椴树属形态演化与生物地理学

MORPHOLOGICAL EVOLUTION AND BIOGEOGRAPHY OF TILIA

椴树属在椴树科中是一个形态很特殊的属，也是唯一的北温带分布属。在属内，由于许多种间缺乏断然可分的形态特征，种内分化和种质渗入现象又交错存在，以至于造成了分类学上的困难。为进行区系比较、分析，各分布区的椴树种数以统一的标准进行了重新评价。现确认该属共有２５种，其中，东亚（含西伯利亚）１８种，欧洲（含西亚）５种，北美２种。以果实为主要特征，椴树属中存在三个易于识别的类群，即浆果组、木果组和壳果组。苞片形态演化及化石证据揭示了三个组间的演化关系。椴树属的现代分布与化石分布有许多相似之处，但至少在以下三个方面存在显著的差异：１．椴树化石分布纬度远较现代分布偏北，达到了北纬８０°附近；２．现今无椴树分布的亚洲大陆腹地有化石记录；３。北美西部椴树至第三纪末完全绝迹，而东部仅在第四纪才有化石记录。结合现代分布、化石证据、地质历史、古气候变迁及形态演化诸方面的资料，文中试图解释上述现象，并推测该属可能在晚白垩纪就已从祖先类群中分化出来，我国东部亚热带山地很可能是该属的最早发源地。

As a special taxon morphologically,Tilia is the only genus thriving in the northerntemperature zone today in the family Tiliaceae.Taxonomically it is hard to identify onespecies from another due to the diversity and the introgression of the genus．However,ac-cording to the revision of the group by means of comparing the recent Floras of various re-gions,25 species have been recognized as good species，of them l8 in East Asia(incl.Siberia),5 in Europe(incl.West Asia),and 2 in North America.With referenceto the fruit patterns,3 sections,i.e.Sect.Trichophilyra Vassil,Sect.Ⅱ. Lindera Reichb．and Sect.Ⅲ.Tilia Linn.havc been distinguished.The Tilia fossils found fromthe Tertiarv strata in the Northern Hemisphere reveal the evidence of the evolution path inthe three sections.Contrasting to the modern distribution pattern fossil distribution dis-plays some differences in the following aspects:tilia has once reached 80°northernlatitude,a little more northward than todav;2.Tilia fossils were reported from thehinterland of Asia where no more Tilia survived now;3.Tilia disappeared from thewestern North America in the late Tertiary,and was recorded in the Quaternary strata ofeastern North America.On the basis of modern distribution,fossil evidence,geologichistory，palaeoclimate changes and morphological evolution,the authors make an attempt to explain the phenomena and infer the eastern subtropic mountains of Asia as anoriginal area where the ancestor Tilia might have developed during the late Cretaceous.