Abstract. The flora and vegetation of the upper reaches of the Hongshui River, where northeastern Yun-nan, southwestern Guizhou and northwestern Guangxi meet, are discussed. This region is on the boundary between the southeastern part of ihe Yunnan-Guizhou Plateau floristic region of the Sino-Himalayan Floristic Subkingdom and the southwestern part of the Yunnan-Guizhou-Guangxi Floristic Region of the Sino-Japanese Floristic Subkingdom in the Holarctic Kingdom. Comparisons made with seven other regions of China, ranging from tropical to temperate, indicate that the upper reaches of the Hongshui River drainage are strongly tropical in nature. The flora is also rich in species and in the number of primitive families and genera it contains. The origin of this flora can perhaps be traced to the Tertiary Paleotropical flora and the region may be considered a conservation center for some primitive groups of seed plants. It also occupies an important transitional position between the Paleotropic flora and the Holarctic flora, and thus might have served as an important corridor for the migration of plants.

Key words. Distribution type, floristic characteristic, floristic region, floristic relationship, statistic analysis of flora.

The upper reaches of the Hongshui River, where northeastern Yunnan, southwestern Guizhou and north-western Guangxi meet, is situated between 22°40' and 27°03'N and between 102° 42' and 109°09'E and covers an area of 11,000 km². According to Wu's floristic regionalization scheme of China (Wu, 1979, 1983), the upper reaches of the Hongshui River occupy a position on the boundary between the southeastern part of the Yunnan-Guizhou Plateau floristic region of the Sino-Himalayan Floristic Subkingdom and the southwestern part of the Yunnan-Guizhou-Guangxi Floristic Region of the Sino-Japanese Floristic Subkingdom in the Holarctic Kingdom. Some xian (counties) such as Lapo, Debao, Jingxi and Pingguo, fall into the Gulf of Tonkin flo-ristic Region in the Paleotropic Kingdom. The demarcation line between the Paleotropic Kingdom and the Holarctic Kingdom traverses the southern part of the Sino-Himalayan Floristic Subkingdom and the Sino-Japanese Floristic Subkingdom along the Tropic of Cancer. According to the vegetational regionalization scheme of China (Wu, 1979), the eastern part of the upper reaches of the Hongshui River are located in the southern subtropical monsoonal evergreen broadleaved forest area of the eastern evergreen broadleaved forest subregion of the subtropical evergreen broadleaved forest region, while the western part is located in the southern subtropical monsoonal evergreen broadleaved forest area of the western evergreen broad-leaved forest subregion. It is noteworthy that in both floristic and vegetational regionalization schemes the eastern and western parts of the upper reaches of the Hongshui River are separated by Qinghuanglao Moun-tain, which is situated in the center of this region. In Takhtajan's (1978) floristic regionalization scheme of the world, the upper reaches of the Hongshui River are assigned to the Eastern Asiatic Floristic Region of the Holarctic, with the western part in the Sikang-Yunnan Floristic Province and the eastern part in the Central China Floristic Province. Takhtajan did not assign the southern part of this region to the Paleotropic Kingdom.

The facts mentioned above demonstrate the great phytogeographical importance of the flora of the upper reaches of the Hongshui River, where the Sino-Himalayan and Sino-Japanese floras have intermixed. Fur-thermore, the karst topography in this region is well developed and provides special habitats for the growth and development of a number of unusual plants, making the flora more peculiar. The flora of the upper reaches of the Hongshui River, therefore, has long been a source of interest and fascination to botanists.

Statistics of the floristic composition

Based on the 11,000 specimens collected by the Wild Plant Resource Exploration Group organized by the Chinese Academy of Sciences, along with some pertinent data previously reported, a checklist of the gymnosperms and angiosperms plants was compiled that lists 5,060 species in 1,292 genera and 177 fami-lies (Table 1) for the upper Hongshui River region. It should be noted that the flora referred to here actually includes only the flora of the area botanized by the exploration group. Although the plants from southeastern Yunnan and southern Guangxi are not included, geographically the two regions are also situated within the upper reaches of the Hongshui River.

Statistics and analysis of some important groups of seed plants

(1) Gymnosperms. Table 1 shows 34 species in 13 genera and seven families of gymnosperms in the upper reaches of the Hongshui River (excluding cultivated genera, such as Cycas, Ginkgo, Cryptomeria, Platycladus, Sabina, etc.). Thus, gymnosperms are rather poorly represented in this region. Of the 13 genera, Podocarpus (3 spp.) and Gnetum (4 spp.) are of pantropical distribution; Fokienia (1 sp.) and Caloce-drus (1 sp.) are of tropical Asian distribution; Taxus (2 spp.). Pinus (6 spp.) and Cupressus (1 sp.) are of north temperate distribution; Pseudotsuga (2 spp.) and Tsuga (2 spp.) are of eastern Asian and North American distribution; Cephalotaxus (3 spp.) and Keteleeria (7 spp.) are of eastern Asian distribution; both Pseudotaxus and Cunninghamia are monotypic genera endemic to China, the former occurring in Longqiang Xian of eastern China's Zhejiang Province, and also in Jiangxi, Hunan, northern Guangdong and central and northern Guangxi. The latter occurs widely in the regions south of the Qinling Range and Taiwan mountainous areas. The genus Keteleeria has 11 species worldwide. Two species are in Vietnam and the remaining nine species are in China, with one endemic to Taiwan and one endemic to Hainan. It is noteworthy that six species and one variety of Keteleeria are in the upper reaches of the Hongshui River, which indicate that this region is within the modern geographical distribution center for the genus.

(2) Statistics of genera of angiosperms with more than 20 species. Of the 1,279 angiospermous genera in the upper reaches of the Hongshui River, there are 32 genera with more than 20 species each, 21 of which are woody, seven are herbaceous, five are lianoid. These genera include 1,028 species and belong to 28 families. They account for only 2.6% of all the genera in this region, but their species account for 20.21 % of all species. Among the 32 genera, six are of cosmopolitan distribution, 14 are of pantropical distribution, six are of other types of tropical distribution, eight are of temperate distribution. Obviously, the genera with a primarily tropical distribution are more numerous than those with temperate distributions; the proportion of the large genera with more than 20 species to the total genera of this region is relatively low, while that of the species of these large genera to the total species is rather high. On the other hand, the number of species in eight genera within this area represent more than 50% of the total species for these genera in China, and they are all of a tropical nature. Some of these large genera, such as Castanopsis, Lithocarpus, Quercus, Machilus, Ficus, Eurya, Lindera, Ilex, Euonymus, Symplocos, Viburnum, Rhododendron and Rubus, are important constituents of the forest, or even the dominant elements in several different layers of the forest in this region.

(3) Statistics and analysis of families of angiosperms with more than 30 species. Of the 177 families of an-giosperms in the upper reaches of the Hongshui River, there are 44 families with more than 30 species (Tables 3 and 4), comprising 24.86% of the total families in the region. Among them, both the Papilionaceae (60 genera, 253 spp.) and the Asteraceae (81 genera, 229 spp.) have more than 200 species. Both of them, as is well known, are of cosmopolitan distribution, although the former are mostly distributed in temperate regions. In the Papilionaceae in this region, two genera are of cosmopolitan distribution, one is endemic to China, 41 are of tropical distribution and 16 are of temperate distribution. In the Asteraceae, eight genera are of cosmopolitan distribution, four are endemic to China, 32 are of tropical distribution and 37 are of temperate distribution. At the specific level, 169 species of Papilionaceae in this region are of tropical dis-tribution and 66 are temperate. In the Asteraceae, 87 species are of tropical distribution and 113 are of tem-perate distribution. Therefore, although the two families are important floristic elements of the upper reaches of the Hongshui River, the distribution patterns of their genera and species are quite different.

There are nine families with 101 to 200 species. Among them, the Poaceae, Rosaceae, Lamiaceae (mostly occurring in the Mediterranean) and Liliaceae (which mostly occur in the temperate and subtropical zones) are of cosmopolitan distribution; the other five, including Rubiaceae, Euphorbiaceae, Orchidaceae, Urticaceae and Lauraceae, are of pantropical, subtropical or temperate distribution. In these nine families, 15 genera are of cosmopolitan distribution, six are endemic to China, 213 are of tropical distribution, 99 are of temperate distribution. Thus, the genera with a tropical distribution pattern are much more numerous than those with temperate distributions. In the Rosaceae (tropical genera/temperate genera=3/22), Lamiaceae (13/20) and Liliaceae (7/19), however, the genera with tropical distributions are fewer, or even much fewer, than those with temperate distributions. This phenomenon may be explained by the fact that these three families occur mostly in the temperate zone. Members of the Rosaceae, as is well known, are very characteristic of the flora and vegetation of temperate areas in China. There are 16 families with 30 to 50 species.

