杨树起源与进化的探讨

On the Origin and Evolution of the Poplars

杨树(杨柳科杨亚科)包含两个属(即胡杨属、杨属)、约50余个天然种(以及繁多的天然杂种和人工杂种),自然分布于热带非洲和大致从北纬19度至70度的北半球;胡杨属约8种,间断分布于赤道非洲、古地中海地区和北美洲东南部;杨属约40余种,大致产于北回归线与北极圈之间的亚洲、欧洲、非洲和北美洲。杨柳科植物的近期祖先可能在白垩纪初期起源于西冈瓦纳古陆,并通过两个中间环节(即古杨群和古柳群)逐步分别发展为现存的杨树(即胡杨属与杨属)和现存的柳树(即原柳属、钻天柳属与柳属)。古杨群(杨树的直接祖先)可能在白垩纪中期起源于西冈瓦纳古陆;后来它扩散至有关各地(特别是北方的劳亚古陆),并在迁移的过程中进一步分化发展。胡杨属可能在白垩纪中、晚期发生于非洲陆块,并逐步扩散至劳亚古陆;后来,统一的胡杨属区系因劳亚古陆的断开和撒哈拉大沙漠的形成而被分割为远离的、独立发展的几个部分。杨属可能在第三纪中、晚期先后发生和分化于古地中海东部地区;后来扩散至欧亚大陆并陆续通过白令地区迁移至北美大陆。文中还提供了有关植物的属、种一览和杨柳科植物亚科与属的检索表。

The poplars （ Salicaceae Mirbel subfam. Populoideae Kimura） contain two genera （ i. e. Balsam- iflua et Populus ） with more than 50 natural species （ as well as a great number of natural hybrids et artifi- cial hybrids）, and distribute naturally in Tropical Africa et in the Northern Hemisphere roughly from 19~ to 70~ north latitude;Balsamiflua of about 8 species disperses disjunctively in Equatorial Africa, Tethys regions et southeastern North America; Populus of more than 40 species widely ranges in Africa, Asia, Eu- rope et North America approximately between the Tropic of Cancer and the Arctic Circle. It is probable that the salicaceous late ancestor originated from the West Gondwana land during the Early Cretaceous, and gradually developed into extant poplars （ i. e. Balsamiflua et Populus） et extant willows （ i. e. Pleia- rina, Chosenia et Salix） through two intermediate links （i. e. Archaic poplar group et Archaic willow group） respectively. The Archaic poplar group （ i. e. the direct ancestor of the poplars） probably emerged from the West Gondwana land during the Middle Cretaceous;subsequently,it seattered over the areas con- cerned （ especially the north Laurasia）, and differentiated et evolved further in the course of its migrating.The genus Balsamiflua possibly occurred in Africa land during the Middle et Late Cretaceous, and dis- persed progressively into the Laurasia;then, the unitary Balsamiflua flora was divided into several distant parts developing alone owing to the breakup of the Laurasia et the formation of the Sahara. The genus Pop- ulus might rose and differentiated in eastern Tethys regions during the Middle et Late Tertiary;afterwardz, it widely spread in Eurasia land and gradually migrated to North America land via Bering regions. Be- sides, a conspectus of all the genera and species of the plants concerned and a key to the subfamilies et genera of Willow Family are provided in the present paper.