Abstract: This paper summarized the different views of former authors about the origin and formation of the Chinese desert floras. Some authors considered that the Chinese desert floras formed with mixed elements of other floras during the Quarternary period because there were very few endemic genera and species in it. Others held that northwestern China had already been a continent since the Late Paleozoic era and the aridity had been enhanced especially during the Tertiary period, which indicates the ancientness of the Origin of the Chinese deserts. Besides, the presence of many monotypic and oligotypic genera in the desert flora can also explain it. There are also some authors, who thought that plants in Chinese desert flora are mostly descendants of the xerophytes of the Tethys, and thus the common species between northeastern China and Middle Asia reflect a more ancient relationship, while others considered that those common species reflect a more recent relationship. For tracing back to the formation of the Chinese desert floras, an analysis of some geographical elements that played a leading role in it is made. 1. Geographyical elements of the Tethys. In the genus Gymnocarpos G. decander is distributed along the Mediterranean coast, and G. przewalskii is distributed in Xinjiang, Gansu, Qinghai, Ningxia and Inner Mongolia in China. In the genus Cornulaca C. monacantha is distributed in Africa and C. alaschanica in the Alashan area, while in the genus Cynomorium C. coccineum is distributed in Sahara and C. songaricum in the sandy areas in China. These disjunctions reflect a more ancient relationship between floras of the Chinese sandy areas and the Tethys. There are five species in Frankenia in Africa, while only one species, E. pulverulenta,occurs from northern Africa to Xinjiang and Gausu via Middle Asia. In the genus Helianthemum there are eight species along the Mediterranean coast, while in Middle Asia and in western China there is only one species, H. songarium. At present many Mediterranean species are still present along the Tethys tracks, some of which stop in Xinjiang, e.g. Atriplex dimorphostegia; some stop in Hexi, Gansu, e.g. Capparis spinosa; some go deep into Ningxia, e.g. Peganum harmala and some extend eastwards even to the Middle Inner Mongolia, e.g. Populus euphratica. All these facts prove that weastern China joined together with the Tethys. After great changes had taken place in the earth history, disjunction occurred in some plant species: some became degraded and extinct, while others preserved. 2. Floristic elements of Middle Asia. The formation of plant genera in the Middle Asian flora is related to the Tethys flora and they further differentiated. The Middle Asian flora is closely related to those of our sandy areas. For example in the large genus Salsola, 22 species are distributed in Junggar of China and 59 in Middle Asia of the former USSR, while 25 are shared by the two floras. In the genus Zygophyllum 17 species are shared by the Middle Asian flora with 38 species of this genus and the Chinese flora with 16 species distributed in Junggar. Thus, a common trend can be recognized that the Middle Asian species are in common mostly with those in the Junggarn flora, and the more eastward, the fewer the common species are present. In the Middle Asian florra there exist many species of ephemeral plants and ephemeroides, but in the Chinese desert flora forty odd such plant species are distributed only in Junggar and mostly in common with those in the Middle Asian flora e.g. Euphorbia turzaninowii, Gagea bulbifera. In sandy areas of Middle Asia Haloxylon ammodendron and H. persicum are two of edificators, of which the former has its distribution in our desert areas, but the latter is distributed in the Junggar Basin only. The Middle Asian flora exerts its strongest influence on those of our sandy areas and the Junggarn flora is a part of the Middle Asian flora. 3. Floristic elements of Mongolia. In the Chinese sandy areas all the floras belong to the Mongolian floras except those of Xinjiang and Qinghai. The western part of the Mongolian flora is the Alashan desert and the eastern part is the sandy area in the steppe zone. The isopleths of certain metereorological factors in northern China show their distribution pattern as area roughly from southeast to northwest and differrent zones of steppe, desert steppe, steppe desert, and desert thus formed. Different geographical vicarious species are present in different zones. For instance, Caragana microphylla is distributed in the middle-temperate steppe zone, C. intermedia in warm-temerate steppe or desert steppe zone, and C. korshinskii is in the Alashan desert. Because of the different influences by the geographical positions, distances from the ocean, the ingression and regression of glaciers, and