The 44 families with more than 30 species mentioned above contain altogether 3,680 species (72.73% of the total species in this region) in 841 genera (65.09% of the total genera), and they have played the most important role in shaping the characteristics of the flora. Of the 841 genera, 44 are of cosmopolitan distribution, 36 are endemic to China (endemism in the Gesneriaceae is the richest, with 13 genera endemic to China), 514 are of tropical distribution and 251 are of temperate distribution. The tropical genera and the temperate genera account for 64.49% and 31.49% respectively of the total genera (excluding cosmopolitan ones).

(4) Analysis of important families. Which families can be considered to be important in a given flora? How should important families be chosen in a floristic analysis? These two questions have long been in dispute and there are not yet satisfactory answers to them. Such a situation may be explained by the fact that different regions have different geological histories and ecological environments, and thus their floras and vege-tation are composed of different associations of plants, and the degree of importance of a family or a genus in a given flora may change in response to fluctuations in the quality and quantity of the family or genus. Therefore, the criteria by which the important families are chosen are often determined by the author him-self who is working on a floristic study of a given flora. In this paper, the important families are chosen mainly based on two criteria: one is their degree of importance in phylogeny, that is to say, primitive families are preferred; the other is whether or not the members of a given family comprise the dominant species of the forest vegetation, especially of the basal zone of the vegetation. According to these two criteria, the Magnoliaceae, Theaceae, Hamamelidaceae, Lauraceae and Fagaceae were chosen for detailed discussion. The members in some of these families are not only primitive taxa, but also the dominant elements of the forest vegetation in this region.

The Magnoliaceae, consisting of 250 species in 15 genera worldwide. are generally considered to be one of the most primitive families of angiosperms. More than 100 species in eleven genera are found in China. Historically, this family once had a broader distribution than it does now. According to paleobotanical data, fossils of the genus Liriodendron have been discovered in North America, Greenland, Europe, Australasia and Japan, although at present this genus occurs disjuncily only in eastern North America and China. Similarly, fossils of the genus Magnoliahave also been found in North America, Greenland, Europe, Australasia and Japan, but at present this genus is disjunctly distributed in eastern Asia and North America and neighboring regions. Manglielia, a genus distributed in eastern Asia, comprises some 32 species, 20 of which are found in China, with 14 in the upper reaches of the Hongshui River. Two species, Manglietia megaphylla and M. grandis, might be among the rather primitive members of this genus. In M. megaphylla, the aggregate fruits are cylindrical or ellipsoid-globose, up to 10 cm in length, 6-7 cm in diameter, and consist of 90-100 follicles each with a long, apical beak. These characteristics are usually considered to be primitive in the Magnoliaceae. The genus Michelia is of tropical Asian distribution, comprising some 55 species, 35 of which are found in China, with 14 species in the upper reaches of the Hongshui River. They belong to the dominant elements of the forest vegetation in this region. The two monotypic genera, Parakmeria and Tsoongiodendron, both endemic to China, are generally considered to be ancient taxa among the angiosperms. though they might not be the most primitive group in the Magnoliaceae. The former is distributed in Guangxi, Guangdong, Hainan, Taiwan, Jiangxi. Fujian, Yunnan, Guizhou, Sichuan and Hunan provinces, while the latter is distributed in Guangxi, Guangdong, Hainan, Jiangxi, Fujian, Guizhou and Yunnan provinces. Law's (1984) study on the geographical distribution and origin of the Magnoliaceae (sensu stricto, excluding Illicium, Schisandra and Kadsura) shows that this family is most richly represented in Yunnan, Guangxi and Guangdong, and that the genus Manglietia, the most primitive genus in the family, and the most primitive species of the most widely distributed genus Magnolia, are found in this region. The endemic genera, monotypic genera, oligotypic genera, and endemic species in the Magnoliaceae are concentrated in these provinces.

The upper reaches of the Hongshui River are situated in the transitional region between eastern and northeastern Yunnan and western Guangxi. This region is within the range of the present differentiation center and the most important center for the conservation of the Magnoliaceae. Thirty-eight species in seven genera of this family are found in this region.

The Theaceae, consisting of 500 species in 30 genera worldwide, with 400 species in 14 genera in China, are widely distributed in tropical Asia and tropical America. Sixty seven species in ten genera occur in the upper reaches of the Hongshui River. Among the ten genera, Temstroemia (2 spp.) and Cleyera (4 spp.) are of pantropical distribution; Eurya (32 spp.) is disjunct in tropical Asia and tropical America; Adinandra is of tropical Asian-tropical African distribution; Schima (3 spp.), Hartia (2 spp.) and Camellia (15 spp.) are of tropical Asian distribution. The genus Tutcheria consists of about 20 species; except for one species in Vietnam, all the other species are found in China, with one species in the upper reaches of the Hongshui River. The species of Schima, Camellia and Eurya are important constituents of the plant communities in this region. For Stewartia and Gordonia, both with temperate distributions, this region is at the southern margin of their distribution range.

The Hamamelidaceae are disjunctly distributed around the world in tropical and subtropical areas in all but Europe and South America, but with the greatest diversity in eastern Asia. Some genera and subfamilies exhibit the classic pattern of disjunction between eastern Asia and eastern North America. Worldwide, the family comprises about 140 species in 31 genera, of which 75 species in 18 genera arc found in China. The phylogeny of this family has long been in dispute. Chang (1979) divided it into six subfamilies. Endress (1989) classified it into four subfamilies, merging Disanthoideae and Mytilarioideae into Exbucklandioideae. In their monographic study of the genus Disanthus Pan et al. (1991), they disagreed with Endress on the treatment of Disanthus and proposed that Disanihus should be placed in its own subfamily, as treated by Chang. The systematic treatment of Mytilaria has also remained a controversial matter for a long time. In this paper, the classification in Florae Reipublicae Sinicae is adopted for the convenience of discussion. In the upper reaches of the Hongshui River, 26 species in eleven genera of Hamamelidaceae are found. Except for Disanthoideae, the remaining five subfamilies are more or less represented. Exbucklandioideae is monogeneric, including only Exbucklandia with four species, of which one species is distributed only in Malaysia and Indonesia, the other three species are all found in China. with one species in this region. Rhodoleioideae is also monogeneric, including only Rhodoleia, which consists of nine species, six of which are endemic to China. Two species, R. championii and R. parvipetala, occur in the upper reaches of the Hongshui River. Both species have wide petals and thus are usually considered to be primitive in the genus (Chang, 1979). Mytilarioideae consist of two monotypic genera, Mytilaria and Chunia, but only Mytilaria is found in the upper reaches of the Hongshui River. Mytilaria ranges from Vietnam (or the Indo-Chinese Peninsula) to southern China (or southwest China), while Chunia is endemic to Hainan. The former has petals and thus is considered to be primitive in this subfamily, while the latter has no petals and is considered to be derived. Liquidambaroideae have three genera. Liquidambar, Semilicqidambar and Altingia. All are found in the upper reaches of the Hongshui River. Liquidambar has five species, two of which occur in China, and both are in this region. Among them, L. formosana seems to be rather primitive because of its dentate calyx, and L. acalycina might be a reduced species because of its non dentate calyx. Semiliquidambar is phylogenetically intermediate between Liquidambar and Altingia, consisting of three species endemic to China, only S. cathayensis occurs in the upper reaches of the Hongshui River. Altingia consists of eleven species, eight of which are found in China. Three species of Altingia, belonging to two different series, occur in the upper reaches of the Hongshui River. The Hamamelioideae consist of 19 genera in five tribes. Of the nine genera and four tribes found in China, five genera in four tribes occur in this region. Among them, Loropetalum is one of the most primitive genera in the subfamily.

As mentioned above, five subfamilies of the Hamamelidaceae are found in the upper reaches of the Hongshui River, and all four tribes in the subfamily Hamamelioideae, are found in this region. These observations demonstrate that this region is one of the modern distribution centers and an important conservation area for the Hamamelidaceae, Some of the plants mentioned above are very common accompanying elements in the forest vegetation of this region. In regard to their geographical distribution, seven genera are of tropical Asian distribution, one is of eastern Asian-eastern North American distribution, two are of eastern Asian distribution, and one is endemic to China.

