Arctic Plants: Geographical Distribution 101 b 



III. Southern, alpine: Erysimum, Thlaspi. 



IV. A weed : Sisymbrium. 



Of these the circumpolar, and the arctic but not circumpolar, with the only- 

 exception oi Cardamine pratensis, may be considered as having originated in 

 the polar regions. _ Erysimum inconspicuum belongs more properly to the Rocky 

 mountains where it is associated with several species and more widely distributed; 



Cardamine pratensis is circumpolar and, moreover, it is widely distributed 

 through the temperate regions of both Worlds. In the arctic region it has only 

 two congeners: C. bellidifolia L., which is circumpolar, andC. digitaia Richards., 

 which seems to be a rare plant, known only, so far, from the islands of the arctic 

 American archipelago, from both coasts of Bering strait, from the Yukon- 

 Alaskan boundary. Long. 141° W., Lat. 60.02 N., from the Hudson bay region 

 and from the territory explored by the expedition. But farther south the 

 genus is quite rich in species, annual or biennial, as well as perennial. Some of 

 these extend as far south as Cape of Good Hope (C. anteniquana Burch. and C. 

 africana L.), and Tierra del Fuego "fere sub nive" (C. glacialis DC). And 

 with respect to .C. pratensis, this species shows a wide distribution in the inounT 

 tains of Europe and Asia, Caucasus, the Himalayas, Altai, and Baikal moun- 

 tains; it is, however, absent from the Rocky mountains. A peculiarity of this 

 species is that it sometimes does not develop mature seeds, not even in Central 

 Europe, but that reproduction may take place by means of bulblets developed 

 upon the leaves. In the arctic region this species sometimes does not even 

 reach the flowering stage, as pointed out by Nathorst, for instance in Spitz- 

 bergen, and this peculiarity it shares vfith Empetrum, Petasites J rigida'. Ranun- 

 culus Pallasii, and R. hyperboreus. Nevertheless, these plants are quite widely 

 distributed in Spitzbergen, and Nathorst explains this by considering the plants 

 to represent remnants of an earlier period when the climate was sufficiently 

 warm to enable them to develop fruit and mature seeds. However, judging 

 from the wide geographical distribution and the abundant representation of 

 the genus in the south, it seems most natural to consider the species as being of 

 southern origin, but that it partook in the migration towards north when the 

 arctic flora retreated. 



But it is impossible to decide with any certainty the location of the original 

 centre of this species; it may, however, have been in Central Europe where the 

 species is most abundant and associated with a number of allied species, and 

 also with several of the closely allied genus Dentaria. In the United States the 

 species is not so common and mostly restricted to the Northern Atlantic States; 

 on the other hand, there are some allied species in the Western States, for instance 

 C. rhomboidea DC. and C. cordifolia Gray in the Rocky mountains, besides that 

 several species of Dentaria are known from the Atlantic and Pacific slope. 



In considering the arctic element in Europe and Asia the Cruciferae are in 

 Finmark represented by not less than 24 species, 8 being introduced weeds, 

 however; in Russia there are 28 species, and in Siberia 20, according to the 

 enumeration in Kjellman's paper (Sibir. Nordkust. Fanerog. Flora). 



By looking over the representation of the family on this continent, it is 

 interesting to see how several genera have developed here, being endemic to 

 North America {Thysanocarpus, Lesquerella, Physaria, Streptanthus, Caulanthus, 

 Thelypodium, etc.); and among the genera,, which are common to both Worlds, 

 several have in America given rise to a relatively large number of endemic 

 species, for instance, Draba with 20 species endemic, Arabis with 30, Dentaria 

 with 9, Erysimum with 7, etc. , 



But from the geographical table (Table 1) it is surprising to see how ex- 

 tremely few alpine, I mean southern alpine, types have entered the polar regions; 

 for the circumpolar must be left out of consideration, and with respect to the 

 simply arctic species, none of these bear any evidence of ever having been more 



