TECHNICAL BULLETIN 17 



centration which permits spore germination. It may be pointed out 

 that both Fusarium and Helminthosporium are chiefly soil organisms. 

 Among the organisms that Webb studied, only Fusarium responded 

 favorably to an alkaline medium. Maximum germination occurred at 

 hydrogen-ion concentrations of pH 2.8 and pH 7.4. From pH 6.2 

 a steady increase in germination occurred with the increase in hydrogen- 

 ion concentration up to pH 2.8. From the same point, a steady increase 

 in germination also occurred with the decrease in hydrogen-ion con- 

 centration and practically the same maximum was reached at a con- 

 centation of pH 7.4. Examining the data of H. sativum again, there 

 is a steady decrease in germination from the neutral point with the 

 increase in hydrogen ions up to a concentration of pH 2.4, where no 

 germination occurred during 18 hours and only very slight germina- 

 tion during 36 hours. However, the usual bimodal curve is obtained, 

 but, in this case, both maxima occur on the alkaline side at hydrogen-ion 

 concentrations of pH 7.8 and pH 9.2. With H. sativum, germination 

 occurred chiefly in the alkaline solutions. 



A series of spore germination tests was also made in Czapek's 

 solution minus the sugar, with various hydrogen-ion concentrations 

 ranging from pH 2.6 to pH 9.8. The results are represented by the 

 curve in Figure 7. In this case also the bimodal curve was obtained. 

 The first maximum, however, occurred on the acid side of neutrality at a 

 hydrogen-ion concentration of pH 6. The second was on the alkaline 

 side at a concentration of pH 8. 



IOO 



80 



3*56789 tC 



Hydrogen ion concentration p ti 



Fig. 7. Percentage Germination of Spores of Helminthosporium sativum in Czapek's 

 Solution Minus the Sugar at Various Hydrogen-Ion Concentrations 