The Lauraceae are a pantropical family, consisting of 2,000 to 2,500 species in 45 genera, of which 423 species in 20 genera are found in China. One hundred twenty two species in 13 genera occur in the upper reaches of the Hongshui River. This family is the ninth largest in this region. The genera with the most species are Lindera (29 spp.), Machilus (23 spp.), Cinnamomum (16 spp.) and Neocinnamomum (12 spp.). The genera of Lauraceae in this region are of six distribution types: pantropical (2 genera), tropical Asiatropical America (2 genera). Old World tropical (1 genus), tropical Asia-tropical Australasia (1 genus), tropical Asia (7 genera), eastern Asia-eastern North America (1 genus). Except for one genus, Lindera, with an eastern Asia-North America distribution, the remainder are tropical. Li (1979) studied the distribution of the Lauraceae in China and discussed the origin and development of the Chinese genera in this family. He pointed out that Lindera and Litsea might have originated in tropical Asia. Since many primitive species in these two genera occur in southern and western China, these regions might be within the range of the place of origin of the two genera. In any case, because of its richness in genera and species in the upper reaches of the Hongshui River, the Lauraceae not only play an important role in the composition of plant communities in this region, but also can not be neglected for the determination of the floristic characteristics in the upper reaches of the Hongshui River. As pointed out by Li (1979), the occurrence of typical temperate tree species in the western and southwestern mountainous regions of China and the existence of an evolutionary series from evergreen trees to deciduous trees in those areas might indicate that the southern and southwestern mountainous region of China is the cradle for the majority of the temperate floras or even the entire temperate flora.

The Fagaceae are a cosmopolitan family, mostly distributed in temperate regions, though sometimes ex- tending their distribution to tropical mountains. The family consists of about 900 species in eight genera; 210 species in six genera occur in China and 88 species in five genera occur in the upper reaches of the Hongshui River. All five genera have a temperate distribution; three have a north temperate distribution and two have an eastern Asia-North America distribution. As is well known, Fagus consists of deciduous trees, Castanea and Quercus consist of both evergreen and deciduous trees, Castanopsis and Lilhocarpus consist of completely evergreen trees.

As to the origin of this family, Fedorov (1959), claimed that the composition of the genera showed that they essentially might have originated in the tropics. While studying the flora of Guangdong, Chang (1965) pointed out that Cyclobalanopsis is very likely to have originated in the mountainous regions of southern China along the Tropic of Cancer, Castanopsis developed in southern China, and Lithocarpus might have its primitive groups concentrated in the islands of eastern Asia near the Equator. In the tropical regions of eastem Asia there is not another genus like Lithocarpus that so closely connects the Malaysian flora and the flora of southern China. In the vast subtropical and tropical regions of China, including the tropical mountainous regions, the Fagaceae are important constituents of the forest vegetation, even in the basal zone of the vegetation. Eighty eight species of Fagaceae occur in the upper reaches of the Hongshui River, making up about one tenth of all the species in the family. Some species are dominant elements of the subtropical evergreen broadleaved forests, of the mixed evergreen and deciduous broadleaved forests and of the deciduous broadleaved forests in the upper reaches of the Hongshui River. Some species even form pure forests.

Distribution types of the genera

As mentioned above, there are about 5,060 species in 1,292 genera of 177 families of seed plants in the upper reaches of the Hongshui River. Based on the modern distributions of the genera and Wu's (1991) division of the distribution types of the Chinese genera of seed plants, the distribution of the 1,292 genera fall into 15 types.

(1) Cosmopolitan. Sixty-seven genera in 37 families, including four woody, 59 herbaceous and four others which contain both woody and herbaceous, or herbaceous lianoid. are of this distribution type. families containing five or more genera are Poaceae (6 genera, 15 spp.). Lamiaceae (5 genera, 36 spp.). Asteraceae (6 genera, 19 spp.) and Cyperaceae (5 genera. 70 spp.): those with three or four genera are Ranunculaceae (3 genera, 42 spp.), Brassicaceae (3 genera, 8 spp.), Apiaceae (3 genera. 8 species). The remaining families contain only one or two genera. The cosmopolitan genera in this region represent 64.4% of the 104 genera in China with this distribution type. The species of these genera number 519. accounting for 15.1 % of the 3,442 species in China with this distribution type, and 10.26% of the all the species in this region. Genera with ten or more species are Carex (43 spp.), Polygonum (35 spp.), Lysimachia (29 spp.), Rhamnus (25 spp.), Hxperlcum (19 spp.). Solanum (19 spp.), Viola (16 spp.), Salvia (15 spp.), Liparis (13 spp.), Cyperus (12 spp.) Scutellaria (10 spp.) and Lobelia (10 spp.). These genera are common plants of the herbaceous layer of the forests in this region. Because information derived from the distribution of cosmopolitan genera are uninformative for the determination of the floristic characteristics of this region, they are excluded from the statistical data.

(2) Pantropical. In the upper reaches of the Hongshui River, 239 genera in 80 families, including 85 woody, 125 herbaceous, seven lianoid, and 22 others, are of pantropical distribution. These genera account for 19.5% of the total genera in this region and 65.5% of the 365 genera in China with this distribution type. These figures reveal the important position pantropical genera occupy in the flora of this region, and that this region might be within the center of this distribution type or at least in the neighborhood of the center. Families with five or more genera with this distribution type are Poaceae (31 genera, 63 spp.), Fabaeeae (24 genera, 166 spp.), Asteraceae (12 genera, 77 spp.), Euphorbiaceae (10 genera, 75 spp.), Rubiaceae (.10 genera, 59 spp.), Convolvulaceae (9 genera, 17 spp.), Cyperaceae (7 genera, 26 spp.) Verbenaceae (5 genera, 52 spp.) and Acanthaceae (5 genera, 5 spp.). These genera contain 1,303 species, which account for 28.7% of the all the species in this region and 32.5% of the 3,997 species of genera in China with this distribution pattern. The number of species is the largest among the 15 distribution types. Thirty nine genera have ten or more species, among which are Celtis (14 spp.), Ficus (75 spp.), Boehmeria (22 spp.), Pilea (33 spp.), Aristolochia (17 spp.), Acacia (10 spp.), Bauhinia (19 spp.), Crotalaria (12 spp.), Dalbergia (13 spp.), Indigofera (17 spp.), Millettia (21 spp.), Smilax (28 spp.) and Dioscorea (21 spp.). As mentioned above, the genera with pantropical distribution, and the species of these genera, make up a high proportion in this flora; even some typical tropical elements are often found to occur in this region. On the other hand, some pantropical genera are only very rarely represented in this flora. Alternanthera, for example, has about 200 species worldwide, but only two species are in the upper reaches of the Hongshui River; Portulaca has 200 species worldwide, but only one species is found in this region; similarly, only one species in each of Eryngium (230 spp.), Planchonella (102 spp.), Strychnos (200 spp.), Aristida (330 spp.), Eutophia (200 spp.) is found in this region. Another noteworthy example is Nertera (Rubiaceae). This genus has ten species distributed in Malaysia, Australasia (New Zealand and Australia), the Philippines and South America (Chile). Three species are found in China, with two in Taiwan and one on the southern Chinese mainland. One species occurs in the upper reaches of the Hongshui River, which is undoubtedly at the northern edge of the distribution of the genus. In the upper reaches of the Hongshui River, such phenomena are often found. That is, many plants in this region are far from their center of distribution. This indicates that the flora of this region occupies a transitional position between the subtropical flora and the warm temperate flora.

(3) Tropical Asia-Tropical America. Twenty-nine genera in 25 families, including 15 woody, eight herba- ceous, one lianoid and four others, are of tropical Asian and tropical American distribution, making up 2.4% of the genera in this region and 40.8% of the 71 genera in China with this distribution type. These genera contain 118 species, accounting for 25.2% of all the species in genera with this distribution type in China and 2.6% of all the species in this region. Some of the plants with this distribution type, such as Eurya (32 spp.) and Meliosma (19 spp.), are the dominant plants in the understory of forests in this region.

