Cooksonia

= Cooksonia = Cooksonia is an extinct grouping of primitive land plants. The earliest Cooksonia date from the middle of the Silurian (the Wenlock epoch);[1] the group continued to be an important component of the flora until the end of the Early Devonian, a total time span of 433 to 393 million years ago. While Cooksonia fossils are distributed globally, most type specimens come from Britain, where they were first discovered in 1937.[4] 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. While reconstructions traditionally depict Cooksonia as a green and red, photosynthesising, self-sufficient stem, it is likely that at least some fossils instead preserve a sporophyte generation which was dependent on a gametophyte for its nutrition – a relationship that occurs in modern mosses and liverworts. However, no fossil evidence of a gametophyte of Cooksonia has been discovered to date.

For some years, it was suspected that Cooksonia and its species were poorly characterized. Thus four different kinds of spore, probably representing four different species, were found in sporangia originally identified as C. pertoni.

A 2010 study of the genus produced the consensus cladogram shown below (some branches have been collapsed to reduce the size of the diagram). This was based on data from an earlier study (by Kenrick and Crane[10]), supplemented by further information on Cooksonia species resulting from the authors' own research. 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 a rhizome that has not been preserved.[2] 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".[4] 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. Cooksonia and similar genera have been placed in a group called "cooksonioids". Originally the term was used for a group of plants fitting the general description of Cooksonia (i.e. simple plants with naked axes showing dichotomous branching and terminal sporangia), but with uncertain evidence of vascular tissue.[11] Boyce restricted the group to forms with axes usually less than 1 mm in diameter, and hence possibly not capable of independent growth. In addition to Cooksonia, he included genera such as Salopella, Tarrantia and Tortilicaulis.[2] Hue and Xao regarded cooksonioids as a group within the rhyniophytes with radially symmetrical sporangia of roughly the same height and width, and included Cooksonia pertoni, ''C. paranensis and C. hemisphaerica, but not C. crassiparietilis and Aberlemnia caledonica'', as they had bilaterally symmetrical sporangia. hes ''C. pertoni,C. paranensis and C. banksii''