162 VEGETATION OF THE PEAK DISTRICT [CH. VI 



conditions, have directly or indirectly an enormous influence on 

 the differential distribution of the flora and vegetation of that 

 district. Precisely what this influence is, however, is a matter 

 on which the present state of knowledge in physiological botany 

 allows no definite statement to be made. 



Clements (1907: 18) opposes the "chemical theory" (cf. 

 Schimper, loc. tit.); and even goes so far as to assert that "it 

 now appears entirely incorrect to ascribe the presence or absence 

 of certain species on limestone soils to the chemical nature of 

 the latter." Without doubt, the " chemical theory " requires 

 close investigation by modern plant physiologists ; and a re- 

 statement of the whole position is urgently needed. However, 

 the study of the vegetation of a district like the present, where 

 highly calcareous soils occur in close proximity with soils 

 extremely poor in lime, where the climatic factors obtaining 

 over the two types of soil are identical, where both habitats 

 exhibit every transition from wet to dry, and yet where the 

 flora and vegetation of the two types of habitat show very 

 marked differences, should convince any observer that the 

 " chemical theory " is far from being effete. 



Hall and Russell (1911 : 54) have recently stated that "soils 

 devoid of carbonate of lime are never fertile, because without 

 it the plant food of the soil cannot readily be brought into a 

 condition available for the plant, and many of the most important 

 bacterial actions in the soil are dependent on the presence of a 

 base like carbonate of lime." 



Kraus (1911) has recently published some valuable statistical 

 data bearing on the question; and not until more work has 

 been performed on these lines will it be possible to reach the 

 solution of this intricate and much-debated problem. 



