SOIL ACIDITY WHERRY. 261 



lutionary history and with the fact that the climatic conditions of 

 the more northern regions, as well as the glacial action which has 

 affected them, are adapted to the accumulation of calcareous soils, 

 whereas in more southern regions there is, on the whole, a tendency 

 for soils to develop acidity. 



Soil reaction and plant relationship. — In several cases listed in 

 the above table related plants show marked differences in their soil 

 preferences. Thus Pellaea glabella is much less tolerant of acid 

 condition than is P. atropurpurea; Asplenium ruta-muraria and 

 A. montanum lie at the opposite extremities of the group in this 

 respect; G&mptosorus rhizophyllus and Asplenium pinnatifidum are 

 also widely separated; the three small Wooddas form a subgroup, in 

 which W. alpina is intermediate both in morphologic characters and 

 soil reaction; and finally the two species of Cystopteris differ dis- 

 tinctly, and the two Polypodiums markedly, in their soil preferences. 



On the other hand, the two ferns listed, which the evidence indi- 

 cates to be recent hybrids, namely, Asplenium ebenoides and A. 

 gravesii 15 do not differ essentially in soil requirements from their 

 parents. It is accordingly reasonable to conclude that the greater 

 the divergence in soil reaction of related species the longer time has 

 been required for their development since their original separation. 



Ferns of woods and swamps. 16 — The ferns to which this essay is 

 devoted are, on the whole, less sensitive to soil acidity and alkalinity 

 than those which grow on rocks, to which attention was directed 

 above. It seems worth while, however, to place on record what 

 data have been obtained on testing the soils surrounding their roots, 

 by the indicator method. The following designations are used in the 

 class column of Table 10 : 



AA, intensely acid ; appearing to thrive only in mediacid soils. 



A, acid ; growing well in soils of practically all degrees of acidity. 



I, indifferent (relatively) ; appearing to thrive in both acid and 

 alkaline soils as long as neither reaction is extreme. 



C, calcareous or circumneutral ; growing best in neutral or nearly 

 neutral soils, though extending throughout what is termed the cir- 

 cumneutral range (specific acidity 10 to specific alkalinity 10). No 

 instance has been observed of a species which will not grow in 

 neutral or slightly acid soils if it thrives and is ordinarily found in 

 actually alkaline ones. 



16 Added to the list since the paper was originally published ; compare Am. Fern Journ., 

 10, 119-121, 1920. 

 16 Abstracted from paper in Amer. Fern Journ., 11, 5-1G, 1921. 



