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''Cooksonia'' is an extinct grouping of primitive land plants. The earliest ''Cooksonia'' date from the middle of the Silurian (the Wenlock epoch);〔 the group continued to be an important component of the flora until the end of the Early Devonian, a total time span of . While ''Cooksonia'' fossils are distributed globally, most type specimens come from Britain, where they were first discovered in 1937.〔 ''Cooksonia'' includes the oldest known plant to have a stem with vascular tissue and is thus a transitional form between the primitive non-vascular bryophytes and the vascular plants. ==Description== Only the sporophyte phase of ''Cooksonia'' is currently known (i.e. the phase which produces spores rather than gametes). Individuals were small, a few centimetres tall, and had a simple structure. They lacked leaves, flowers and roots — although it has been speculated that they grew from an unpreserved rhizome.〔 They had a simple stalk that branched dichotomously a few times. Each branch ended in a sporangium or spore-bearing capsule. In his original description of the genus, Lang〔 described the sporangia as flattened, "with terminal sporangia that are short and wide", and in the species ''Cooksonia pertoni'' "considerably wider than high". A 2010 review of the genus by Gonez and Gerrienne produced a tighter definition, which requires the sporangia to be more-or-less trumpet-shaped (as in the illustration), with a 'lid' or operculum which disintegrates to release the spores.〔 Specimens of one species of ''Cooksonia'' have a dark stripe in the centre of their stalks, which has been interpreted as the earliest remains of water carrying tissue.〔 Other ''Cooksonia'' species lacked such conducting tissue. ''Cooksonia'' specimens occur in a range of sizes, and vary in stem width from about 0.03 mm to 3 mm.〔 Specimens of different sizes were probably different species, not fragments of larger organisms: fossils occur in consistent size groupings, and sporangia and spore details are different in organisms of different sizes.〔 The organisms probably exhibited determinate growth (i.e. stems did not grow further after producing sporangia).〔 Some ''Cooksonia'' species bore stomata, which had a role in gas exchange; this was probably to assist in transpiration-driven transport of dissolved materials in the xylem, rather than primarily in photosynthesis, as suggested by their concentration at the tips of the axes.〔 These clusterings of stomata are typically associated with a bulging in the axis at the neck of the sporangium, which may have contained photosynthetic tissue, reminiscent of some mosses.〔 As the genus is circumscribed by Gonez and Gerrienne, there are six possible species. ''C. pertoni'',〔Two spellings are in use: the spelling used by the original author of the name, ''C. pertoni'', and the spelling ''C. pertonii'' (e.g. in ). The second is appropriate in botanical Latin when an epithet relates to a person, according to Article 60.11 of the International Code of Nomenclature for algae, fungi, and plants.〕 ''C. paranensis'' and ''C. banksii'' are all relatively similar with flat-topped, trumpet-shaped sporangia; stems are somewhat narrower in ''C. paranensis'' than in ''C. pertoni''. Only one specimen of ''C. bohemica'' is known. It has stouter, more branched stems; the original shape of the sporangia is unclear because of poor preservation. ''C. hemisphaerica'', described from the same locality as ''C. pertoni'', differs in having sporangia of which the tops, at least as preserved, are hemispherical rather than flat. ''C. cambrensis'' also has spherical sporangia, but without the gradual widening at the base characteristic of the other species. Preservation of the sporangia is again poor.〔 抄文引用元・出典: フリー百科事典『 ウィキペディア（Wikipedia）』 ■ウィキペディアで「Cooksonia」の詳細全文を読む スポンサード リンク
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