GROWTH 309 



can also stimulate it, if applied in weaker dilutions. The differ^ 

 ence must therefore be considered to be rather quantitative than 

 qualitative, and only substances which inhibit growth in very weak 

 concentrations should be regarded as poisonous. The action of all 

 cations in a pure state is also poisonous. Naturally only those 

 solutions are good nutrients for plants in which the ions are in 

 equilibrium. An especially poisonous influence is exhibited by the 

 H + ion. For the majority of plants, with the exception of some 

 molds, a considerable acidification of the medium is lethal. The 

 different plants are not equally susceptible to the concentration of 

 hydrogen ions and this (Art. 32), explains to a considerable extent 

 the distribution of plants in relation to soil acidity. 



The mold fungi are especially convenient objects for the study 

 of both the stimulating and the poisonous effect of various sub- 

 stances. In his classical work on the conditions of growth of 

 Aspergillus niger, Raulin has shown that the yield of this fungus 

 depends not only upon the presence in the solutions of definite 

 mineral and organic compounds serving directly as nutrients, but 

 also on the presence of special stimulants, which can alter growth 

 considerably. Thus, for example, the addition of very small quan- 

 tities of zinc sulphate (0.0005 per cent) caused a two to threefold 

 increase of the yield. The same experiments have been confirmed 

 by later work. In growing microorganisms, the possibility of the 

 stimulating influence of organic and inorganic compounds is now 

 always taken into account. These observations have lately found 

 application in agriculture. By his investigations of the distribu- 

 tion and role of manganese salts in plants, Bertrand laid the foun- 

 dation of the doctrine of the so-called complementary or catalytic 

 fertilizers. These fertilizers, chiefly manganese salts, produce 

 better crops not as direct nutrient substances, but as stimulants of 

 growth and development. The role played by iron in plant life also 

 appears to be largely of a catalytic nature. 



Catalytic fertilizers have not found wide practical applica- 

 tion. The addition of manganese sometimes produces an increase 

 in yield, while at other times it remains without influence. The 

 conditions under which it may be definitely effective have not been 

 established as yet. Growth stimulants have recently received a 

 new emphasis through the efforts of the Bulgarian professor, 

 M. Popoff, who is of the opinion that it is not absolutely necessary 

 that the stimulant should act continuously upon the plant during 



