Oct. ai, 1918 



Angular-Leaf Spot of Cucumber 



207 



because the sensitiveness of the organism to sunlight was known and 

 because moisture, such as dew on the leaves, wa« thought to be the most 

 important factor in infection. A few infections were nearly always ob- 

 tained in this way, but the number was consistently smaller than occurred 

 on leaves naturally infected. A suggestion that the factors limiting the 

 number of leaf infections were in some way involved with the time of 

 inoculation was obtained when, from a series of inoculations made in the 

 field at intervals of 2 and 4 hours during the day and night of a 24-hour 

 period, more abundant infections resulted from the inoculations made 

 during the day. Evidence that infection occurs more abundantly when 

 inoculations are made during the day was confirmed by other tests. 



The idea that duryig the process of photosynthesis enough oxygen was 

 given off through the stomata to exert a chemotactic action on the caudal 

 bacteria was first conceived as a possible explanation of the different 

 results from night and day inoculations. The hypothesis was abandoned 

 after experimental tests. Plants which were kept in darkness for 24 

 hours before and after inoculation became infected to about the same 

 extent as the controls. 



The idea that stomatal movement might be a factor was next hit 

 upon. Pool and McKay (11) found that there was a relation between 

 stomatal movement in sugar-beet leaves and infection by Cercospora 

 heticola. This fact suggested that in the case of the disease under con- 

 sideration a similar relation might hold true. To study the behavior of 

 the stomata the method described by Lloyd {10) of direct visual observa- 

 tion of the stomata in situ was utilized. It was found that the stomata 

 on the lower surfaces of the leaves were generally open during the day 

 and closed at night. The movement of the stomata on the upper sur- 

 faces was not always the same as those on the undersides, but this fact 

 is of no special significance here. It was then found by repeated tests 

 that inoculations on the under surfaces made in the morning, when the 

 stomata were observed to be opep, gave much more abundant infec- 

 tions than did similar inoculations made at night, when the stomata 

 were seen to be closed (PI. 14). The following table gives a comparison 

 of the number of infections from night and day inoculations. The two 

 plants used were of the same age, each having seven leaves at time of 

 inoculation. Leaf 7 is the youngest leaf of each. They were treated 

 similarly except for time of inoculation. 



On the plant inoculated in the evening the youngest leaves. No. 6 and 

 7, showed many more infections than did the older leaves, and this has 

 been repeatedly found in other inoculations. Why this difference in in- 



