33 



the margiiuii growth and the individual rods on the margin were not 

 ver}' distinct (Zeiss 16 mm. and 8 mm. apochromatics with compensat- 

 ing oculars up to No. 18). 



Potato. 



More than 100 cultures of 7^s-. hy<(ct)ithl have been made on potato. 

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

 cm. long b}' 1 to 1.3 cm. thick) in well plugged clean test tubes of 

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

 use of drier cylinders, onl}" the curved bottom of the test tu])e 

 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 cleanly way, this short cooking on 

 3 consecutive da3^s is suihcient to render them sterile. 



The color of the organism on this substratum varies from bright 

 yellow to pale or dirty j^ellow. Usuall}^ the color is distinctly l)righter 

 than in corresponding cultures of Ps. campestris or Ps. 2)h(f'S&oli. 

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

 approximate Ridgway's Indian yellow (VI-5); i. e. , it is nearly as l)right 

 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. Frequentl}' 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 distinctly visible in 

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

 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 yellow or bright yellow growth, 

 covering a part only or nearly 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 aroimd the 

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

 cipitate which does not increase (14, 24, 41 davs). All distinctly 

 \'ellow growth is restricted to that part of the cyliMder above the 

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

 of the surface of the sul)stratum are not obscured and. as the lluid evap- 

 orates, the bacterial layer shows no tendency to follow down flic sides 

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

 is never any lilling up of the fluid with yellow slime, such as always 



21788— No. 28—01 3 



