term phanerogams or phanerogamae is derived from the Greekφανερός, phanerós meaning "visible", in contrast to the cryptogamae from Greek κρυπτόςkryptós = "hidden" together with the suffixγαμέω, gameein, "to marry". These terms distinguished those plants with hidden sexual organs (cryptogamae) from those with visible sexual organs (phanerogamae).
The living spermatophytes form five groups, the first four of which were traditionally grouped as "gymnosperms":
- cycads, a subtropical and tropical group of plants with a large crown of compound leaves and a stout trunk,
- Ginkgo, a single living species of tree,
- conifers, cone-bearing trees and shrubs,
- gnetophytes, woody plants in the genera Ephedra, Gnetum, and Welwitschia
- angiosperms, (or magnoliophyta) the flowering plants, a large group including many familiar plants in a wide variety of habitats.
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 ecological importance, and representatives of modern gymnosperm groups were abundant and dominant through the end of the Cretaceous, when angiosperms radiated.
Main article: Evolutionary history of plants § Seeds
A whole genome duplication event in the ancestor of seed plants occurred about 319 million years ago. This gave rise to a series of evolutionary changes that resulted in the origin of seed plants.
A middle Devonian (385-million-year-old) precursor to seed plants from Belgium has been identified predating the earliest seed plants by about 20 million years. Runcaria, small and radially symmetrical, is an integumented megasporangium surrounded by a cupule. The megasporangium bears an unopened distal extension protruding above the mutlilobed integument. It is suspected that the extension was involved in anemophilous (wind) pollination. Runcaria sheds new light on the sequence of character acquisition leading to the seed. Runcaria has all of the qualities of seed plants except for a solid seed coat and a system to guide the pollen to the seed.
Relationships and nomenclature
Further information: Gnetophyta § Classification
Seed-bearing plants were traditionally divided into angiosperms, or flowering plants, and gymnosperms, which includes the gnetophytes, cycads, ginkgo, and conifers. Older morphological studies believed in a close relationship between the gnetophytes and the angiosperms, in particular based on vessel elements. However, molecular studies (and some more recent morphological and fossil papers) have generally shown a clade of gymnosperms, with the gnetophytes in or near the conifers. For example, one common proposed set of relationships is known as the gne-pine hypothesis and looks like:
However, the relationships between these groups should not be considered settled.
Other classifications group all the seed plants in a single division, with classes for the five groups:
A more modern classification ranks these groups as separate divisions (sometimes under the Superdivision Spermatophyta):
An alternative phylogeny of spermatophytes based on the work by Novíkov & Barabaš-Krasni 2015 with plant taxon authors from Anderson, Anderson & Cleal 2007 showing the relationship of extinct clades.
Unassigned spermatophytes:
- ^Jiao Y, Wickett NJ, Ayyampalayam S, Chanderbali AS, Landherr L, Ralph PE, Tomsho LP, Hu Y, Liang H, Soltis PS, Soltis DE, Clifton SW, Schlarbaum SE, Schuster SC, Ma H, Leebens-Mack J, Depamphilis CW (2011) Ancestral polyploidy in seed plants and angiosperms. Nature
- ^"Science Magazine". Runcaria, a Middle Devonian Seed Plant Precursor. American Association for the Advancement of Science. 2011. Retrieved March 22, 2011.
- ^ abPalmer, Jeffrey D.; Soltis, Douglas E.; Chase, Mark W. (2004). "The plant tree of life: an overview and some points of view". American Journal of Botany. 91 (10): 1437–1445. doi:10.3732/ajb.91.10.1437. PMID 21652302.
- ^James A. Doyle (January 2006). "Seed ferns and the origin of angiosperms". The Journal of the Torrey Botanical Society. 133 (1): 169–209. doi:10.3159/1095-5674(2006)133[169:SFATOO]2.0.CO;2. ISSN 1095-5674.
- ^Zi-Qiang Wang (2004). "A New Permian Gnetalean Cone as Fossil Evidence for Supporting Current Molecular Phylogeny". Annals of Botany. 94 (2): 281–288. doi:10.1093/aob/mch138. PMID 15229124.
- ^Chaw, Shu-Miaw; Parkinson, Christopher L.; Cheng, Yuchang; Vincent, Thomas M.; Palmer, Jeffrey D. (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 (8): 4086–4091. doi:10.1073/pnas.97.8.4086. PMC 18157. PMID 10760277.
- ^Bowe, L. M.; Michelle, L.; Coat, Gwénaële; Claude (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 (8): 4092–4097. doi:10.1073/pnas.97.8.4092. PMC 18159. PMID 10760278.
- ^Soltis, Douglas E.; Soltis, Pamela S.; Zanis, Michael J. (2002). "Phylogeny of seed plants based on evidence from eight genes". American Journal of Botany. 89 (10): 1670–1681. doi:10.3732/ajb.89.10.1670. PMID 21665594.
- ^Chung-Shien Wu, Ya-Nan Wang, Shu-Mei Liu and Shu-Miaw Chaw (2007). "Chloroplast Genome (cpDNA) of Cycas taitungensis and 56 cp Protein-Coding Genes of Gnetum parvifolium: Insights into cpDNA Evolution and Phylogeny of Extant Seed Plants". Molecular Biology and Evolution. 24 (6): 1366–1379. doi:10.1093/molbev/msm059. PMID 17383970.
- ^Won, Hyosig; Renner, Susanne (August 2006). "Dating Dispersal and Radiation in the Gymnosperm Gnetum (Gnetales)—Clock Calibration When Outgroup Relationships Are Uncertain". Systematic Biology. 55 (4): 610–622. doi:10.1080/10635150600812619. PMID 16969937.
- ^Novíkov & Barabaš-Krasni (2015). "Modern plant systematics". Liga-Pres: 685. doi:10.13140/RG.2.1.4745.6164. ISBN 978-966-397-276-3.
- ^Anderson, Anderson & Cleal (2007). "Brief history of the gymnosperms: classification, biodiversity, phytogeography and ecology". Strelitzia. SANBI. 20: 280. ISBN 978-1-919976-39-6.
- Kron, Kathleen A; Chase, Mark W. Molecular systematics and seed plant phylogeny: a summary of a parsimony analysis of rbcL sequence data. pp. 243–252. , in Gibbs et al (1995)
- Gibbs, Adrian J.; Calisher, Charles H.; García-Arenal, Fernando, eds. (1995). Molecular basis of virus evolution. Cambridge: Cambridge University Press. ISBN 9780521022897.
- Soltis, D. E.; Soltis, P. S.; Zanis, M. J. (1 October 2002). "Phylogeny of seed plants based on evidence from eight genes". American Journal of Botany. 89 (10): 1670–1681. doi:10.3732/ajb.89.10.1670. PMID 21665594.