EFFECT OF CHEMICAL ENVIRONMENT I33 



that is, the fertile hyphse, show in general a negative hydro- 

 tropism. This is the reason why the conidiophores and spor- 

 angiophores of the various molds generally rise at right 

 angles to the surface of the medium upon which the organ- 

 ism is growing. As a result the spores are developed in an 

 environment relatively dry, and the conditions are, there- 

 fore, much more favorable for their distribution by cur- 

 rents of air. It will be noted that hydrotropism is much 

 more important in controlling the direction of growth 

 of molds than it is in higher plants. For the latter, 

 gravity is frequently the agency most active in deter- 

 mining growth direction. 



Effect of Chemicals on Eapidity of Growth 



Various chemicals exert a marked influence upon the 

 rapidity of growth of microorganisms, that is, upon the 

 length of the average generation time. Among the more 

 important of these may be mentioned the concentration 

 of hydrogen ions, the concentration of oxygen, the con- 

 centration of nutrient substances, the kinds of nutrient 

 substances present, the stimulating effect of dilute 

 poisons, and the effect of substances of autogenic origin 

 not belonging to any of the preceding groups. 



Effect of Hydrogen Ion Concentration on Rapidity of 

 Growth. — For every organism there exists that concentra- 

 tion of hydrogen ions in the medium which is most 

 favorable for development, that is, the optimum; likewise 

 a maximum concentration beyond which it cannot grow; 

 and a minimum concentration, that is, a concentration of 

 hydroxyl ions likewise in which it cannot develop. The 

 growth limits vary widely with different species. Most 

 species of bacteria prefer to grow in hydrogen ion concen- 

 trations represented by the Pg values 7 to 8.- Yeasts and 

 certain species of molds prefer solutions which are some- 

 what more acid. It has been found by laboratory tests that 