(4) Old World Tropical [Tropical Asia. Africa (or East Africa. Madagascar)]. In the upper reaches of the Hongshui River, 109 genera in 54 families. including 42 woody, 45 herbaceous, 18 lianoid and four others, are of the Old World tropical distribution type. They account for 8.9% of all genera in this region and 61.9% of the 176 genera in China with this distribution type. These genera contain 504 species. accounting for 11.1% of the total species in this region and 33.6% of the species of the genera in China with this distribution type. The species of some of these genera, such as Melia, Alangium, Piltosporum, Albizzia, Mallotus, Viscum and Elatostema, have distributions that extend northward into temperate regions. Some genera with this distribution type, such as Elatostema (39 spp.), Stephania (16 spp.), Pittosporum (18 spp.) and Syzygium (15 spp.), are common or dominant elements in this region.

(5) Tropical Asia and Tropical Australasia. Sixty-nine genera in 43 families, including 26 woody, 32 her- baceous and eleven lianoid are of this distribution type. They account for 5.6% of the genera in this region and 46.3% of the genera in China with this distribution type. Only one family, Orchidaceae (9 genera), has more than five genera with this distribution type in the region. The 69 genera contain 242 species, accounting for 5.4% of all the species in this region and 29.0% of the 843 species of the genera with this distribution type in China. In some genera, nearly half or more of the species found in China occur in this region. These genera include Helicia (11/ 18= species in the upper reaches of the Hongshui River/species in China), Cinnamomum (16/45), Tetrastigma (20/45), Mitrasacme (10/18), Hoya (12/22), Wendlandia (12/23), Trichosanthes (15/40) and Zingiber (10/14). Some woody genera with this distribution type, such as Helicia and Cinnamomum. are very important constituents in the evergreen forests in tropical and subtropical regions. Other genera with this distribution type. such as Ailanthus (2/6), Toona (1/3), Mazus (5/30), Trichosanthes and Leptopus (2/9), have ranges that extend northward into temperate regions. Some members have disjunct distributions. Horsfieldia hainanensis, for example, is disjunctly distributed on Hainan and along the Gulf of Tonkin in Guangxi. The genus Nothopanax might be taken as another ex- ample. This genus includes about 13 species, with two species endemic to China and 11 species in Australasia (mainly in New Zealand). The distributions of some of the above genera indicate that many of the components of the flora of the upper reaches of the Hongshui River are closely related to the tropical flora.

(6) Tropical Asia and Tropical Africa. Seventy-six genera in 35 families, including 23 woody, 41 herbaceous, 11 lianoid and one other, are of this distribution type. They account for 6.2% of the total genera in this region and 45.0% of the 169 genera with this distribution type in China. The families with five or more genera of this distribution type are Acanthaceae (5 genera, 13 spp.), Asteraceae (9 genera, 23 spp.) and Poaceae (7 genera, 20 spp.). These 76 genera contain 177 species, accounting for 3.9% of the total species in this region and 28.1% of the 630 species in genera with this distribution type in China. Only one genus, Premna (11 spp.) of the Verbenaceae, has more than ten species. It is noteworthy that the upper reaches of the Hongshui River include the distribution center of some groups with this distribution type. For example, Debregeasia (Urtfcaceae) has five species worldwide and is distributed in North Africa and southeastern Asia, with four species in southwest and south-central China and Taiwan, three species of which, i.e. D. longifolia, D. orientalis and D. squamata, are found in the upper reaches of the Hongshui River. Debregeasia orientalis is disjunctly distributed in south-central China, Taiwan and southern Japan. The genus Roureopsis (Connaraceae) has tea species and is distributed in tropical Africa, Malaysia and Southeast Asia, with one species in China and also in the upper reaches of the Hongshui River, where it might be at the northern edge of the distribution range for the genus. Both Toddalia (Rutaceae) and Parochetus (Fabaceae) are naonotypic genera and are found in this region. The genus Woodfordia(Lythraceae) has two species, one in Ethiopia and the other in Madagascar, India and Yunnan and Guangxi provinces, China. Neyraudia (Poaoeae) has three species and is distributed in tropical and subtropical regions of the eastern hemisphere, with two species in China, and also in the upper reaches of the Hongshui River. Myrsine (Myrsinaceae) has seven species and has a distribution in the Azores, Africa, Madagascar. Arabia, Pakistan, Afghanistan, northeastern India and China, with four species in the Chang Jiang (Yangtze River) region in China, three of which are found in the upper reaches of the Hongshui River.

(7) Tropical Asia (Indo-Malaysia). Tropical Asia has one of the richest floras in the world. The northern border of this region extends to southern China and southern Taiwan, with extensions into more northern regions. Thus, there are a great number of genera with a tropical Asian distribution within the Chinese flora. In the upper reaches of the Hongshui River, 277 genera in 71 families, including 127 woody, 110 herbaceous, 37 lianoid and three others, are of tropical Asian distribution, accounting for 22.6% of the genera in this region and 44.62% of the genera with this distribution type in China. These genera contain 760 species, or 16.7% of all species in this region and 28.8% of the species in genera with this distribution type in China. Families with five or more genera belonging to this distribution type are Orchidaceae (19 genera/61 spp.), Acanthaceae (16 genera/26 spp.), Fabaceae (13 genera/35 spp.), Euphorbiaceae (13 genera/33 spp.), Gesneriaceae (12 genera/49 spp.), Apocynaceae (12 genera/25 spp.), Rubiaceae (10 genera/40 spp.), Araceae (9 genera/30 spp.), Asclepiadaceae (8 genera/16 spp.), Melastomataceae (7 genera/30 spp.), Lamiaceae (7 genera/25 spp.), Menispermaceae (7 genera/ 13 spp.), Araliaceae (6 genera/23 spp.), Annonaceae (6 genera/8 spp.), Hamamelidaceae (5 genera/10 spp.) and Poaceae (5 genera/8 spp.). Most of the families are tropical. Other typically tropical families, such as Dipterocarpaceae, Sarcospermataceae, Myristicaceae, Passifloraceae, Sapotaceae, Proteaceae and Dilleniaceae, are also found in the upper reaches of the Hongshui River. For example, the genus Vatica (Dipterocarpaceae) has about 65 species and is distributed from India and Malaysia through Indochina to southern China. Three species of the genus occur in China: V. xishuangbannaensis is endemic to Xishuangbanna, Mengia Xian, Yunnan, V. mangachapoi has its distribution on Hainan Island, China, Vietnam, Thailand, the Philippines and Indonesia, and V. guangxiensis is endemic to Lapo County in the upper reaches of the Hongshui River where it is at the northern limit of the range of distribution for the genus. Bennettiodendron (Flacourtiaceae), which has a tropical Asian distribution, contains six species, two of which are found in the upper reaches of the Hongshui River at the northern border of the distribution range for the genus. The center of distribution for the genus extends from the upper reaches of the Hongshui River to southeastern Yunnan. The genus Chukrasia (Meliaceae), with two species, also has the northern border of its distribution within the upper reaches of the Hongshui River.

Within this distribution type, some genera are monotypic or oligotypic and their distributions are of special interest. The monotypic family Rhoipteleaceae includes only Rhoiptelea, and is distributed in northern Vietnam and southernmost China. The genus Annamocarya(Juglandaceae) is monotypic and ranges from southeast Guizhou to western Guangxi and southeast Yunnan to northern Vietnam. The genus Erythropalum (Olacaceae) has three species and is distributed in India, Burma, Vietnam, the Philippines, Malaysia and Indonesia; the only species occurring in China is distributed in the southern mountainous regions, including the upper reaches of the Hongshui River. The genus Mytilaria (Hamamelidaceae) is monotypic and is distributed in Guangdong, Guangxi, Yunnan and Vietnam. The genus Acrocarpus (Fabaceae) includes two species and is of typical tropical Asian distribution, with A. fraxinifolius occurring in western Guangxi and in Xishuangbanna, Yunnan. The genus Urariopsis, also in the Fabaceae, is monotypic and distributed in India, Guangxi and southern Yunnan. The genus Tirpitzia (Linaceae) has two species and is distributed in Vietnam, Guangxi, Guangdong, Guizhou and Yunnan. The genus Cladogynos (Euphorbiaceae) has one species with several varieties and is mostly distributed in Malaysia, with one variety oc- curring in China and also in the upper reaches of the Hongshui River. The genus Mappianthus (Icacinaceae) has two species, one in Kalimantan and the other in southern China. The genus Boniodendron (Sapindaceae) is another monotypic genus growing in limestone mountains in the upper reaches of the Hongshui River. The genus Sladenia (of uncertain family status, although near Actinidiaceae and Theaceae) is monotypic and distributed in Thailand, Burma and Guangxi and Yunnan in China. In Asclepiadaceae, Dischidanthus includes only D. urceolalus and is distributed in Vietnam, Laos, Cambodia and southern China; Graphistemma has only C. pictum and occurs in Vietnam and southern China: Myriopteron is of typical tropical Asian distribution and contains only M. extensum. which is found also in the upper reaches of the Hongshui River; Stelimitocrypton has only S. khasianum and is distributed in India and Guangxi, Guizhou and Yunnan, China. Paralamium (Lamiaceae) includes only P. gracile and is distributed in Burma, Vietnam and southern Yunnan in China. Legazpia (Scrophulariaceae) includes two species and is distributed in Southeast Asia and Australasia, with L. polygonoides in Guangdong and Guangxi. Picria (Scrophulariaceae) has only P. felterrae and is distributed in India and in Hainan and Guangxi, China. Mayodendron (Bignoniaceae) includes only M. igneum, and occurs in Burma and southern China.

