Charophyta
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Mesostigmatophyceae | ||||||||
The Charophyta are a division of green algae,[1] that includes the closest relatives of the embryophyte plants.[2] In some groups, such as conjugating green algae, flagellate cells do not occur. The latter group does engage in sexual reproduction, and motility does not involve flagella, since they are totally lacking. Flagellate cells in the form of sperm are found in stoneworts (Charales) and Coleochaetales.
Classification
Because they exclude the embryophytes, the Charophyta make a paraphyletic group[3] (although the division Charophyta is occasionally restricted to simply the Charales or stoneworts, which are monophyletic). The Charophyta plus the embryophytes make up the Streptophyta, which is a monophyletic group.
Description
Charophyta are a small but important group of plants which show marked differences from both the Thallophyta and the Bryophyta. They are all specialized water plants with a highly peculiar structure and complex reproductive organs. Chlorophyll is the only pigment they possess, and they may be related to the Chlorophyceae; in fact some authorities have relegated them to that group as a separate order. The older view, and one which still has much to recommend it, was to regard them as a separate phylum, of equal rank with the Thallophyta, 'whose relationship to other phyla was unknown. The species are distributed throughout the world, but are most common in the warm and cool temperate zones. It may be mentioned that plants with a wide geographical distribution are often known to be of greater antiquity than those of restricted distribution, and the Charophyta are no exception to this rule, for remains of them are found as far back as the Devonian and Carboniferous rocks. Such forms differ little from the present-day types and throw no light on the ancestry of the group. Because of the fact that they are common and widely distributed they have been known from early times and were used by many early botanists as material for study and demonstration. For example, the cyclosis of cytoplasm was first demonstrated in the cells of Chara by Amici in 1818. The Charophyta are plants whose stems are either green or grey; the latter occurs in many species, resulting from the masking of the green colour by incrustations of lime on the walls. The main stems are slender and slightly branched. Lateral branchlets occur in whorls at regular intervals up the stem. The reproductive organs consist of antheridia and oogonia, though the structure of these organs differs considerably from the corresponding organs in the Algae. As a result of fertilization a protonema is formed from which the sexual plant is developed. The plants are submerged, and occur widely in fresh water or water containing less than I per cent. of salt. Thus they are found in the Baltic Sea, but not in other more saline waters. The water must be still, or only slowflowing. There must be no pollution due to sewage, and the plants must be exposed to bright light. They are frequently found in water charged with calcareous material. The phylum contains only one family, Characeae, with six genera and about two hundred species. We shall consider only one example, Chala fragilis; since the differences between the genera are small this species may be considered typical of the whole group.
Cell structure
The cells of the nodes are relatively small, with dense, granular protoplasm and a single nucleus. There are numerous small discoid chloroplasts, which are disposed around the periphery of the cells. No pyrenoids are present. The large internodal cells are sometimes multinucleate, and their nuclei often possess large nucleoli and scanty chromatin. In these cells the cytoplasm forms only a peripheral layer with a large central vacuole. The cell walls are composed of cellulose, though there may be also a superficial layer of a more gelatinous material of unknown composition. The storage material is starch, except in the oospore, where oil also occurs. This starch also accumulates in special storage structures, termed bulbils, which consist of rounded cells of varying size which are developed in clusters on the lower stem and root nodes. They are mainly developed when plants are growing in fine slimy mud.
Sexual reproduction
In their reproductive organs the Characeae show a high degree of specialization, and the structure of these bodies is unlike anything in other phyla of the Plant Kingdom. The female organ is a large oval structure with an envelope of spirally arranged, bright green filaments of cells. It is termed an oogonium. The male organ is also large, bright yellow or red in colour, spherical in shape, and is usually termed an antheridium, though some workers regard it as a multiple structure rather than a single organ. The sex organs are developed in pairs from the adaxial nodal cell at the upper nodes of the primary lateral branches, the oogonium being formed above the antheridium. They are sufficiently large to be easily seen with the naked eye, especially the bright orange or red antheridium. Many species are dioecious. In others the monoecious condition is complicated by the development of the antheridium before the formation of the oogonium, thus preventing fertilization by antherozoids of the same plant. In this case the two types of sex organs usually arise from different points on the lateral branches
Vegetative propagation
At almost any stage in the life of Characeae, vegetative propagation can occur. Secondary protonemata may develop even more rapidly than primary ones. Fragments of nodes, dormant cells of plants after hibernation or the basal nodes of primary rhizoids may all produce these secondary protonemata, from which fresh sexual plants can arise. It is probably this power of yegetative propagation which explains the fact that species of Chaaceae are generally found forming dense mats in the beds of ponds or streams, coverirtg quite large areas. It will be seen that very little comparison can be made between this and any other group of plants. That the Characeae are highly specialized is obvious, yet it is noteworthy that a fundamentally simple system of cellular nodes and internodes has been modified and adapted to serve very varied purposes in a striking and unique manner.
References
- ↑ Lewis, Louise A.; Richard M. McCourt (2004). "Green algae and the origin of land plants" (abstract). American Journal of Botany 91 (10): 1535–1556. doi:10.3732/ajb.91.10.1535. PMID 21652308.
- ↑ Karol KG, McCourt RM, Cimino MT, Delwiche CF (December 2001). "The closest living relatives of land plants". Science 294 (5550): 2351–3. Bibcode:2001Sci...294.2351K. doi:10.1126/science.1065156. PMID 11743201.
- ↑ http://tolweb.org/Green_plants/2382
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