晚古生代同处于赤道或低纬度地带的中国大陆植物群与欧美地区同期植物群之间总存在一定的差异。中泥盆世—早石炭世维宪期,中国大陆以石松类的大量繁育和高度分异为显著特色,而同期的欧美地区植物群以古蕨类和前裸子植物为其主体;纳缪尔期—二叠纪,大量东亚型鳞木类植物及许多地方性植物属相继在中国大陆出现,使它和欧美植物群之间的差距进一步拉大。造成两地植物组分差异的主要原因可能与当时气候的经向变化有关(主要是两地距离古特提斯洋远近不同,从而导致湿度有别)。

Differences are obviously existed between the Chinese and Euroamerican floras in the late Palaeozoic.The Chinese middle-late Devonian floras were mainly occupied by the primitive lepidophytes includingLepidodendropsis,Protolepidodendron,Lepidosigillaria,SublepidodendronandLeptophloeum. In contrast,the synchronous Euroamerican floras were dominated by ferns and progymnosperms,the common members of which are Hyenia,Reimannia,Rellimia,Archaeopteris,RhacophytonandAneurophyton.While primitive lepidophytes were relatively rare.In Tournaisian-Visean,the Chinese floras were characterized by flourishing of the lepidophytes.Besides some primitive elements such asLepidodendron volkmannianumandLepidodendron Lossenii,a few oriental type species with relatively large leaf scars (i.e.Lepidodendron ninghsiaense,Bothrodendron ruchengenseandBothrodendron criculare)also appeared.In addition,the so-called Euroamerican Westphanian Flora members ofParipteris,LinopterisandEusphenopterishave been found in the Chouniukou Formation of North Qilian Mountains.However,both the Tournaisian and the Visean floras in the Euroamerican massland were dominated by the primitive ferns with radiate venation,the most common elements areAdiantites,Anisiopteris,FryopsisandSpathulopteris.
According to the above mentioned,“Procathaysian Flora”is suggested to represent the Chinese middle Devonian-Visean floras in this paper.
From Namurian to Permian,many typical oriental type lepidophytes(includingLepidodendron ninghsiaense,Lepidodendron oculus-felis,Lepidodendron posthumiiandCathaysiodendron nanpiaoense)and some endemic genera(Tingia,Conchophyllum,Gigantoperis,Emplectopteris,Lobatannularis)appeared one after another on the Chinese Composite Continents,suggesting that the gap between the Chinese and the Euroamerican floras enlarged and the famous Cathaysian Flora occured in this time.
The present authors believe to a great extent that it was the east-west climatic differentiation(mainly the change of humidity)that caused the differences between the two floras.Since the Caledonian orogeny at the end of Silurian,the North American plate,the European plate and the Gondwana continent collided in succession,and thus resulted in continuous growth of the united continent(Pangea).In this case ,the equatorial current and the trade wind of the ancient Tethys ocean had fewer and fewer effect on the hinterland and dry continental climate strengthened there.As a response,the mesophilous plants(ferns)were extremely prosperous.On the contrary,the Cathaysian continent was constituted by certain water surrounded small islands in the overwhelming majority of late Palaeozoic,so the humid maritime climate developped and the lepidophytes lushed.