In the upper reaches of the Hongshui River, some plants are indicators of limestone topography. Such plants include Parashorea chinensis var. kwangsiensis (Dipterocarpaceae), Excentrodendron hsienmu, Burretiodendron esquirolii and B. longistipitatum (Tiliaceae) and Cleidiocarpon cavaleriei and Cephalo- mappa sinense (Euphorbiaceae), all of which are constituents of tropical rain forests. Cauliflorous species of the genus Ficus (Moraceae) with buttressed roots are also included in this group.

As mentioned above, this distribution type not only represents the richest floristic component in the upper reaches of the Hongshui River, but also contains some ancient or primitive monotypic and oligotypic genera. These features reveal a close relationship between the flora of the upper reaches of the Hongshui River and the Indo-Maiaysian flora.

(8) North Temperate. Genera widely distributed in the temperate regions of Europe, Asia and North America are considered to belong to the North Temperate type of distribution. In the upper reaches of the Hongshui River, 128 genera in 54 families, including 39 woody, 87 herbaceous, one lianoid and one other, are of this distribution pattern. These genera account for 10.4% of all genera in the upper reaches of the Hongshui River and 32.2% of the 303 genera with this distribution type in China. Among them, several genera are disjunctly distributed in the temperate regions of both the northern and southern hemispheres. They are Myrica, Urtica, Thalictrum, Sedum. Vaccinium, Swertia, Veronica, Adenocaulon, Sagittaria and Trisetum. There are 580 species in the 128 genera with this distribution type, accounting for 12.8% of all the species in the upper reaches of the Hongshui River and 7.3% of all species in genera in China with this distribution type. Many tree genera with typical north temperate distribution are represented in the upper reaches of the Hongshui River, such as Populus (3 species in this region/62 species in China), Salix (11/240), Alnus (3/10), Carpinus (11/22), Fagus (3/5), Quercus (34/11O), Ulmus (5/23), Morus (5/16), Berberis (4/200), Cerasus (2/68), Sorbus (5/60), Acer (27/150), Rhododendron (40/650) and Fraxinus (5/27). The herbaceous genera number 81, and are more numerous than the woody genera, but the species in these genera are usually very few, such as in Aconltum (1/80), Delphinium (2/135), Thalictrum (7/73), Corydalis (7/200), Rhodiola (1/73), Sedum (8/124), Saxifraga (1/40), Veronica (7/64), Artemisia (19/170) and Polygonatum (6/31). These figures show that many herbaceous genera with typical north temperate distribution are only poorly represented in the upper reaches of the Hongshui River. These herbaceous genera are usually not the dominant constituents of plant communities, and only occasionally become the dominant elements of the understory herbaceous plant communities within the forests in the upper reaches of the Hongshui River. Instead, many of the woody genera with this distribution type are the dominant elements of the conifer forests, deciduous broadleaved forests and of the shrub layer in this region

(9) East Asia and North America. Since Asa Gray (1846) first focused attention on the close floristic rela- tionship between eastern Asia and eastern North America, many botanists have discussed this phytogeographical phenomenon. In the upper reaches of the Hongshui River, 55 genera in 30 families, including 34 woody, 16 herbaceous and five lianoid, are of this distribution type, accounting for 4.5% of the genera in this region and 43.7% of the genera with this distribution type in China. These genera include 255 species, accounting for 5.6% of the total in this region and 28.4% of the species in genera with this distribution type in China. Some oligotypic genera with this distribution type are noteworthy, such as Saururus, Antenoron, Liriodendron, Penthorum, Tiarella. Liquidambar, Gymnocladus, Cladrastis, Pachysandra, Gelsemium, Campsis, Phryma and Acorus. The intercontinental distributions of' these genera indicate that eastern Asia and North America might have been under the influence of similar climatic events throughout their geological histories, and in the origin and development of their floras. Caslanopsis, Lithocarpus, Photinia and Aralia are the dominant constituents or common elements of the forests in the upper reaches of the Hongshui River. These latter genera, except for Ara!ia. have their North American distributions restricted to the West Coast.

(10) Old World temperate and its subtypes. In the upper reaches of the Hongshui River, 48 genera in 22 families, including seven woody and 41 herbaceous, belong to the Old World temperate distribution type, or to one of its subtypes, accounting for 3.9% of the total genera in this region and 28.7% of the genera with this distribution in China. The important families are Asteraceae (11 genera, 29 spp.), Lamiaceae (6 genera, 24 spp.) and Apiaceac (6 genera, 15 spp.). These three families are cosmopolitan or mainly distributed in temperate zones. The 48 genera with this distribution type contain about 132 species in the upper reaches of the Hongshui River, accounting for 2.9% of all the species in this region and 9.6% of the species in genera with this distribution type in China. Most of the species in genera with this distribution type are either common or rare plants on forest margins or on roadsides, and only a few are important elements of plant communities. Thus, genera belonging to this distribution pattern are not important floristic elements in the upper reaches of the Hongshui River.

(11) Temperate Asia. In the upper reaches of the Hongshui River, only eight genera in six families, including two woody and six herbaceous. are of this distribution type. They account for 0.7% of the total in this region and 13.8% of the genera with this distribution type in China. These eight genera contain 23 species, accounting for 0.5% of the total species in this region and 6.5% of the species in genera with this distribution type in China. Most of the species are very rare. Thus, genera with this distribution type in Hongshui region are poor indicators of the floristic characteristics of the area.

(12)Mediterranean, West Asia to Central Asia. In the upper reaches of the Hongshui River, only seven genera in six families, including two woody and five herbaceous, are of this distribution type. They account for 0.6% of the total genera in this region and 4.1% of genera with this distribution type in China. The seven genera contain eleven species, accounting for 0.2% of the total species in this region and 2.2% of species in genera with this distribution type in China. Most of the species, except for species of Spinacia and Pisum, which are cultivated as vegetables in this region, are scarcely represented. This basically reflects the obvious; that the upper reaches of the Hongshui River are floristically only distantly related to the Mediterranean region and western and central Asia.

(13) Central Asia. In the upper reaches of the Hongshui River, three herbaceous genera with ten species are of this distribution type. The monotypic genus, Cannabis. which is endemic to Central Asia, is widely cultivated in China, and also in the Hongshui region as an important fiber plant. The genus Incarvillea (Bignoniaceae) has 16 species worldwide, 13 species of which occur in China, but only one is in this region and might be at the southernmost edge of the distribution range for the genus.

(14) Eastern Asia. Genera distributed from the Himalaya through China to Japan are included in this distri- bution type. In the upper reaches of the Hongshui River, 122 genera in 58 families, including 45 woody, 67 herbaceous and ten lianoid, are of this distribution type, accounting for 10.0% of the total genera in this region and 39.7% of genera with this distribution type in China. These genera contain about 360 species, accounting for 7.9% of the total species in this region and 21.1% of the species in genera with this distribution type in China. Since the eastern part of the upper reaches of the Hongshui River belongs to the Yunnan-Guizhou-Guangxi Floristic Region of the Sino-Japanese Floristic Subkingdom, and the western part belongs to the Yunnan Plateau Floristic Region of the Sino-Himalayan Floristic Subkingdom, 61 of the 122 genera with eastern Asia distribution are distributed throughout eastern Asia, 32 are of the Sino Himatayan distribution subtype, 29 are of the Sino-Japanese distribution subtype. Among the 24 monotypic and oiigotypic genera (Table 6). 13 are of the Sino-Himalayan distribution subtype, 11 are of the Sino- Japanese distribution subtype. Thus. genera with a Sino-Himalayan distribution and those with a Sino- Japanese distribution are almost equally represented in this region.

