450 



Journal of Agricultural Research 



Vol. XX, No. 6 



tion they were at the temperature of the cases. The inocolum used in all 

 instances was from a 48-hour-old culture of Pseudomonas citri in beef 

 bouillon. While the plates were being inoculated precautions were taken 

 to maintain them at the same temperature as that of the case. At the 

 end of every 24 hours two plates were taken from each case, and the 

 increased diameter of the colonies was measured. 



In studying the rate of enzym action, an iodin solution 1 was poured 

 over the plate to be tested, was allowed to remain a few moments, and 

 was then poured out. The result was that the colonies stood out as a 

 lemon- yellow color, surrounded by a clear zone which came next showed 

 the disappearance of the starch and its conversion into maltose and 

 achroo-dextrin. This was followed by a reddish band, indicating erytho- 

 dextrin, an intermediate product, which merged into a light blue band 

 and finally into the dark blue color of the remaining agar. Thus, on 

 one plate, both the growth of the colonies and the rate of the enzym 

 action, as indicated by the iodin test, could be measured. 



Table I gives the diameter of the colonies in millimeters for each day 

 and temperature. Each reading represents an average of 28 measure- 

 ments. 



Table I. — Diameter in millimeters of colonies of Pseudomonas citri on soluble starch 

 agar at various temperatures 



When the time factor, or length of exposure, is considered, the opti- 

 mum temperature for the growth of Pseudomonas citri on soluble starch 

 agar is between 20 and 30 C. There is evidence of a decided lag in the 

 growth of the organism between 15 and 20 . In other words, while the 

 amount of growth at 20 is just one day behind that produced at 25 

 and two days behind that at 30 , the growth at 15 is much slower, being 

 two days behind the growth made by the organism at 20 . At 20 , growth 

 starts the first day, while at 15 , growth is just starting at the end of the 

 second day. This point is very well brought out in figure 1 , where the 

 rate of enzym action at the various temperatures is plotted. 



1 The solution was composed of 0.5 gm. potassium iodid and 1.0 gm. iodin, allowed to stand overnight 

 together in 10 ce. of water. It was then diluted to 100 ec. (stock solution). As needed, the stock solution 

 was diluted to abmit one-half or less, depending on the material tested. 



