Dec. i 5 . 1920 nfject of Temperature and Humidity on Citrus-Canker 453 



At 5 C, 33 to 35 , and 38 to 40 a light blue color was given with 

 the iodin, indicating that only a partial hydrolysis of the starch took 

 place. At io° the light blue zone persisted for several days, followed by 

 a wide reddish zone. It was not until the fourth day that a clear zone 

 was formed. Likewise, at 15 no clear zone was formed until the third 

 day. At 20 , 25 , and 30 the clear zones were present at the end of 24 

 hours, increasing in diameter in proportion to an increase in temperature. 

 The curves for the rate of enzym action are shown in figure 1 . Especially 

 noticeable are the differences in the rate of enzym action represented by 

 the 1 5 and 20 curves. The l#g mentioned under the rate of growth of 

 the organism at these temperatures is very well shown. Further in- 

 vestigations must be carried out before the explanation of this lag can 

 be given. 



Potato plugs.— The first trial with the growth of the organism on 

 potatoes was attempted with blocks of raw potatoes cut under aseptic 

 conditions and placed in Petri dishes with plain agar poured into the 

 dishes even with the top of the blocks t^keep them moist. However, 

 this method had to be abandoned because the surface of the blocks 

 oxidized and dried out too rapidly. Therefore in the following trials, 

 steamed potato cylinders were used. The same procedure was followed 

 as in the tests with soluble starch agar to bring the cylinders to the tem- 

 perature of the cases prior to and during inoculation. They were inocu- 

 lated by means of a shallow stab, and the organism was allowed to grow 

 out over the surface. The inoculum was taken from a 5-day-old culture 

 of Pseudomonas citri on potato plugs. The results are not as comparable 

 as those obtained for starch agar because of the variation in the amount 

 of inoculum and the physical differences in the potato cylinders them- 

 selves. However, in general the growth of the organism and the rate of 

 enzym action, as determined by the iodin test, followed the curves shown 

 in figure 1. As the red and blue zone was very narrow on the potato 

 cylinders, the total diameter of the zone is represented in Table III, 

 together with the growth of the organism. This table gives the average 

 of two trials of four readings each. 