In regard to the physical conditions in the eastern and western parts of the upper reaches of the Hongshui River, including the geology, topography, soil, water and temperature and vegetation, the differences between the two parts are very obvious. In the western part, where a karst topography is well-developed, endemism is especially rich.

Consequently, it might be appropriate to assign the western part of the upper reaches of the Hongshui River to the Yunnan Plateau Floristic Region of the Sino-Himalayan Floristic Subkingdom and the eastern pan to the Yunnan-Guizhou-Guangxi Floristic Region of the Sino-Japanese Floristic Subkingdom.

The phytogeographical patterns of some genera and species with this distribution type are noteworthy. Platycodon and Semiaquilegia, for example, both with a Sino-Japanese distribution, have the western border of their distribution range in the upper reaches of the Hongshui River; Dipentodon and Leycesteria, both with a Sino-Himalayan distribution type, have the eastern border of their distribution range in this region. These observations imply that the upper reaches of the Hongshui River are located in a transitional zone between the Sino-Himalayan flora and the Sino-Japanese flora.

(15) Chinese Endemics. In the upper reaches of the Hongshui River. 55 genera in 37 families, including 28 woody, 25 herbaceous and two lianoid, are endemic to China, accounting for 4.5% of the total in this region and 20.6% of the endemic genera in China. These genera contain 66 species, accounting for 1.5% of the total species in this region and 12.3% of the species of all genera endemic to China.

Among the 55 genera, eight are endemic to the upper reaches of the Hongshui River, accounting for 14.55% of the total. Thamnocharis (Fig. 2) in the Gesneriaceae, has only one species, T. esquirolii, known from only Xingren and Zhenfeng xian in southwest Guizhou. This genus has nearly actinomorphic flowers and completely fertile stamens and might be considered to be one of the most primitive genera in the Gesneriaceae. The genus Metabrlggsia, also Gesneriaceae, has two species, M. ovalifolia with three staminodes occurs in Lapo Xian, Guangxi; M. purpureotincta with two staminodes occurs in Landan Xian, Guangxi. The monotypic Pseudochirita in the same family is distributed in western Guangxi (in Jingxi, Longzhou, Shangliang and Laibing). Three other monolypic genera in this family, Dotlcholoma, Allocheilos and Gyrogyne, are also very restricted in distribution, occurring respectively in Lapo in Guangxi, Xingyi in southwest Guizhou, and Baise in Guangxi, This evidence, together with the fact that several other genera in the Gesneriaceae, such as Pseudochirita, Petrocodon, Rhabdothamnopsis, Chiritopsis, Calcareoboea and Gyrocheilos, are also found in the upper reaches of the Hongshui River or its adjacent regions, indicate that this region, along with its adjacent regions, might be the distribution and the largest diversity center of this subfamily (W. T. Wang, 1982).

The distributions of some other Chinese endemic genera are of also great interest. Malania (Olacaceae) is a genus of evergreen trees of a strongly tropical nature, and is endemic to Funing and Guangnan in Yunnan and to Longzhou, Daxing, Debao, Jingxi, Leye, Tianling, Tianyang, Fengshan, Lingyun, Baise, Pingguo, Tiandong, Longling and Fengshan in Guangxi. The genus Dolichopetahum (Asclepiadaceae) is monotypic and is endemic to Aniong in southwest Guizhou and Longling in northern Guangxi.

Two genera, Scorpiothyrsus (Melastomataceae.) and Whytockia (Gesneriaceae), both with disjunct distri- butions, are of special significance in phytogeography. The former was considered to be endemic to Hainan until a new species, S. shangszeensis, was found (Chen, 1984). This genus is undoubtedly tropical in nature. consisting of six species, five of which occur in Hainan, one species, S. shangszeensis, as mentioned above, is distributed in Shangsi and Jingxi in southern Guangxi. It is obvious that Hainan is the modem distribution and differentiation center of this genus. From a phylogenetic viewpoint, however, Hainan is not the place of origin of this genus. Scorpiothyrsus shangszeensis retains some primitive characters, such as the wide petals up to about 5 mm long and the absence of tubercles or spurs on the dorsal side of the connectives, suggesting it might be a primitive species. Hainan did not separate from the mainland until the Quartemary when the Xiongzhou Strait sank. Furthermore, floristically Hainan is closely related to other areas around the Gulf of Tonkin. From these facts we postulate that the genus Scorpiothyrsus might have originated in the Gulf of Tonkin, and then might have undergone strong differentiation in Hainan after the island became isolated from the mainland.

The genus Whytockia has three species. One species, W. chirillflora, occurs in Mengzi in southeast Yun- nan. The other. W. tsiangiana, has three varieties: var. tsiangiana in southeast Yunnan and northwest Guangxi, northwest Hunan, southwest Hubei, Guizhou and southern Sichuan; var. minor in Pingbian in southeast Yunnan, var. wilsonii on Emei Mountain, Erbian and Leibo in Sichuan. Another species, W. sasakii, is in Taiwan. The two species on the mainland have two stigmas, while the species in Taiwan has only one stigma, which may be the consequence of fusion of two stigmas, suggesting this species might be an advanced one (W. T. Wang, 1989).

Endemic genera can sometimes provide some information for the regionalization of the Chinese flora. For example, Handeliodendron bodinieri (Sapindaceae) and Tapiscia (Staphyleaceae) have the southern end of their distribution range in the upper reaches of the Hongshui River, and Hilliella (Brassicaceae) reaches its southwestemmost limit in this region.

Floristic relationships with neighboring regions

In order to determine the floristic relationships of the upper reaches of the Hongshui River with other regions, we have chosen seven neighboring regions for comparison (Fig. 3). These regions include Taiwan and Wuyi Mountain, Fujian, in the east, Xishuangbanna and Yulong Mountain, both in Yunnan, in the west, Hainan Island in the south, Shennongjia, Hubei, and Jinfo Mountain, Sichuan, in the north. The justification for choosing these seven is that they have been studied floristically and thus their floristic characteristics have been, at least to some degree, determined. Hainan Island and Xishuangbanna are assigned to the Paleotropic Kingdom. Taiwan is also assigned to the Paleotropic Kingdom by most scholars, although others have different opinions on this placement. The remaining five regions are assigned to the Holarctic Kingdom. The comparison is made from three aspects, including the distribution types, the number of the shared genera, and the similarity coefficient of the genera of these regions.

1) Statistical comparison of the distribution types of the eight regions.

As shown in table 8, in five of the eight regions, the tropical genera (excluding cosmopolitan ones) are obviously much more (or only slightly more) numerous than the temperate genera. According to their R/T (tropical genera/temperate genera) ratio, the degree of the tropical nature of the eight regions are arranged in descending order as follows: Hainan, R/T=6.25; Xishuangbanna, R/T=4.71; upper reaches of the Hongshui River, R/T =2.16; Taiwan, R/T =1.97; Wuyi Mountain, R/T =1.10; Jinfo Mountain, R/T= 0.87; Yulong Mountain, R/T=0.65; Shennongjia region, R/T=0.47.

From the above calculations, it can be seen that among the eight regions compared both Hainan Island and Xishuangbanna are strongly tropical in nature. Taiwan, the region with the highest mountain, Yu Shan, at 3,997 m, and influenced strongly by an oceanic climate, is strongly tropical in nature, but temperate to some extent. The Shennongjia region is clearly temperate in nature, with Yulong Shan and Jinfo Shan following. These figures show that, as expected, temperate genera increase in number with an increase in latitude, which is in accordance with an altitudinal zonation of plant distribution.

2) Analysis of the floristic relationships of the eight regions.
[1] Statistics and comparison of the genera shared by these regions. The floristic relationships of the eight regions are determined by the number of genera shared by these regions. There are 131 genera in 73 families shared by all eight regions. These genera belong to 11 distribution types. Twenty genera are cosmopolitan, of which five genera, Polygonum, Clematis, Viola, Lysimachia and Senecio, have more than 100 species in China, two genera, Rorippa and Bidens, have less than ten species in China. Thirty-eight genera are pantropical in distribution, accounting for 29.0% of the 131 genera. Some, such as Cocculus, Celosia, Schoepfia, Achyranthes, Siegesbeckia, lmperata, Oplismenus and Pennisetum, which all have less than five species in China, extend their distribution to subtropical and temperate regions. Two genera, Meliosma and Sageretia, are of tropical Asian-tropical American distribution. Eight genera, such as Elatostema, are of Old World tropical distribution. Seven genera, such as Cinnamomum, are of tropical Asian-tropical Australasian distribution. Six genera, such as Lindera and Pueraria, are of tropical Asian distribution. Twenty one genera, including Quercus, Acer, Vitis, Rhododendron, Viburnum and Artemisia, are of north temperate distribution; 13 genera, including Lithocarpus and Castanopsis, are of eastern Asian-North American distribution. Five genera, such as Oenanthe and Ligustrum, are of Old World temperate distribution. Four genera, such as Actinidia and Acanthopanax, are eastern Asian in distribution.

