A seed plant or spermatophyte (lit. 'seed plant'; ), also known as a phanerogam (taxon Phanerogamae) or a phaenogam (taxon Phaenogamae), is any plant that produces seeds. It is a category of embryophyte (i.e. land plant) that includes most of the familiar land plants, including the flowering plants and the gymnosperms, but not ferns, mosses, or algae. The term phanerogam or phanerogamae is derived from the Greek φανερός (phanerós), meaning "visible", in contrast to the term "cryptogam" or "cryptogamae", together with the suffix γαμέω (gaméō), meaning "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 gymnosperms, plants that have unenclosed, "naked seeds": The fifth extant division is the flowering 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 many extinct taxa of seed plants, among those: 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 whole genome duplication event in the ancestor of seed plants occurred about. This gave rise to a series of evolutionary changes that resulted in the origin of modern 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. 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 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

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 which qualify as "seed ferns":