SOIL ACIDITY — WHERRY. 263 



Circumneutral reactions are shown by soils which either contain 

 considerable amounts of undecoruposed carbonate minerals, are 

 bathed by alkaline spring waters, or are so situated as to favor the 

 accumulation of leaf mold. An acid reaction, on the other hand, 

 tends to develop in soils which either lack carbonate minerals, are ex- 

 posed to the action of rain water so that basic constituents become 

 leached out, or are so located that peat can accumulate. 



In northern latitudes, or at high elevations, rocks disintegrate 

 more rapidly than they decompose, and so, if the rocks at any locality 

 thus situated contain suitable minerals in the first place, circumneu- 

 tral soils may develop. Glacial deposits are especially likely to con- 

 tain undecomposed carbonate minerals, which the ice has ground 

 from rock ledges; and actual tests of the soils derived from such 

 deposits in Pennsylvania, New Jersey, and the New England States, 

 have shown that even after exposure to the weather for many thou- 

 sands of years, since the last ice sheet retreated, sufficient quantities 

 of undecomposed minerals are still present in many places to keep 

 the reaction circumneutral. 



The territory left bare by the retreat of the great ice sheet must at 

 first have presented an almost unbroken expanse of circumneutral 

 soils, and the vegetation which first occupied it accordingly com- 

 prised only plants which thrive best in such soils. Although acid 

 soils have developed subsequently in many places, and permitted in- 

 vasion by plants adapted to growth under acid conditions, a con- 

 siderable number of the original occupants still persist, and are 

 to-day classed as " northern " species. 



In more southern regions, on the other hand, decomposition usually 

 outstrips disintegration, so that soils containing undecomposed car- 

 bonate minerals are relatively rare. Except where limestone out- 

 crops or where leaf mold accumulates, therefore, the dominant soil 

 reactions are inclined to be acid, and the plants, established there 

 since long before the glacial period, have become adapted to growth 

 in such soils. The favoring of circumneutral soils by northern species, 

 and of acid soils by southern ones, is thus connected with the geo- 

 logical history of the respective regions. 



STUDIES ON ORCHIDS. 



The results which have been obtained in the study of the native 

 orchids are here summed up in a table similar to those used for ferns, 

 the data from three previous publications being combined. 17 The 

 species are arranged for convenience in several more or less natural 

 groups, and are listed in each group in the order of increasing acidity 

 of their soils. 



" The reactions of the soils supporting the growth of certain native oi-chids, Journ. Wash. 

 Acad. Sci., S, 591, 1918 ; Table IV in Soil tests of Ericaceae, etc., Rhodora, 22, 47, 1920 ; 

 Table 3 in Observations on the soil acidity of Ericaceae, etc., Proc. Acad. Nat. Sci., 

 Philadelphia, 1920, 110. A few subsequent additions have also been made. 



