Systematic Studies On The Genus Of Aristida

The genus Aristida L. which belongs to the family Poaceae, about 300 species, widely distributed in tropical and warm-temperate regions of the world and ten species (six endemic) in China. This genus is found on poor dry soils in areas of low rainfall, but does not usually penetrate into true desert. Due to its unique morphology and leaf anatomy structure, the systematic status of Aristida has still a great mater of debate.In this thesis, the systematic status of the Aristida has been documented by means of leaf abaxial epidermis, caryopsis morphology, and molecular systematics, which provide new evidences for systematic position of Aristida. The results are summarized as follows:Leaf abaxial epidermis. Light microscopes are used to examine leaf epidermal features of 40 species of the genus Aristida and related genera of the family Poaceae. Based on the leaf epidermis features, two types and five subtypes can be recognized. The results show that the type of silica-bodies of the genus Aristida shares the same type with the subfamily Panicoideae and Oryzoideae besides the basic feature such as long cell, stomata, however the same type of silica-bodies also is in some genera of the subfamily Arundinoideae. Other features of Aristida, such as prickle-hairs, micro-hairs, papillae, are also found in other subfamily. Then the results show the systematic position of Aristida has its peculiar status, which has close relationships with other subfamilies, and it is not reasonable that be transferred it to subfamily Arundinoideae or Chloridoideae.Caryopsis morphology. Light anatomical microscopes are used to examine caryopsis features of 36 species of the genus Aristida and related genera of the family Poaceae. Three caryopsis types and five subtypes can be recognized according to differences in ventral face and hilum morphology, First is the groove ventral face, corresponding with a needle-shaped hilum, Second type is the concave ventral face, corresponding with a linera hilum, the third type is the flat ventral face, corresponding with a round, elliptic or oblong hilum,. The results indicate that the genus of Aristida share the same features with the genera of Stipa, Achnatherum, Oryzopsis of subfamily Pooideae, and close hilum proportion and hilum morphology with subfamily Arundinoideae. All of these show that Aristida has its peculiar status which has close relationship with other subfamilies, and become the ligament among the species.Molecular phylogeny. The systematic rank of Aristida is discussed and examined the phylogenetic relationships between this genus and related genera in the Poaceae using ITS and trnL-F DNA fragment. The results show that Poaceae formed monophyletic group comprised of two large clades. Cladeâ… contains 4 subclades labeled A, B, C, D. Of these, subclades A, B, C correspond with the subfamily Chloridoideae, Panicoideae, Arundinoideae. And cladeâ…¡corresponds with subfamily Pooideae. However, the subclade D, that is Atistida clade, is the sister to these clades. The result indicate that it be proposed to remove this genus Aristida from the subfamily Arundinoideae or Chloridoideae, and its rank of Aristida should be promoted to the subfamily rank, that is Aristidoideae.In conclusion, Based on Leaf abaxial epidermis, Caryopsis morphology, Molecular phylogeny, the present study indicate that genus rank of Aristida should be promoted to the subfamily rank, that is Aristidoideae, which is sister to the subfamilies: Chloridoideae, Panicoideae, Arundinoideae...