Abstract:

1.  The distribution of Salix species among the continents.  There are about

526 species of Salix in the world, most of which are distributed in the Northern Hemisphere

with only a few species in the Southern Hemisphere.  In Asia, there are about 375 species, mak-

ing up 71.29 percent of the total in the world, including 328 endemics; in Europe, about 114

species, 21.67 percent with 73 endemics; in North America, about 91 species, 17.3 percent with

71 endemics; in Africa, about 8 species, 1.5 percent, with 6 endemics.  Only one species occurs

in South America.  Asia, Europe and North America have 8 species in common (excluding 4

cultivated species).  There are 34 common species between Asia and Europe, 14 both between

Europe and North America and between Asia and North America, 2 between Asia and Africa.

Acording to the Continental Drift Theory, the natural circumstances which promoted speciation

and protected newly originated and old species were created by the orogenic movement of the

Himalayas in the middle and late Tertiary.  Besides, the air temperature was a little higher in

Asia than in Europe and North America (except its west part) and the dominant glaciers were

mountainous in Asia during the glacial epoch in the Quaternary Period.  Then willows of Eu-

rope moved southwards to Asia.  During the interglacial period they moved in opposite direc-

tion.  Such a to-and-fro willow migration between Asia and Europe and between and North

America occurred so often that it resulted in the diversity of willow species in Asia.  Those

species of willows common among the continents belong to the Arctic flora.

     2.  The multistaminal willows are of the primitive group in Salix.  Asia has 28 species of

multistaminal willows, but Europe has only one which is also found in Asia.  These 28 species

are divided into two groups, “northern type” and “southern type”, according to morphology of

the ovary.  The boundary between the two forms in distribution is at 40°N.  The multistami-

nal willows from south Asia, Africa and South America are very similar to each other and

may have mutually communicated between these continents in the Middle or Late Cretaceous

Period.  The southern type willows in south Asia are similar to the North American multista-

minal willows but a few species.  The Asian southern type willows spreaded all over the conti-

nents of Europe, Asia and North America through the communication between them before the

Quaternany Period.   Nevertheless, it is possible that the willows growing in North America

immigranted through the middle America from South America.  The Asian northern type mul-

tistaminal willows may have originated during the ice period.

     The multistaminal willows are more closed to populars in features of sexual organs.  They

are more primitive than the willows with 1-3 stamens and the most primitive ones in the ge-

nus.

     3.  The center of origin and development of willows Based on the above discussion it is re-

asonable to say that the region between 20°-40°N in East Asia is the center of the origin and

differentiation of multistaminal willows.  It covers Southern and Southwestern China and nor-

thern Indo-China Pennisula.

Key words: Salix, Distribution, Origin