33 



the Jiiarginal growth jind the iiidividiiMl rods on the margin were not 

 very distinct (Zeiss U> nini. and 8 mm. apochromatics with compensat- 

 ing oculars up to No. 18). 



Potato. 



More than 100 cultures of J*^. JiydchdJil have been made on potato. 

 This medium was usualh' prepared by steaming slant cylinders (5 to 6 

 cm. long by 1 to 1.8 cm. thick) in Avell plugged clean test tubes of 

 resistant ghiss, in 1 to 3 c. c. of distilled water. Occasionally I made 

 use of drier cylinders, only the curved ))ottom of the test tube 

 being tilled with water. The potatoes found in the Washington 

 markets usually bear three steamings of 15 or 20 minutes each without 

 cracking open or losing their smooth surface and white color, and, if 

 they are prepared beforehand in a cleanl}^ way, this short cooking on 

 3 consecutive days is sufficient to render them sterile. 



The color of the organism on this substratum varies from bright 

 yellow to pale or dirty yellow. Usually the color is distinctly brighter 

 than in corresponding cultures of Ph. campestris or Ps. phaseolL 

 During the first week or two in most cases the color may be said to 

 approximate Kidgway's Indian yellow (VI-5); i. e. , it is nearly as bright 

 as gamboge. As the culture becomes old the color dulls. In well- 

 grown cultures not too old the color approximated Ridgway's wax 

 3'ellow (VI-7). The color of the slime from a typical potato culture 

 30 days old was exactly Ridgway's gallstone yellow. Frequently the 

 germs from very old cultures were brownish yellow in mass. The 

 slime from a culture 18 days old was between ocher yellow and tawny 

 olive. 



Usually, at temperatures of 20^ to 25^ C, in inoculations made from 

 broth cultures, the bacterial mass was not plainly visible along the 

 streak until after 2 or 3 days. In one case it was distincth^ visible in 

 24 hours, but then the temperature was 28° C., and the inocvilation 

 was with a mass of yellow slime from the surface of a potato culture. 

 After a week or two the germ appeared in potato cultures as a thin, 

 rather feeble, wet-shining, pale j^ellow or bright yellow" growth, 

 covering a part only or nearty the whole of the exposed potato, but 

 showing no inclination to fill up the water. 



There is, of course, a moderate clouding of the fluid around the 

 cylinder, and after some days or weeks there is a scanty yellow pre- 

 cipitate which does not increase (14, 24, 41 days). All distinctl}^ 

 yellow growth is restricted to that part of the cylinder above the 

 water. This growth is so thin that very often the slight irregularities 

 of the surface of the substratum are not obscured and, as the fluid evap- 

 orates, the bacterial layer shows no tendenc}^ to follow down the sides 

 of the cylinder and occupy the exposed surface of the potato. There 

 is never any filling up of the fluid with yellow slime, such a»s always 

 21788— No. 28—01 3 



