acadkmt or sciences.] CENTERS OF DISTRIBUTION. 21 



Montana near Stanton Lake at 7,500 feet (by R. S. Williams). Papaver nudicaule evidently 

 developed in the Arctic regions, wandering from there to the mountains farther south, where 

 it is now more or less frequent. An important geographical center is undoubtedly the Altai 

 and Baikal Mountains, where it occurs in several characteristic forms, but I believe the species 

 developed in the Arctic mountains. The occurrence of P. pyrenaicum L. in the northern 

 Rocky Mountains, so very remote from the European stations, the Alps and Pyrenees, is diffi- 

 cult to account for unless we adopt the hypothesis of the species having developed both in 

 Europe and on this continent, rather than having formerly been more widely distributed, but 

 exterminated except at these two very remote stations. 



Draba crassifolia Grah. 



According to Macoun (1. c.) this plant is "abundant on the sides of ravines and grassy 

 slopes above the limits of trees on Cathedral, Castle, and other mountains in the Rocky Moun- 

 tains, latitude 51°." It is very rare farther south and is known only from a few stations in Col- 

 orado and in the Sierra Nevada. It is known also from Greenland between latitude 64° and 

 72° and from Finmark in the valley of Tromsoe. This geographical distribution may at a 

 first glance seem very strange, especially considering the very distant station in Finmark. 

 However, Draba crassifolia is not the only plant of unquestionable American origin which 

 has found its way to Norway. There are a few others the geographical distribution of 

 which is about the same: Carex scirpoidea Michx., C. nardina Fr., C. f estiva Dew., Platanthera 

 obtusata Lindl., and Artemisia Norvegica Fr. Of these the three Carices are not only recorded 

 from Greenland but also from Iceland and Finmark. Platanthera obtusata, so very frequent 

 in cool, mossy woods throughout the forest region of Canada, does not occur in Greenland 

 or Iceland but in Finmark. The Artemisia, on the other hand, is not, so far, reported from 

 any station intermediate between Norway and the Rocky Mountains and does not occur 

 north of the Arctic Circle. These plants, including the Draba, have their widest distribu- 

 tion on this continent and south of the Arctic regions. They all are associated with allied 

 types and I presume, therefore, that they developed in the Rocky Mountains. Their 

 occurrence in Norway, partly also in Greenland and Iceland, seems to indicate that they 

 belong to that category of plants which, although of southern origin, accompanied the Arctic 

 northwards, gaining foothold on some of the mountains in Greenland, Iceland, and Norway. 

 It may be mentioned also that in Finmark the very rare Carex holostoma Drej. has been discov- 

 ered and this species has so far only been recorded from a few places on the west coast of 

 Greenland. 



Chrysodraba, of which D. crassifolia is a member, is altogether well exemplified in the 

 mountains of this continent, and several are endemic: D. Mogollonica Grne., D. montana Wats., 

 D. Howellii Wats., D. Lemmonii Wats., etc. Beside these the circumpolar D. alpina L., and 

 the subarctic (Greenland) D. aurea Vahl are also known from these mountains. Very few 

 yellow-flowered Draba are known from the European Alps, and none of these belong to the 

 section Chrysodraba, but to Aizopsis: D. aizoon Walenb. and D. aizoides L. Three species, 

 all Chrysodrabae, are recorded from the Caucasus: D. repens M. B., D. incompta Stev., and D. 

 molli-ssima Stev. Of these D. repens is the only member of the section that has been found in 

 Altai and Baikal Mountains, and this species is also Arctic. 



It would thus appear as if especially the Rocky Mountains constitute a most important 

 center of distribution and origin of Chrysodraba, of which D. crassifolia and D. aurea have 

 reached the Arctic regions. On the other hand, one of the Old World species, D. repens, has 

 from the Altai Mountains become distributed through northern Siberia, reaching the Arctic 

 coast near Ural Mountains, and finally the east coast of Greenland, with no intermediate stations. 



The genus Thlaspi is represented by a few species, of which one was found abundant on 

 the very summit of Pikes Peak: TM. Coloradense Rydbg. However, the American species are 

 not well understood, and the statement that " TM. alpestre L. is common throughout the West, 