orogenesis, the origin and formation of floras vary in the different sandy areas. (1) Junggar Basin Here was still in ingression period during the Palaeozoic era. On account of the lifting of the Tienshan Mountains and Altai Moutains, lakes and seas migrated to the centre of the basin. During the Late Tertiary and Early Quarternary period the greater part of Eurasia and Siberia was covered by glaciers, so that the extent of desert diminished and a part of the xerophytic flora disappeared. It is clear, therefore, that the flora formed during the Quarternary period. Junggar is an inland basin surrounded by moutains from three directions. However, in the west many rivers lead to Middle Asia thouth there are also mountains. Therefore, the climate and flora are similar to those of Middle Asia. The Hexi Corridor located on the east of the basin is a migration channel for elements of the Alashan desert flora. The plant species in the basin is rather abundant but simpler than that of Middle Asia. Many endemic species of Zaisan and Balk-hash regions do not exist in Junggar where there are no endemic genera. Although several endemic species exist there, they are all Quarternary ones. (2) Tarim Basin It is surrounded by mountains in the south, north and west with Taklamakan desert at its central part. The longest inland river in the world, the Tarim River runs through the basin. It is located deep in the inland, very far from the ocean, with a warm-temperate climate and very little precipitation, and is rather poor in plant species. The tugayi forest is distributed only along the river. With the intensification of aridity halophytes increase. The floristic elements here are very different from those in the Junggar Basin. There are neither ephemeral and ephemeroid plants nor Haloxylon persicum. The species of Calligonum are different from the ones in the Junggar Basin. The Ammopiptanthus nanus, an endangered species, distributed northwest of Kashgar City, exhibits disjunction with A. mongolica in the Alashan desert. This is the only evergreen shrub genus in our sandy areas, obviously a subtropical relict, which may prove that there already xerophytes in the Tertiary period. In western Tarim there are Middle Asian elements, such as Populus euphratica. In the eastern part of Tarim there exist elements of the Mongolian flora , such as Sympegma regelii. In the Tarim basin there are many endemic species, e.g. Aristida grandiglomis, Caragana polourensis. The relic nature, disjunctions, and endomism may explain that there already existed xerophytes in the Early Tertiary period. There are either endemic elements or elements of Mediterranean, Middle Asia and Alashan, which may indicate that even in the Early Tertiary period plants already occurred and new developments took place in the Late Tertiary period in the basin. (3) Tsaidam Basin The basin was a part of the Tethys before the Mesozoic era and became land during the orogenesis. Its flora has evolved from the xerophytic tropical flora along the southern coast of the Tethys. The present-day dominant species of lowland and shifting sands such as Tamarix hispida, Nitraria sibirica, Haloxylon ammodendron, are similar to those in the Tarim Basin. There are only a few endemic species in the Tsaidam Basin, such as Salsola zaidamica, Calligonum zaidamense, that all originated in the Quarternary species. Some Mongolian elements had penetrated the eastern part. (4) Alashan Desert It lies deep in our hinterland and is abundant in endemic genera,such as Potaninia, Tetraena, Stilpnolepi, Elachanthemum, and oligotypic genera, such as Gymnocarpos, Cornulaca, Cynomorium, Pugionium, Ammopiptanthus, which reflects its ancient nature. Furthermore, there are 20-odd endemic species, such as Ephedra rhytidosperma, Synstemon petrovii Which have very narrow area, and they are all ancient in origin. The presence in the desert of these monotypic and oligotypic genera, and endemic species may prove that in the Tertiary period there were already xerophytic plants. Owing to the less influence of ingression and glaciation on the flora of this region more ancient elements could be preserved. However there have also been new developments during the Quarternary period. For example, certain species of Calligonum and Nitraria are newly-developed ones. (5) Eastern sandy areas in the steppe zone These areas obtain greater influence of the ocean; they have more precipitation, abundant floristic elements and lower topography. After the glaciation in the Tertiary and Quarternary periods plant species from several distribution centres, Mongolia, Europe, Middle Asia, East Asia, and North China, migrated towards this region simultaneously. So, there are no ancient genera here and the floristic elements are all rather young.

Key words: Chinese deserts, Flora genesis, Flora formation