PAPERS OX BIOLOGY AND AGRICULTURE 22o 



In summarizing the various theories of bog xerophy- 

 tism, Rigg (2) regards four as of chief importance, and 

 considers that these are : acidity, difference between air 

 and soil temperature, lack of aeration of the substratum, 

 and toxic substances in the substratum. It will be seen 

 that the conditions in these four theories fit in with the 

 observed relation between the distribution of bog plants 

 and the depth of the substratum. In the broad shallow 

 portions of the depressions the water is stirred up con- 

 tinually by the wind and mixed with finely divided matter 

 from the uplands, thus insuring aeration, neutralization 

 of acids, absorption of toxic substances, and a tempera- 

 ture more nearly the same as that of the air. The situa- 

 tion in the deep substratum would be just the opposite 

 and would favor the development of conditions which 

 would permit the growth only of plants with a structure 

 more or less xerophytic in character. 



The absence of sphagnum and other bog plants from 

 depressions with an alkaline substratum has been re- 

 ported before, but the experiments of Transeau ( 3 ) who 

 succeeded in growing sphagnum in the laboratory in 

 water containing 100 parts of calcium carbonate to the 

 million, have usually been interpreted as overthrowing 

 the theory that presence of calcium salts is the reason 

 for the absence of sphagnum. This experimental evi- 

 dence does not seem to the writer to be conclusive, for 

 either conditions in the field might be different in other 

 important respects from those in the laboratory, or the 

 conditions governing the germination and establishment 

 of sphagnum might be different from those affecting the 

 mature plant. 



In the bogs under consideration all other conditions 

 except the acidity or alkalinity of the substratum seem to 

 be the same in Xos. 7 and S as in the remaining bogs, so 

 it would seem as if the presence or absence of calcium 

 must be the limiting factor for the sphagnum. Experi- 

 ments in transferring sphagnum and other bog plants to 

 parts of bogs from which they are now absent are in 

 preparation, and it is hoped they may throw some light 

 on this question. 



