SOIL ACIDITY WHERRY. 257 



to be classed as acid-soil plants. In the course of geological field 

 trips and vacation outings for several years past the writer has been 

 collecting information upon these relationships. The first plan tried 

 was to carry samples from the field to the laboratory and there de- 

 termine the percentage of calcium oxide (lime) present, both the 

 total amount and the soluble portion; and a brief account of 

 some results thus obtained has been published. 13 Subsequently it 

 proved possible to work out the above-described method for meas- 

 uring, in the field, the soil reaction (acidity or alkalinity), which is 

 much simpler as well as more instructive than the determination of 

 lime. 



The writer's field work on rock ferns has extended from Vermont 

 and New Hampshire on the north to West Virginia and Virginia on 

 the south, and all of the common species, as well as a few of the 

 rarer ones, occurring within these limits have been studied. The 

 results obtained are presented in Table I. The correctness of pre- 

 vious classifications has been confirmed in most cases, but consider- 

 able new data have been obtained on many species. As pointed out 

 in the above-cited paper on rock ferns, it is the soil rather than the 

 rock which affects the growth of plants ; acid humus sometimes coats 

 limestone ledges to such a thickness that species not normally favor- 

 ing calcareous soils flourish there ; and on the other hand, while the 

 soils over sandstone, schist, granite, etc., are usually more or less 

 acid in reaction, alkaline (calcareous) soils may accumulate on these 

 rocks through the decomposition of vegetable debris, and typical cal- 

 careous soil species thrive there. Accordingly, actual tests have been 

 made of the soils at the roots of the plants investigated. It is prob- 

 able that further work will result in extending somewhat the ranges 

 of reaction here recorded, although it seems unlikely that the classi- 

 fication of many of the species will be changed. It is hoped, in par- 

 ticular, that species which the writer has been unable to study fully 

 will be worked up by others. 



Method of recording data. — For recording data on individual 

 species the following plan has been proposed: 14 Arrange numbers 

 representing specific acidities in a horizontal line, decreasing from 

 left to right. At the left of this line place a column of numbers, 

 increasing upward, to show how many observations have been 

 made. Then place X's above each acidity opposite the number 

 representing how many times such a degree of acidity has been ob- 

 served at the roots of flourishing plants. A curve may be regarded 

 as drawn through the X's thus placed, and from its shape the be- 

 havior of a plant with respect to soil acidity may be seen at a 



ls Amer. Fern Journ., 7, 110-112, 1917. 



14 Proc. Acad. Nat. Sci., Philadelphia, 1920, p. 96 et seq. 



42803°— 22 17 



