THE FERN BULLETIN 



39 



Trichomanes Pctcrsii in our own country may be cited 

 as an example, but very rarely do we find an outlying- 

 colony of some well known fern at any great distance 

 from the center of distribution, and there never has 

 been, nor will there ever be a record of a greater 

 separation between two colonies of plants for the 

 simple reason that the earth is not large enough to 

 allow the distance to be greater. 



Our own fern flora, again, affords several illustra- 

 tions of ferns separated by wide stretches of land or 

 water from others of their kind, but most of them oc- 

 cur in such a way, or are species of such affiniiies mar 

 the method of their distribution is not difficult to make 

 out. Leaving aside those cosmopolitan species that 

 are spread nearly throughout the world, we find that 

 all the others are limited on all sides by temperature, 

 moisture or shade or all three combined. Tropical 

 species are limited by low temperatures, while north- 

 ern ferns are affected by extremes of heat. The Arc- 

 tic species may extend equatorward along mountain 

 tops which make a congenial habitat and Tropical 

 ferns may extend toward the poles where warm winds 

 sheltered valleys, or mountain barriers to the cold 

 make a favorable locality for growth. 



Thus we can account for the occurrence of Adian- 

 Iwm C apillus-vcucvis in Great Britain and in Dakota 

 in our own country. Nowehere else in the United 

 States does the venus hair fern go north of Kentucky, 

 but certain hot springs in Dakota make a habitat in 

 which it can exist. 



In the case of several other ferns that are common 

 to North America, Europe and Eastern Asia, such as 

 the forked spleenwort, green spleenwort and rusty 

 woodsia, the distribution is easily explained by the 

 fact that they are boreal ferns and extend southward 



