academy of Sciences.] CENTERS OF DISTRIBUTION. 25 



ones of which the geographical distribution extends to the Arctic regions of this continent and 

 of Siberia, where the former, S. glacialis R. Br. is quite widely distributed and where it has 

 evidently developed. With regard to S. Rossii R. Br. this species appears to be of North 

 American origin, presumably within the Arctic Circle, from where it took part in the migration 

 toward the south, leaving some remnants on the summits of the Rocky Mountains. S. triflora 

 R. Br. and S. Peckii R. Br. are also of North American origin, but were evidently developed 

 in regions farther south. Besides these geographical centers of the genus in northern Asia 

 and America the European Alps constitute a third center, where S. reptans Spreng. and S. montana 

 Willd. have had their origin, and finally the Himalayas are the home of S. data Royle. Although 

 the species are very much alike in their habit and principal structure, S. Rossii R. Br. is quite 

 distinct "carpellorum Cauda glabra," a character, however, of merely specific importance. We 

 have thus in the genus Sieversia an example of a very characteristic genus having been devel- 

 oped at stations very remote and having given rise to species of a habit peculiar to each of 



these centers. 



Potentilla fruticosa L. 



In Sweden this plant is known only from the island Oeland, in the Baltic Sea. It occurs 

 also in the Caucasus, in Baikal and Altai Mountains, and in the temperate and subalpine Hima- 

 layas at an elevation of 8,000 to 12,000 feet. On this continent the species has been reported 

 from a number of stations in Canada, from Labrador to the Pacific, and northward to the Arctic 

 Sea. Very interesting is the following statement by Macoun (1. c, p. 141): "Besides being 

 frequent in eastern Canada at low altitudes it becomes truly Alpine in the Rocky Mountains 

 and is found almost at the snow line." Southward the geographical distribution extends to 

 Michigan, Pennsylvania, Arizona, and California. It seems strange that the species does not 

 occur in Greenland, although it has such a wide distribution in the northern part of North 

 America. Being Arctic, so far as concerns this continent, and exhibiting such wide geo- 

 graphical range between the two oceans, I presume the original center of the species was located 

 within these regions but south of the Arctic. The scattered occurrence of the species in the 

 Old World — Sweden, the Caucasus, Altai, and the Himalayas — seems to indicate a formerly much 

 wider distribution, and a second geographical center may have been located there; for instance, 



in Altai. 



Sibbaldia procumbens L. 



This plant shows the same very wide distribution in the northern, but not Arctic, regions 

 of North America as Potentilla fruticosa L. It occurs also in Greenland, as far north as latitude 

 70° 20'. In Europe the plant has been reported from Iceland, Faeroe Islands, Scandinavia, 

 Arctic Russia, the Alps, Pyrenees, and Caucasus, while in Asia it is reported from Baikal Moun- 

 tains and the Himalayas (above 17,000 feet). Sibbaldia is evidently not of Arctic origin. It 

 may have developed in the mountains farther south, for instance, in North America. 



THE GENUS SAXIFRAGA. 



Most of the species are mountain plants, and quite a large number have been reported 

 from the Alpine region of the Northern Hemisphere. Several species are Arctic, some are even 

 circumpolar, and according to Feilden 25 the following species belong to the flora growing 

 nearest the Pole: S. oppositifolia, S. cernua, 8. flagellar is, S. caespitosa, S. tricuspidata , and 

 S. nivalis; these have! been gathered in Grinnell Land and islands to the north of Greenland 

 between 82° and 24' N. By Hart (1. c.) S. rivularis has been reported froni 81° 40', S. aizoon 

 from 69° 55', and S. steUaris from 68° 46'. The Alpine species of Colorado are by Engler 26 

 classified under the sections as follows: Trachyphyllurn Gaud. (S. dirysaniha Gr., S. hronchialis 

 L., and flagellaris Willd.); Nephrophyllum Gaud. (S. cernua L.), Boraphila Engler (S. punctata 

 L. and #. nivalis L.). 



» Feilden, H. W.: The flowering plants of Novaja Zemlya, etc. (Journ. of Bot., 1898). 

 » Engler, A.: Monographie der Gattung Saxifraga L. Breslau, 1S72. 



