wherry: reactions of soils 595 



alongside of Microstylis uni folia and Spiranthes cernua. It 

 occurs also on sphagnum hummocks, the relations of which 

 will next be described. 



The hummocks of sphagnum and other mosses which develop 

 in many swamps or bogs support a number of characteristic 

 orchids especially Arethusa, Calopogon, Pogonia ophioglossoides, 

 and the above mentioned Cypripedium acaule. Tests of the 

 partially rotted vegetable matter which surrounds their roots 

 have shown that the acidity is often greater than that of the 

 water in the hollows between the mounds. Even in calcareous 

 swamps, where the water is alkaline, a definite and rapid increase 

 in acidity is found on ascending through the moss, and an in- 

 tensity of 300 is often present at the top. The more acid the 

 water in the first place, the less rapid is this increase, but the 

 same upper limit is almost ahvays reached. The orchids rooted 

 in this moss may therefore be livingi in highly acid conditions, 

 even though others growing between the hummocks may have 

 their roots bathed in alkaline waters. 



The habitat in which the most acid conditions of all may 

 develop appears to be the peat overlying clay which usually 

 occurs around the margins of bogs. In it grow^ Habenaria 

 blephariglottis, H. ciliaris, H. clavellata, H. cristata, and Liparis 

 loeselii. The intensity of acidity of such material has been 

 found to vary in general from 100 to 1000. In one case, how- 

 ever, a soil supporting Habenaria blephariglottis, near Hyatts- 

 ville, Maryland, the extreme value of 3000 has been observed. 

 It may be noted here that the peat used for growing tropical 

 orchids also shows an intensity of acidity of 100 to 1000. 



The results obtained with all the species studied are collected 

 in table 2.^ 



^It is not claimed that some of these plants may not occasionally grow in soils 

 with reactions outside of the ranges here given, but these ranges are based on 

 measurements in sufficient number to justify regarding them at least as normal. 

 Points of special significance in the tabulated series of reactions appear to be: 

 alkaline 10 (PhS.O), the greatest alkalinity observed; acid 100 (PhS.O), the 

 upper acid limit for many species, and the lower limit for a few; acid 300 

 (Ph 4.5>, the acid limit for a further large number; and acid 3000 (Ph 3.5) the 

 greatest acidity observed. 



