On the Origin and Evolution of the Willows

Key words: 
• Willows /
• Pleiarina /
• Chosenia /
• Salix /
• Archaic willow group /
• Origin /
• Evolution

Abstract: The willows (Salicaceae subfam. Salicoideae) are composed of three genera with about four hundred and fifty natural species (as well as a great number of natural hybrids) and distribute naturally in Africa,South America,Asia,Europe et North America (not in Oceania and Antarctica) from about 52 degrees south latitude to 82 degrees north latitude et from near sea level to altitude more than 5 000 m et from the south temperate zone to the north frigid zone. i. e. Pleiarina of about fifty species and approximately on the south of the Arctic Circle,Chosenia of about three species and in the neighbouring areas of both the Sea of Ochotsk et the Sea of Japan,and Salix of about four hundred species and roughly on the north of Tropic of Cancer. It is quite evident that there is a long and tortuous evolutionary process from primitive angiosperm to the contemporary willows. Archaic willow group probably originated from the West Gondwanaland during the Middle Cretaceous and then evolved further along the various direction in the course of its spreading everywhere; and the three genera before-mentioned are its extant descendants. The genus Pleiarina possibly occurred in Africa of the West Gondwanaland during the Late Cretaceous; it first radially scattered over this locality (and it existed in Africa,Madagascar and South America respectively owing to the breakup of the archaic land afterward) and then spread to the Laurasia; subsequently,the unitary Laurasia's Pleiarina flora was divided into two parts developing alone because of wholly separating each other of the Eurasia land mass and the North America land mass. The genus Chosenia most likely arose in the northeastern Eurasia land during the Late Tertiary. The genus Salix might emerge from eastern Tethys regions during the Middle and Upper Tertiary,and then widely spread in Eurasia land et progressively migrated to North America land via Bering regions. Besides,a conspectus of all the genera and species of the plants concerned is provided in this paper,including their correct names,bibliographic references,geographic distributions etc.