432 MINNESOTA BOTANICAL STUDIES. 



to west. A third, Cypripedium acaule Ait., seems to be able to 

 exist under various thermal conditions since it is found near 

 the Arctic circle and extending southward appears in regions 

 but a few degrees north of the tropic of Cancer. Seven are 

 found east of the twentieth meridian, the area of greatest con- 

 centration apparently finding its western limit in Minnesota, 

 while the remaining species appear most prominently on the 

 western coast. Five have a general range there, between the 

 fortieth and fiftieth parallels and extend eastward through Mon- 

 tana. Another species is found in Alaska and one of the west- 

 ern forms ranges from New Mexico and Texas down to Panama. 



Such an arrangement is best explained when it is remem- 

 bered that these plants are essentially forest dwellers. The 

 areas may then be considered similar to the forest areas as di- 

 vided by Sargent. By this writer, the continent is separated 

 into two regions, the Atlantic and the Pacific. "Both regions 

 are more or less dissimilar, but united at the north by a broad 

 belt of sub- arctic forests extending across the continent. ""^^ 

 The trans continental belt of Cypripedium is not so wide as the 

 forest belt but extends a few degrees north of the fiftieth par 

 allel. The peninsular tract formed by Cypripedium acaule 

 Ait in reaching almost to the limits of the forest areas is the 

 most pronounced departure from the general distribution of the 

 American species. "On the south, the two main divisions are 

 united by a narrow strip of the flora peculiar to northern 

 Mexico, here extending northward into the United States. 

 Typical North American species, peculiar to the forests of the 

 Atlantic or of the Pacific mingle upon the Black Hills of Da- 

 kota, and upon the Guadalupe and other mountains of western 

 Texas, and the outposts between the Atlantic and Pacific re- 

 gions." 



The two species forming, the trans- continental belt and Cy- 

 pripedium candidum Muhl., one of the species of the Atlantic 

 region appear upon the plains of North America. This por- 

 tion of the continent is considered to be "debatable ground 

 where a continuous struggle between forest and plain takes 

 place." There is a sufficient precipitation of moisture to cause, 

 under normal conditions, a growth of forest but "so nicely bal- 

 anced is the struggle that any interference quickly turns the 

 scale." The following conclusion is then drawn, that it is not 

 improbable that the forests of the Atlantic region once extend- 

 ed continuously as far west at least, as the ninety-fifth meridian 



(58) Sargent, C. S. Kept. Forests N. Am. Census Rept. 10:4. 1881. 



