268 Journal of Agricultural Research voi. xm, no. s 



when grown in culture (a.bout six hours). The germ tube of some of 

 the spores could be seen to enter the cell walls of the epidermal cells. 

 Penetration took place at a point of contact of the epidermal cells; 

 hence, the germ tube probably grew between the cells throughout. In 

 no case was the germ tube observed within a cell (fig. i, D, showing 

 penetration). In order to determine whether penetration had actually 

 taken place, water was introduced under the cover glass by means of a 

 capillary tube. In the slight currents which were set up, the free ends 

 of the spores whose germ tubes had penetrated swayed back and forth, 

 but did not wash away, as did those whose germ tubes had not entered 

 the host tissue. 



RELATION OF TEMPERATURE TO INFECTION 



As the season progressed it was found increasingly difficult to infect 

 plants in the greenhouse, and after June i this was quite impossible. 

 The assumption that temperature had considerable influence on the 

 ability of the fungus to infect healthy plants was proved by the following 

 experiment. 



Twenty-five plants were inoculated from a 2 2 -day-old culture, by 

 smearing mycelium on the upper and lower surfaces of the leaves. Ten 

 plants were left uncovered in the greenhouse, and 10 were covered with 

 bell jars. The temperature of the greenhouse varied from 25° to 38° C. 

 Five plants were placed in a low-temperature thermostat at 15°. The 

 air conditions in the thermostat were humid, being comparable to the 

 plants under the bell jars. 



After three days the plants were examined. Those in the greenhouse 

 were apparently unaffected, but the plants which had been placed in the 

 refrigerator had developed the characteristic lesions of M. panattoniana. 

 The greenhouse plants were subsequently watched for 10 days, but no 

 infection could be observed. Evidently temperature is an important 

 factor in the inception of this disease. These results indicate that 

 infection periods occur in cool weather rather than in hot, bright weather. 

 This is in accordance with the observations of Appel and Laibach (j), 

 who say : 



... It may happen in warm dry weather that the attacked leaves fall and yet 

 the plants in spite of that fact go on to the formation of usable lettuce. Damp weather 

 is especially favorable to the development of the fungus, and therefore it is clear that 

 due to this cold wet spring (1907) the disease has taken on such a threatening char- 

 acter. — Translation. 



RELATION OF MOISTURE TO DEVELOPMENT OF LESIONS 



Twenty plants were inoculated and placed in a Wardian case equipped 

 with a fine jet for water adjusted to form a spray. A tested ^ hair hygrom- 

 eter registered 70 per cent. The saturated plants were allowed to remain 



1 Compared with sling psychrometer. 



