The Xizang (Tibetan) flora with numerous endemics is of importance in Chi-
nese flora. According to recent statistics there are in Xizang 27 genera of spermatophytes
endemic to China, being only 2.25% percent of the total number of genera in the Xizang flora.
Four of them are regarded as palaeoendemics (14.81%) and the others as neoendemics (85.19%).
These endemic genera, of 30 species and 3 varieties, belong to 17 families, of which, Umbelli-
ferae contains 6 genera, 7 species and 3 varieties; Compositae has 6 genera and 7 species, and
Gentianaceae 1 genus and 2 species. All the other families each comprises one genus with a
single species.
The cosmopolitan families together comprising 14 genera with 15 species have the highest
perecentage (52.92%) and the tropical ones (5 families, 5 genera with 5 species) come to the next
(29.42%), followed by the temperate ones (3 families, 10 genera with 10 species) (17.66%). It
shows that these endemic genera are obviously related to the tropical flora and temperate one
in essence.
According to the number of species, the genera endemic to China and occurring in Xi-
zang flora may be grouped as fallows.
Monotypic endemic ones 14 (51.85%)
Ditypic endemic ones 6 (22.22%)
Oligotypic endemic ones 4 (14.81%)
Small endemic ones 3 (11.11%)
The formation of the endemic genera is correlated with the topography, climate and en-
vironmental conditions, and they may have resulted from the diversification in geography and
climatic influence for a long time. The southeastern part of Xizang Plateau is of very diverse
ecological conditions, with the adequate precipitation, which may explain the concentration of
these endemic genera in this region.
The largest similarity coefficient (38.30%) of the genera endemic to China and occurring
in Xizang is with those in Qinghai Plateau, next, with those in Yunnan and in Sichuan pro-
vinces (both 27.60%), which shows that these endemic genera are related to the floras of the
regions mentioned above.
The difference in the horizontal distribution of these endemic genera is obviously between
the southern and northern parts of Xizang Plateau. The vertical distribution of the genera is
also rather obvious, from 800 m to 5200 m above sea level, but concentrated in the zone of 3000
m to 4500 mm. Therefore their occurrence in Xizang is not only affected by the historical
environmental conditions but also controlled by the horizontal and vertical distribution.
The origin and evolution of some endemic genera, such as Psammosilene, Parateropyrum,
Sphaerotylos, Salweenia, Ajaniopsis, Xizangia, Sinoleontopodium, are discussed in this paper.
Parateropyrum, a monotypic palaeotropic endemic, belongs to the tribe Atraphaxideae in-
cluding Atraphaxis, Calligonum and Pteropyrum. It may be a comparatively advanced group
in the tribe, and is closely related to the genus Pteropyrum which is distributed in western
Asia. The genus Parapteropyrum has possibly survived as a palaetropic-tertiary relic in this
region.
Sphaerotylos, a member of the subtribe Sphaerotylinae, the tribe Boehmerieae in the family
Urticaceae, is a comparatively primitive genus in the tribe Boehmerieae so far known. As the
other subtribes, such as Boehmerinae, Sarconchlamydinae, Orecnidinae and Maoutinae, are dis-
tributed in the tropics, rarely in the subtropics, the genus is no doubt a palaetropic -tertiary
relic.
Sinoleontopodium, belonging to the tribe lnuleae in Compositae, is also related to the ge-
nus Leontopodium. It is probable that the genus Sinoleontopodium arised later than the other.
We come to the conclusion that the southern part of Xizang Plateau is also one of the
centres of the origin and differentiation of genera endemic to China.
