316 IOWA ACADEMY OF SCIENCE Vol. XXVI, 1919 



frequent to common and widely distributed over the state. Pteris 

 aquilina, the nearly cosmopolitan species, does not occur, neither 

 does Polypodium vulgarc nor Camptosorns rhisophyllus. There are 

 no Lycopodiums. The nearly cosmopolitan species, Athyrium filix- 

 foemina, is found in Nebraska only in two or three favorable 

 locations. 



SUMMARY AND CONCLUSIONS 



From this it is readily seen that the fern flora of Nebraska is 

 conspicuous for its poor development. The reasons for this paucity 

 may be due to the efifects or mutual reactions of the effects of 

 several causes. Some of these are : 



(1) The greater portion of the state is too arid for a rich 

 development of fern growth ; much of the surface is vmsuitable for 

 any ferns, the suitable areas being few and restricted, thus limiting 

 the possible number of species. 



(2) Entire absence of endemic forms; there are no species 

 peculiar to Nebraska. 



(3) Lack of development of features favorable to distributional 

 adaptation ; ferns are rather rigid in their requirements and do not 

 readily overcome new environmental difficulties. 



(4) Ferns are comparatively old from the standpoint of evo- 

 lution, being far beyond their period of culmination, while the' 

 soil of Nebraska is comparatively new. 



(5) The centers of migration for the ferns into this area are 

 the Appalachian and the Rocky Mountains. The broad prairies of 

 northern Missouri, of Iowa, and of southern Minnesota prevent 

 migration from the east. A number of species of ferns that are 

 absent from similar localities in eastern Nebraska, are found along 

 the eastern border of Iowa. The aridity of the western portion of 

 the state prevents migration from the Rocky Mountains. 



(6) The line of easiest migration into Nebraska is by way of 

 the Missouri river valley, a route suitable to only a limited number 

 of ferns. 



(7) Lack of development of a mountain range wathin the state 

 or near by with a humid climate, the submontane region of western 

 Nebraska being too arid. 



(8) l"hc area of fern distribution tends to lessen, many species 

 are now quite limited in range, and ferns as a whole have little 

 migrating tendency. 



