Seed plant

Aseed plantorspermatophyte(lit.'seed plant'; fromAncient Greekσπέρματος(spérmatos)'seed' andφυτόν(phytón)'plant'), also known as aphanerogam(taxonPhanerogamae) or aphaenogam(taxonPhaenogamae), is anyplantthat producesseeds. It is a category ofembryophyte(i.e. land plant) that includes most of the familiar land plants, including theflowering plantsand thegymnosperms, but notferns,mosses, oralgae.
The termphanerogamorphanerogamaeis derived from theGreekφανερός(phanerós), meaning "visible", in contrast to the term "cryptogam" or "cryptogamae" (fromAncient Greekκρυπτός(kruptós)'hidden', andγαμέω(gaméō), 'to marry'). These terms distinguish those plants with hidden sexual organs (cryptogamae) from those with visible ones (phanerogamae).
The extant spermatophytes form five divisions, the first four of which are classified asgymnosperms, plants that have unenclosed, "naked seeds":
The fifth extant division is theflowering plants, also known as angiosperms or magnoliophytes, the largest and most diverse group of spermatophytes:
In addition to the five living taxa listed above, the fossil record contains evidence of manyextincttaxa of seed plants, among those:
By theTriassicperiod, seed ferns had declined in ecological importance, and representatives of modern gymnosperm groups were abundant and dominant through the end of theCretaceous, when theangiospermsradiated.
A series of evolutionary changes began witha whole genome duplication eventin the ancestor of seed plants occurred about319million years ago.
A middleDevonian(385-million-year-old)precursorto seed plants fromBelgiumhas been identified predating the earliest seed plants by about 20 million years.Runcaria, small and radially symmetrical, is an integumentedmegasporangiumsurrounded by a cupule. Themegasporangiumbears an unopened distal extension protruding above the mutlilobedintegument. It is suspected that the extension was involved in anemophilous (wind)pollination.Runcariasheds new light on the sequence of character acquisition leading to the seed.Runcariahas all of the qualities of seed plants except for a solidseed coatand a system to guide the pollen to the seed.
Runcariawas followed shortly after by plants with a more condensed cupule, such asSpermasporitesandMoresnetia. Seed-bearing plants had diversified substantially by theFamennian, the last stage of the Devonian. Examples includeElkinsia,Xenotheca,Archaeosperma, "Hydrasperma",Aglosperma, andWarsteinia. Some of these Devonian seeds are now classified within the orderLyginopteridales.
Seed-bearing plants are acladewithin thevascular plants(tracheophytes).
The spermatophytes were traditionally divided intoangiosperms, or flowering plants, andgymnosperms, 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 onvessel elements. However, molecular studies (and some more recent morphologicaland fossilpapers) have generally shown acladeofgymnosperms, with the gnetophytes in or near the conifers. For example, one common proposed set of relationships is known as thegne-pine hypothesisand looks like:
However, the relationships between these groups should not be considered settled.
Other classifications group all the seed plants in a singledivision, withclassesfor the five groups:
A more modern classification ranks these groups as separate divisions (sometimes under theSuperdivision Spermatophyta):
Unassigned extinct spermatophyte orders, some of them formerly grouped as "Pteridospermatophyta", the polyphyletic "seed ferns".