448 Journal of Agricultural Research vol. xx, No. 6 



APPARATUS USED 



A complete description of the temperature and humidity cases used 

 in this investigation will soon be published by Prof. Hottes. It is 

 sufficient to state here that the cases were large, well ventilated, well 

 lighted, and most important of all, supplied with accurate and reliable 

 controls. The temperature cases remained constant to within 0.5 C. 

 and were controlled at 5 intervals from 5 to 30 . For work above 30 

 ordinary bacteriological incubators and one large case held at 35 °, but 

 varying several degrees, together with constant-temperature water baths, 

 were used. The cases used for the humidity work were accurate to 

 within 2 to 4 per cent and could be regulated for any desired percentage 

 of relative humidity. The temperature of these cases could also be 

 readily regulated and controlled. Thus, the writer has had the extreme 

 good fortune of working with well-regulated temperature and humidity 

 controls, which were not a continual worry or source of error. 



INFLUENCE OF TEMPERATURE ON GROWTH OF THE ORGANISM 



Little work has been done on the temperature relations 01 Pseudomonas 

 citri. Doidge (1) states that — 



it grows well at 3o°C, rather more slowly at 25 C, and very slow progress is made 

 at 20 C. 



Wolf (17) in preliminary tests found that — 

 the thermal death point was between 58 C. and 70 C. 



and further that — 



no growth occurred in tubes exposed for temperatures above 65 C. 



Stevens (12) reports that — 



bacteria (P. citri) have been killed by temperatures ranging from 55 C.-60 C, 

 when exposed for a period of five minutes. 



Three types of culture media were tested — a liquid, a liquefiable solid, 

 and a solid. These furnished a means of comparing the growth of the 

 organism on different types of media, and if any differences existed 

 between the rate and amount of growth on the different media at various 

 temperatures they could be easily detected. Beef bouillon was used as 

 the liquid, soluble starch agar as the liquefiable solid, and steamed 

 potato cylinders as the solid. Since the most comparable results were 

 obtained with soluble starch agar, they will be taken up first. 



Soluble starch agar.— Hasse (2), Wolf (iy), and Jehle (5) have 



noted the characteristic growth of Pseudomonas citri on potato plugs, 



and especially the formation of a narrow white zone along the margin of 



the bacterial growth. Doidge (7), however, says: 



I have failed to perceive, except in one or two doubtful instances, the narrow white 

 zone on the uninfected surface following the line of the streak in young cultures, 

 which have been recorded both by Hasse and Wolf. 



