58 THE SCOTTISH BOTANICAL REVIEW 



increasing sterility and loss of available nutriment in the soil. 

 In tropical rain-forest, in spite of the greater rainfall and deep 

 leaching of the sub-soil, excessive humus accumulation and 

 surface-acidity are checked by the rapidity of plant decay that 

 follows a greater degree of warmth (i). This not only results 

 in a more rapid oxidation of decaying plant-tissues and 

 removal from the surface of organic acids, but probably also 

 in a more complete return of mineral food to the soil. 



Agricultural experts have proved that lime benefits the soil 

 in various ways, mechanical and chemical, but the latter 

 appear of more consequence from the ecological standpoint. 

 The importance of lime, as a chemical base, has been 

 demonstrated as an agent of neutralisation in the acidity 

 that follows the biological processes of humification and 

 nitrification, and also for setting free potash from inert, 

 zeolitic combinations in the soil. It is further considered 

 essential for the working of the most widely distributed, 

 known form of free-nitrogen-fixation (22). 



Plant physiologists believe that calcium performs important 

 duties in certain metabolic processes in the higher plants, but 

 the great differences in the amounts of lime in the soil, 

 leading to the choice of habitat in various groups of calciphile 

 plants, would negative far-reaching relations in this direction. 

 In the case of heath and bog plants, there are several inde- 

 pendent experimental results proving that calcium carbonate 

 is not in itself prejudicial to their growth (23). Further, that 

 a number of plants should be distinctly calciphile in one 

 district, though growing elsewhere where lime is deficient, 

 suggests that the presence of lime is not the only factor 

 involved. 



We should therefore look for relations between calcium 

 carbonate and other substances in the soil for a full under- 

 standing of the conditions underlying the segregation of 

 calciphile and calciphobe species, and though further ex- 

 perimental work is needed before we can arrive at a solution 

 of the problem, it may be well to define those physiographical 

 factors which seem to have bearings on the subject. 



In cold-temperate, humid regions the so-called calciphobe 

 species are confined to habitats deprived of calcium carbonate 

 through leaching of the surface or stagnation of the soil waters. 



