A seed plant or spermatophyte (; from Ancient Greek , ( spérma), meaning "seed", and ( phytón), meaning "plant"),
The term phanerogam or phanerogamae is derived from Ancient Greek φανερός]] (), meaning "visible", in contrast to the term "cryptogam" or "cryptogamae" (, and γαμέω]] (), 'to marry'). These terms distinguish those plants with hidden sexual organs (cryptogamae) from those with visible ones (phanerogamae).
Description
The extant spermatophytes form five divisions, the first four of which are classified as
, plants that have unenclosed, "naked seeds":
-
Cycadophyta, the cycads, a subtropical and tropical group of plants,
-
Ginkgophyta, which includes a single living species of tree in the genus Ginkgo,
-
Pinophyta, the conifers, which are conifer cone trees and shrubs, and
-
Gnetophyta, the gnetophytes, various woody plants in the relict genera Ephedra, Gnetum, and Welwitschia.
The fifth extant division is the , also known as angiosperms or magnoliophytes, the largest and most diverse group of spermatophytes:
-
Angiosperms, the flowering plants, possess seeds enclosed in a fruit, unlike gymnosperms.
In addition to the five living taxa listed above, the fossil record contains evidence of many
extinct taxa of seed plants, among those:
-
Pteridospermae, the so-called "seed ferns", 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 the
angiosperms radiated.
Evolutionary history
A series of evolutionary changes began with a whole genome duplication event in the ancestor of seed plants occurred about .
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.
Runcaria was followed shortly after by plants with a more condensed cupule, such as Spermasporites and Moresnetia. Seed-bearing plants had diversified substantially by the Famennian, the last stage of the Devonian. Examples include Elkinsia, Xenotheca, Archaeosperma, " Hydrasperma", Aglosperma, and Warsteinia. Some of these Devonian seeds are now classified within the order Lyginopteridales.
Phylogeny
Seed-bearing plants are a
clade within the
vascular plants (tracheophytes).
Internal phylogeny
The spermatophytes were traditionally divided into
, or flowering plants, and
, 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
. However, molecular studies (and some more recent morphological
and fossil
papers) have generally shown a
clade of
, 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
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):
Unassigned extinct spermatophyte orders, some of them formerly grouped as "Pteridospermatophyta", the polyphyletic "seed ferns".
Further reading
-
Thomas N. Taylor, Edith L. Taylor, and Michael Krings. 2008. Paleobotany: The Biology and Evolution of Fossil Plants, 2nd edition. Academic Press (an imprint of Elsevier): Burlington, MA; New York, NY; San Diego, CA, USA, London, UK. 1252 pages. .
