Charophyta (UK: /kəˈrɒfɪtə,ˌkærəˈfaɪtə/ ) is a group of freshwater green algae, called charophytes (/ˈkærəˌfaɪts/), sometimes treated as a division,[2] yet also as a superdivision[3] or an unranked clade. The terrestrial plants, the Embryophyta emerged within Charophyta, possibly from terrestrial unicellular charophytes,[4] with the class Zygnematophyceae as a sister group.[5][6][7][8]
The clade Streptophyta may be formed by placing Embryophyta within the Charophyta.[9][10][11] The Embryophyta may already be included in the Charophyta, in which case it is a synonym of the Streptophyta.[12] The sister group of the charophytes are the Chlorophyta. In some charophyte groups, such as the Zygnematophyceae or conjugating green algae, flagella are absent and sexual reproduction does not involve free-swimming flagellate sperm. Flagellate sperm, however, are found in stoneworts (Charales) and Coleochaetales, orders of parenchymatous charophytes that are the closest relatives of the land plants, where flagellate sperm are also present in all except the conifers and flowering plants.[13] Fossil stoneworts of early Devonian age that are similar to those of the present day have been described from the Rhynie chert of Scotland.[14] Somewhat different charophytes have also been collected from the Late Devonian (Famennian) Waterloo Farm lagerstätte of South Africa. These include two species each of Octochara and Hexachara, which are the oldest fossils of Charophyte axes bearing in situ oogonia.
The Zygnematophyceae or, as they used to be called, Conjugatophyceae, generally possess two fairly elaborate chloroplasts in each cell, rather than many discoid ones. They reproduce asexually by the development of a septum between the two cell-halves or semi-cells (in unicellular forms, each daughter-cell develops the other semi-cell afresh) and sexually by conjugation, or the fusion of the entire cell-contents of the two conjugating cells. The saccoderm desmids and the placoderm or true desmids, unicellular or filamentous members of the Zygnematophyceae, are dominant in non-calcareous, acid waters of oligotrophic or primitive lakes (e.g. Wastwater), or in lochans, tarns and bogs, as in the West of Scotland, Eire, parts of Wales and of the Lake District.[15]
Klebsormidium, the type of the Klebsormidiophyceae, is a simple filamentous form with circular, plate-like chloroplasts, reproducing by fragmentation, by dorsiventral, biciliate swarmers and, according to Wille, a twentieth-century algologist, by aplanospores.[16] Sexual reproduction is simple and isogamous (the male and female gametes are outwardly indistinguishable).[16]
The various groups included in the Charophyta have diverse and idiosyncratic reproductive systems, sometimes with complex reproductive organs. The unique habit among the algae of protecting the overwintering zygote within the tissues of the parent gametophyte is one of several characteristics of Coleochaetales that suggest that they are a sister group to the embryophytes.[17]
The Charales or stoneworts are freshwater algae with slender green or grey stems; the grey colour of many species results from the deposition of lime on the walls, masking the green colour of the chlorophyll. The main stems are slender and branch occasionally. Lateral branchlets occur in whorls at regular intervals up the stem, they are attached by rhizoids to the substrate.[18] The reproductive organs consist of antheridia and oogonia, though the structures of these organs differ considerably from the corresponding organs in other algae. As a result of fertilization, a protonema is formed, from which the sexually reproducing algae develops.