| The swamp is one of the richest environments which algae grow in, especially desmids. The Aershan located in the northwest of the Daxing' anling's northeast region, the latitude is high, the air temperature is low, the humidity is high , the swamp distribution is broad but concentrated. Two third of the swamp types are in Aershan. Desmids is a very big population of Chlorophyta, the majorities are produced in fresh water, most are worldly living kind. As the factors mentioned above, we chose the algae which have broad distribution and large population for investigation and research. From August in 2005 to May in 2006, three times' collections were carried out successively in the Country Forest Park of Aershan, mainly aimed at observing and identifying 260 specimen. Cosmarium was totally identified as 128 classification units, including 117 species, 9 varieties and 2 forms, which was subjected to 2 order 20 genus of Zygnematales, among them , Desmidiaceae has 2 genus 2 species, Desmidiales has 18 genus 115 species 9 varieties 2 forms, including 7 species 2 varieties recorded newly in China.Desmids is a unique group, which includes Mesotaniaceae and Desmidiaceae of Zygnematales. According to the cell wall and the way of division, the Desmids are divided into saccoderm desmids and placoderm desmids. saccoderm desmids is a kind of Mesotaniaceae, the primitive type of desmids, except only one genus is connected by cells loosely into the filiform body with single-row cells, others are unicellular, cells are in the shape of cylinder , oval, spindly, rodlike etc; the placoderm desmids is species of Desmidiaceae, most of which are unicellular, minorities are filiform of single-row cell and colony of many cells, except few genus of the Closterium, the central section of every cell is concave more or less, forming sinus, the cells take on circular, oval, spindly, cylinder, rodlike, crescent, from triangle to polygon. Thereinto. it was reported that saccoderm desmids had 7 genus in the world, while placoderm desmids had 37 Genus, the total of the two is more than 6000 species. 6 genus 17 classification units of saccoderm desmids and 27 genus and more than 1000 classification units of placoderm desmids were reported in our country, among them. Cosmarium had 1300 species, 350 species were recorded in China.According to the numbers of species, Cosmarium is the most, it includes 37 classification units, which accounts for 28.9% of the total; then follows Staurastrum, it includes 26 classification units, which appropriately accounts for 20.3%; the next is Closterium , it includes 22 classification units, which accounts for 17.2%; the other 17 genus includes 43 classification units which accounts for 33.6%. This paper made a preliminary identification and carried out some simple analyses on the species composition and the ecological characteristics of desmids from the collection site mentioned above. The conclusions can be drawn as follows: (1) the desmids in this area have great varieties, the lower repetitiveness in different habitats, comparatively high diversities; (2) the area with various small habitats, major desmids grow in swamp, species in other habitats are relatively less; (3) in this area, the diversities and distribution characteristics of algae weren't affected by factors such as regional climate or geographic location etc., it depends on the microenvironment which it belongs to, thus different habitats have different species.Although desmids is the richest species in the Chlorophyta, so far only a handful people is engaged in desmids classification, both starting and research result appear a bit weak, therefore, this thesis firstly carried out a preliminary study on desmids in swamp areas of Arshan in Inner Mongolia, in order to provide a few fundamental materials to the investigation of desmids resources in our country.
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