260 ANNUAL REPOKT SMITHSONIAN INSTITUTION, 1920. 



some way of classifying; them on this basis. Those the dominant 

 reactions of which lie toward the left-hand side may be termed 

 " acid-soil plants." It should be noted that the degree of acidity 

 represented by habitats supporting these ferns is for the most part 

 less than that of the sphagnum bogs and sandy barrens where so- 

 called " oxylophytes " grow, so the latter term is not desirable for 

 application here. This class is designated by an A, for acid, in the 

 next to the last column of the table. 



The complementary term " alkaline soil plants " is unsuitable for 

 those showing dominant reactions toward the right-hand side of the 

 table, since the degree of alkalinity represented is at most but slight, 

 and moreover no species of this class has been found which will not 

 grow also in neutral and even slightly acid soils. The evidence 

 indicates that the important factor in the case of plants avoiding the 

 most acid soils is the relative abundance of calcium compounds, and 

 accordingly " calcareous soil plants " will be used. The terms " calci- 

 phile" (lime lover) and " calcicole " (lime grower) are often applied 

 to this class of plants. Since plants may grow in calcareous habitats 

 for various other reasons than " love of lime " the latter term is the 

 preferable one ; but neither is really necessary. This class is marked 

 in the table by a C, for calcareous. I, for indifferent, is used in one 

 instance. 



It is evident from the table that no sharp line can be drawn between 

 the two classes, as marked overlapping occurs in the central columns, 

 especially in those of specific acidity 30, 10, and 3. Laboratory tests 

 for calcium compounds have shown these to be present in practically 

 all the soils concerned, their amount and especially their solubility 

 diminishing markedly as the reactions approach mediacidity. By no 

 means all species showing calcium compounds in their soils are calca- 

 reous soil plants; for when the specific acidity exceeds about 30 the 

 physiological effect of the acid appears to predominate over that of 

 the calcium ; and although when the specific acidity is 10 or below, 

 the effect of the calcium is dominant, some acid soil plants can still 

 thrive even at the neutral point. In soils termed minimacid, plants 

 of both classes may flourish side by side ; but if enough occurrences 

 of each species can be studied, the dominant reaction is always found 

 to lie definitely toward one side or the other, and the plant can be 

 assigned to the corresponding class. 



In the final column of the table letters are used — N for northern 

 and IS for southern — to bring out the relation between the range of 

 each species and its soil reaction. The calcareous soil species prove 

 to be dominantly northern in their distribution, the acid soil ones 

 dominantly southern. This is evidently connected with their evo- 



