E. V. Wulff 



—178- 



Historical Plant Geography 



shrubs of dormant buds, which remain closed during unfavorable 

 periods of the year and renew development with the onset of favorable 

 climatic conditions. In plants of the Carboniferous period, despite the 

 existence of large trees, such as Lepidodendron, Cordaites, Catamites, 

 etc., no such buds are found. As another even more characteristic 

 feature of periodicity we may mention the annual rings in the stems of 

 woody plants. In Carboniferous plants, distributed over a considerable 

 part of the northern hemisphere and, to a less extent, in the southern 

 hemisphere, no annual rings are found. This circumstance and also 

 the absence of dormant buds testify to continuous and uniform growth, 

 which could have taken place only under uniform climatic conditions, 

 characterized by the absence of alternating seasons. Such dimatic 

 conditions correspond to those found at present in the equatorial zone 

 with its tropical vegetation. But the plants of the Carboniferous 



Fig. 25. — Distribution of swamps and deserts and location of tlie equator in the Jurassic 

 period (K = coal, S = salt, G = gypsum, W = desert sandstone; stippling = arid regions. 

 (After KopPEN and Wegener). 



period, though lacking periodicity, grew throughout the entire length 

 and breadth of Europe. 



In the flora of the Carboniferous period it is possible, according to 

 POTONI^, to distinguish between elements of tropical origin, typified 

 by the fern Pecopteris, and subtropical elements, typified by Lepidoden- 

 dron, Sigillaria, etc. Whereas the former flora occupied in the Carbonif- 

 erous period a quite Hmited, comparatively narrow strip, passing 

 through North America and Europe and ending in eastern Asia, the 

 subtropical {Lepidodendron) flora lay on both sides of this strip and 

 had a considerably more extensive distribution. It is known as far 

 north as Spitsbergen and as far south as the southern part of South 

 America, occupying a latitudinal range of 120°. 



Another riddle, which seemed no less difficult to solve, is the 

 growing of trees near the poles as at present located. Moreover, as 

 seems entirely incomprehensible, the long polar nights apparently had 

 no effect on this vegetation. 



