52 



Levi Otto Gratz 



tures were comparatively higher than when the other trials were made. 

 Here is shown the intimate relationship between the air and soil tempera- 

 tures. Careful air-temperature records were kept at all times. Such 

 records are given in table 16. These temperatures were not so high as 

 the maximum for the growth of the fungus, but were nevertheless much 



TABLE 16. The Different Maximum, Minimum, and Average Air Temperatures 



UNDER WHICH TRIALS VI, XI, AND XII WERE PERFORMED 



beyond the optimum temperature. They were a deciding factor both 

 directly in the growth of the fungus, and in drying out the surface layer 

 of the soil, thus delaying growth. There is no need of giving further data, 

 but similar figures could be given where a similar increase in the air tempera- 

 ture the previous year was accompanied by a corresponding decrease in 

 the rate of damping-off. The supposition that such decreases were due 

 to the inoculum can hardly hold, as the inoculum for all of these later 

 trials was the same as that used previously and had been prepared a longer 

 time before inoculation than in the earlier experiments. This might 

 have insured even a more thorough infection of the wheat grains with the 

 mycelium, and more sclerotial formation, if that were possible. 



Bottom as compared with top watering. 



The foregoing experiments emphasize the importance of the water 

 content of the surface soil as a factor in determining the health of plants 

 growing in infected soil. This is especially illustrated in trial VIII (table 

 11), where the plants even in a dry soil damped-off forthwith, because 

 the moisture content in the immediate vicinity of the seedlings had been 

 increased several days previous to inoculation. This is in accordance 

 with the recommendations of Jones (1908) and of Johnson (1914), who 

 suggest keeping the surface soil dry in order to delay damping-off. To 

 obtain some definite data, three culture vessels with a total of thirty-three 

 seedlings, watered by sprinkling the surface, were inoculated, and were 

 compared with two vessels containing twenty-one plants which were 

 given the same amount of water added to the bottom of the soil. All other 

 conditions were similar. There was considerably less damping-off in 

 the latter case, as well as a decrease in the rate (table 17). This experi- 

 ment was repeated with sixty plants (four culture vessels) for each_of 