Thus, among the 131 genera above enumerated, 20 are cosmopolitan, 68 are tropical and 24 are temperate in nature. These conclusions show that many tropical elements extend into subtropical and temperate regions, indicating the tropical affinities of the temperate flora. It should be noted that temperate genera occurring in tropical regions are generally alpine elements in tropical mountainous regions.

[2] Analysis of the genera shared between the upper reaches of the Hongshui River and each of the seven neighboring regions. In order to reveal the floristic affinities between different floras, it is necessary to determine as much as possible about the characteristics they have in common, just as when a taxonomist evaluates the relationships between two taxa. Seven regions have been selected with which to compare the genera occurring in the upper reaches of the Hongshui River.

Table 9 shows the genera shared by the upper reaches of the Hongshui River with each of the seven neighboring regions, and the distribution types of these genera, the number of tropical genera and of temperate genera as well as the R/T radio. From this table, it can be seen that the genera shared by the upper reaches of the Hongshui River with Xishuangbanna, Hainan Island and Taiwan number 777, 766 and 771 respectively, of which 582, 597 and 466 genera are tropical. Those shared by the upper reaches of the Hongshui River region with Jinfo Mountain and Wuyi Mountain total 633 and 604 in number respectively, of which 297 are tropical. The Shennongjia region and Yulong Mountain have 514 and 510 genera, respec tively, that also occur in the upper reaches of the Hongshui River.

3) Comparison of similarity coefficients of genera in the eight regions.

The similarity coefficients of the genera in the eight different regions can show directly the floristic af- finities of these regions. There are different methods for calculating similarity coefficient of genera. In this paper, the coefficient is calculated using the following formula: similarity coefficient of genera of A region and B region = number of genera shared by A region and B region/total number of genera of A region + total number of genera of B region-number of genera shared by A region and B region. As shown in Table 10, the similarity coefficients of genera shared between the upper reaches of the Hongshui River and each of the seven regions mentioned above are as follows: 50% for Xishuangbanna, 47% for Taiwan, 46% for Hainan, 43% for both Jinfo Mountain and Wuyi Mountain. 35% for Shennongjia, and 33% for Yulong Mountain. From the coefficients of similarity of genera of each distribution type between the upper reaches of the Hongshui River and the other seven regions, it can be seen that those for tropical genera between this region and Hainan and Xishuangbanna are larger than those for temperate genera. The coefficients of similarity for temperate genera are larger than those for tropical genera between tins region and Shennongjia, Jinfo Shan, Yulong Shan, Wuyi Mountain and Taiwan. For Hainan and Xishuangbanna, the coefficients for tropical genera are larger than the coefficients for the total genera, being 57% and 53% respectively. For Shennongjia, Yulong Mountain, Jinfo Mountain and Wuyi Mountain. the coefficients for tropical genera are smaller than the coefficients for total genera, being 22%, 27%, 36% and 36% respectively. For Taiwan, the coefficient for tropical genera is closer to that for temperate genera, indicating the upper reaches of the Hongshui River might be more closely related to Taiwan floristically.

Discussion and Conclusion

From the above analysis, the floristic characteristics of the upper reaches of the Hongshui River are sum- marized as follows:
1) As shown in Table 3, of the 1292 genera of seed plants in the upper reaches of the Hongshui River, 799 genera have a tropical distribution, accounting for 65.2% of the total genera in this region and 51.6% of all tropical genera in China; 370 genera are of a temperate nature, accounting for 30.2% of the total genera in this region and 29.6% of the all temperate genera in China. These calculations show that the flora of the upper reaches of the Hongshui River, which occupies a position of overlap between the Holarctic Floristic Kingdom and the Paleotropical Floristic Kingdom, is strongly tropical in nature. Among the tropical genera, the most important ones are those with a tropical Asian distribution, accounting for 22.6% of the total genera in this region, some of which, furthermore, belong to strictly tropical families. They include 25 monotypic genera and 69 oligotypic genera. These figures indicate that the flora of the upper reaches of the Hongshui River has strong affinities and shares a history and floristic origin with the Indo-Malaysian flora, especially with the flora extending from Vietnam (or Indochina) to southern (and southwestern) China. Pantropical genera in the upper reaches of the Hongshui River number 239, accounting for 19.5% of the total genera in this region. Numerous genera with this distribution type, which are widely distributed in both the eastern and western hemispheres, are only rarely represented in this region, and thus have their distributional borders in this region. Some genera with this distribution type extend their distribution range to subtropical or even temperate regions, indicating that the upper reaches of the Hongshui River might occupy a transitional position between the Paleotropical Floristic Kingdom and the Holarctic Floristic Kingdom, and that the Chinese seed plant flora might have affinities, to some extent, with the tropical flora.

Temperate genera are mostly those with a north temperate distribution and those with an eastern Asia distribution, each accounting for 10.4% and 9.9% respectively of al the genera in this region. Among the tropical elements in this region, the geographical distributions of the tropical genus Vatica (Dipterocarpaceae) and the genus Scorpioihyrsus (Melastomataceae), endemic to China, provide strong evidence for the division of the Paleotropic Floristic Kingdom and the Holarctic Floristic Kingdom. Some genera with an eastern Asian distribution type in this region offer clues for the division of the eastern Asian flora into the Sino-Japanese and the Sino-Himalayan flora.

2) Wu (1991) did not give an opinion on whether the genera endemic to China should be assigned to tropical genera or to temperate genera. Little attention has been paid to this question in previous discussions on the Chinese flora. It is not easy to determine the floristic nature of a given genus endemic to China, because China is a vast country, having tropical, subtropical, warm temperate, temperate and frigid temperate climates, and furthermore, there exist many mountains with obvious altitudinal distributions of plants, which result in distinct vertical changes in vegetation from the foothills to the mountain summits. The plants of the genus Pteroceltis, for example, are deciduous trees widely distributed from southern China and southwestern China to northern China; the genus Biondia is widely distributed from southern China to north of the Qinling Mountain Range. The floristic nature of these two genera is difficult to determine. Consequently in the floristic analysis, the 55 genera endemic to China in the upper reaches of the Hongshui River are included neither among the temperate genera nor among the tropical genera. Some genera, however, such as Scorpiothyrsus, Stapfiophyton, Styrophyton, Barthea (Melastomataceae), Tsoongiodendron (Magnoliaceae), Delavaya (Sapindaceae) and Malania (Olacaceae), can be considered to be tropical genera, because they are found mainly in the lower parts of river valleys in areas with a karst topography. The proportion of tropical genera in the upper reaches of the Hongshui River, therefore, might be actually a little higher than indicated in Table 3.

3) An analysis of the genera shared by the upper reaches of the Hongshui River with seven other regions, together with the similarity coefficients of genera of these regions, shows that the flora of the upper reaches of the Hongshui River is closet to those of Xishuangbanna, Taiwan and Hainan in constituent genera, with 777, 771 and 776 shared genera and similarity coefficients of shared genera of 50%, 47% and 46% respectively, and next closet to Jinfo Mountain and Wuyi Mountain, with 633 and 604 shared genera respectively, and similarity coefficients of 43% for both. The Shennongjia region and Yulong Mountain follow, with 514 and 510 shared genera and similarity coefficients of genera of 35% and 33% respectively.

4) The floristic characteristics of the upper reaches of the Hongshui River are strongly influenced by climate, and also to some degree by the karst topography. The rich endemism in this region might be partly the influence of the karst topography. As pointed out by Takhtajan, the determination of a "floristic region" is mainly based on whether or not there are numerous endemic species and genera. sometimes even families and orders, in that region. In the upper reaches of the Hongshui River, there are 55 genera endemic to China, eight of which are endemic to this region itself. In western and northwestern Guangxi there are 139 endemic species (Lu et al., 1989). These figures, together with the fact that two monotypic families, Rhoipteleaceae and Sargentodoxaceae, occur in this region, suggest that the upper reaches of the Hongshui River are floristically very peculiar and significant for understanding the floristic regions of China.

