288 NINETEENTH REPORT. 



pump. Tlie air was passed through continuously for seven days at the 

 rate of about forty bubbles per minute (in H^SO^). This constantly 

 moving stream of air besides a dish of CaCl., under the bell jar served 

 to carry off tlie transpiration moisture. A polymeter was susjjcnded in 

 the bell jar for the purpose of determining the amount of humidity. 

 Humidity and temperature (in the room) were recorded at about eight 

 liour intervals. The former ranged from 10 to 23% and the latter from 

 17 to 21'^C. during the entire seven days. There were no signs of infec- 

 tion at the end of this period;, though one of the plants was badly wilted. 

 An inoculated check plant kept under a humid bell jar developed the 

 disease within the usual time. 



From these experiments it is evident that the moisture requirements 

 for germination and subsequent infection are high. While this has not, 

 so far as the author is aware, been determined for fungi similar to 

 M. sarcinae forme, fungi of analogous structure would probably have 

 similar high moisture requirements. 



It would seem that a parasite, one having entered the host would be 

 independent of the humidity of the atmospliere, because it has already 

 reached a more or less saturated environment. To verify tliis inference, 

 plants were inoculated and kept under a humid bell jar for 28 hours. 

 A few of the leaflets were then killed in 95% alcohol, cleared in phenol- 

 turpentine, stained, mounted in balsam, and examined under the micro- 

 scope. The fact of the penetration of the host being established, the 

 plants were then placed under a bell jar containing CaCL and a Lam- 

 brecht polymeter. In spite of the dryness of the atmosphere which con- 

 tained 54-57% relative humidity, disease spots developed within six 

 days after being removed to the dry atmosphere. This and the previous 

 experiment would indicate that the humidity of the atmosphere is an 

 inhibiting factor only until the fungus has entered the host. Once having 

 entered, the external humidity is immaterial. 



Relatio7i of light to infection: 



Under field conditions, the lower leaves of the plant are usually 

 infected first, and suffer the most destruction. Aside from their favorable 

 location near the ground, (i. e., favorable with regard to the possibility of 

 infection) this is due for the most part, to the fact that the lower leaves 

 retain more moisture. There is in addition, the factor of light, which 

 might play a more or less important role. To determine this relation, 

 the following exj^eriment was performed: 



Four healthy, well developed, potted plants were thoroughly sprayed 

 with a heavy suspension of spores. Each plant was placed under a 

 separate bell jar, and the lower half of two of the jars was covered with 

 black paper, so as to prevent any light from a lateral direction reaching 



