24 Harold Thurston Cook 



definitely the hygroscopic requirements of the fungus for fruiting, a 

 series of experiments was run at the New York State Agricultural 

 Experiment Station at Geneva, where a chamber in which the humidity 

 could be controlled was made available through the kindness of Dr. 

 J. G. Horsfall. The plants used in these experiments were inoculated 

 in the greenhouse at Ithaca and were taken to Geneva after the period 

 of incubation had passed. Accident reduced the number of plants that 

 could be used successfully in the first experiments, but the test was 

 repeated later for the writer by Dr. Horsfall with a different set of 

 inoculated plants. 



The experiments were conducted at night, since humidities could not 

 be maintained in the chambers under conditions existing during the day. 



The following plan of experiment was adopted. As many plants as 

 the chamber would hold were thoroughly watered and placed in the 

 chamber late in the afternoon. Some of these were atomized with water 

 and covered with bell glasses to make sure that the water would remain 

 on the leaves all night. The hydrostat was set for as near 100 per cent 

 humidity as was possible. A hygrothermograph placed in a box gave a 

 complete record of the temperature and humidity, and a sling psychrom- 

 eter placed where it would be exposed to the current of air set up 

 by a fan furnished an additional check on the temperature and 

 humidity. 



The plants under the bell glasses were in an atmosphere of 100 per 

 cent humidity with water on the leaves, while the others were in an 

 atmosphere of slightly lower humidity and without visible water on 

 the leaves. Plants that did not produce conidia under the conditions 

 of the experiments were afterward tested to make sure that they were 

 infected. This was accomplished by spraying the plants with water 

 and placing them under bell glasses on, the greenhouse bench or in a 

 chamber with high humidity. Only those that were proved to have 

 been infected are included in the experimental results. 



The plan of the second experiment was changed on February 21. 

 On that date the plants under the bell glasses were not watered, and 

 so their leaves remained dry, while those in the humidity chamber 

 were sprayed continually with water. 



The results of the experiment on humidity are shown in table 3. 

 An examination of this table shows that twenty-one of the twenty-two 

 plants produced conidia when they were exposed to high humidities 

 (98 to 100 per cent) and when there was visible water present on the 

 leaves ; thirteen of these twenty-one had previously failed to produce 

 conidia when exposed to high humidities but without water on their 

 leaves. The results of these tests indicate that water must actually 

 be present on the leaves for conidiophores and conidia to be formed. 

 Two of the twenty-two plants formed conidiophores and conidia when 