5) In the upper reaches of the Hongshui River, gymnosperms are scarcely represented at both the specific and generic levels, but some noteworthy species and genera, such as Nageia nagi, Podocarpus macrophyllus. P. neriifolius. P. pilgeri, Pseudotsuga brevifolia, P. sinensis, Cephalotaxus and Fokienia, do occur there. all of which are ancient groups or so-called Tertiary relicts. In particular, this region is within the distribution range of the modem distribution center of Keteleeria. Additionally, some primitive woody angiosperms, such as Magnoliaceae, Lauraceae, Fagaceae, Theaceae, Hamamelidaceae are more or less represented in this region. Here also occur numerous primitive genera and species of the herbaceous Gesneriaceae, some of which are endemic to China.

In the upper reaches of the Hongshui River, two monotypic families endemic to China, Sargentodoxaceae with only Sargentodoxa and Eucommiaceae with only Eucommia, are also found. Numerous other primitive monotypic or oligotypic genera, such as Annamocarya (Juglandaceae) and Diplopanax (Araliaceae), also occur in this region.

The observations discussed above suggest that the flora of the upper reaches of the Hongshui River might be very ancient, and that this region might be an important conservation area, or have been a center of development, for some plant groups.

In conclusion, the flora of the upper reaches of the Hongshui River is characterized by its richness in spe- cies as well as in the number of primitive families and genera that occur there, and also by its strong tropical nature. The origin of this flora might be traced to the Tertiary Paleotropical flora. The region might be a conservation center for some primitive groups of seed plants, and might occupy an important transitional position between the Paleotropic flora and the Holarctic flora, and thus might have served as an important corridor for the migration of plants. Therefore, the flora of the upper reaches of the Hongshui River is of considerable importance in phytogeographical research.

References

Chang. H. T. 1961.

The characteristics of Kwangtung flora. Acta Sci. Nat. Schol. Super. Sin. (Biol.). 1965(4): 252- 268.

———. 1979.

Hamamelidaceae. In: Fl. Reipubl. Pop. Sin. 35(2): 36-116. Science Press. Beijing.

———. 1986.

The continental drift and the development of the flowering plants. Acta Sci. Nat. Univ. Sunyatseni. 1986(3): 1-11.

———. 1989.

Tiliaceae. In: F1. Reipubl. Pop. Sin. 49(1): 47-123. Science Press. Beijing.

Chen, C. 1984 a.

Materia ad flora Melastomataceae Sinensium. Bull. Bot. Res. 4(3): 33-68.

———. 1984 b.

Melastomataceae. In: Fl. Reipubl. Pop. Sin. 53(1): 135-293. Science Press. Beijing.

Endress, P. K. 1989.

A suprageneric taxonomic classification of the Hamamelidaceae. Taxon 38: 371-376.

Good, R. 1974.

The Geography of the Flowering Plants, ed. 5. London. Longman Group Limited.

Gray, A. 1846.

Analogy between the flora of Japan and that of the United States. Amer. J. Sci. Arts. 2: 135, 136. (Reprinted in Graham, 1972, and in Stuckey, 1978).

How, F. C. 1984.

A Dictionary of the Families and Genera of Chinese Seed Plants, ed. 2., 1-632. Science Press. Beijing.

Ko, W. C. 1989.

A preliminary study on the floristic relationships between Hainan and its adjoining regions. Gui- haia9:211-219.

Law, Y. W. 1984.

A preliminary study on the taxonomy of the family Magnoliaceae. Acta Phytotax. Sin. 22: 89- 109.

Li, H. L. 1952.

Floristic relationships between eastern Asia and eastern North America. Amer. Philos. Soc. 42: 371- 429. [Reprinted. with Forward and additional references. as a Morris Arboretum Monograph. 1971.]

Li, H. W. 1979.

The geographical distribution of Chinese Lauraceae Plants. Acta Phylotax. Sin. 17(3): 24-40. (in Chinese. English summary)

———. 1984.

Lauraceae. In: Fl. Reipubl. Pop. Sin. 3: 1-463. Science Press. Beijing.

———. 1985.

A florislic study of Yunnan Province. Acta Bot. Yunnan. 7: 361-382.

Li, X. W. (=Li, H. W.) and D. Walker. 1986.

The plant geography of Yunnan Province, southwest China. J. Biogeogr. 13: 367-397.

Li, L. Q., C. F. Zhang, S. Y. Song and Y. Chen. 1991.

The flora of the Huping mountains in W. Hunan Province. Acta Phytotax. Sin. 29: 113-130.

Lin, Y. R., X. W. Wang and G. C. Chang. 1981.

A preliminary study in the Wuyishan Nature Reserve (WYNR) in Fujian. W Sci. J. 1: 57-82.

Lu, A. M. 1982.

On the geographical distribution of the Juglandaceae. Acta. Phytotax. Sin. 20:257-271.

Lu, Y. X., G. B. Huang and C. F, Liang. 1989.

Study on the endemic plants from Guangxi. Guihaia 9(3): 201-210

Pan, K. Y., A. M. Lu and J. Wen. 1991.

A systematic study on the genus Disanthus Maxim. (Hamamelidaceae). Cathaya3: 1-28.

Qi.C.J. 1984.

The characteristics of Hunan flora. Bull.Bot.Res.4(1): 130-145.

Takhtajan, A. 1978.

Floristicheskye oblasty Zemli (Floristic Regions of the world). Nauka. Leningrad.

Tong, S. Q. and G. D. Tao. 1990.

Dipterocarpaceae. In Fl. Reipubl. Pop. Sin. 50(2): 113-131. Science Press. Beijing.

Wang, H. S. 1985.

Quantitative analysis of genera endemic to China. Acta. Phytotax. Sin. 23: 241-258.

Wang, W. T. 1965.

Preliminary study of the genera Helicia Lour. and Heliciopsis Sleum. of China. Acta Phytotax. Sin.5:285-309.

———. 1989.

Notes on disjunction in the flora of China. Bull. Bot. Res. 9(1): 1-16.

———. 1990.

Gesneriaceae. In: Fl. Reipubl. Pop. Sin. 69: 125-581. Science Press. Beijing.

———. 1992.

On some distribution patterns and some migration routes found in the eastern Asiatic region. Act. Phytotax. Sin. 30: 1-24:97-117.

———. 1992. The Phytogeography of Gesneriaceae within China. Edinb. J. Bot. 49: 6-8.

Wood, C. E., Jr. 1972.

Morphology and phytogeography: the classical approach to the study of disjunctions. Ann. Missouri Bot. Gard. 59: 107-124.

Wu, C. Y. 1965.

Floristic affinity of China with tropics. Scientia 1965 (1): 25-33.

———. 1979.

The regionalization of Chinese flora. Acta Bot. Yunnan. 1: 1-22.

———. 1983.

The Physical Geography of China-Phylogeography (Vol. 1). Science Press. Beijing.

———. 1984. Index Florae Yunnanensis. pp. 1-2259. The People's Publishing House. Yunnan, China.

———. 1991.

Theareal-types of Chinese genera of seed plants.Acta Bot.Yunnan.Supp.IV: 1-139.

———. and W. T. Wang. 1957.

Preliminary reports on studies of the plants of tropical and subtropical regions of Yunnan. Acta. Phytotax. Sin. 6: 183-254: 267-300.

Ying, T. S., C. G. Ma and C. S. Chang. 1979.

Observations of the flora and vegetation of Mt. Shennongjia in western Hupeh. Acta Phytotax. Sin. 17(3): 41-60.

——— and C. S. Chang. 1984.

Endemism in the flora of China-studies on the endemic genera. Acta Phytotax. Sin. 22:259-268.

———, Y. F. Li, Q. F. Guo and H. Gui. 1990.

Observations on the flora and vegetation of Taibai Shan, Qinling Mountain Range. Southern Shaanxi, China. Acta Phytotax. Sin. 28i: 261-293.

———, D. E. Boufford and Y. L. Tu. 1991.

Phytogeographical relationships of the genera of Angiospenns in the Fanjing Shan mountain range, northeastern Guizhou, China. Ann. Missouri Bot. Gard. 78: 338-358.

———, Y. L. Zhang and D. E. Boufford. 1993.

The Endemic Genera of Seed Plants of China.

Science Press. Beiiing.