142 



As we have alread}^ seen, H.^S is given off promptly from coconut 

 cultures of Ps. /ii/r/cinthi and Fs. campestris^ which do not become 

 grayed or browned, and is not given off from potato or carrot cultures, 

 which do become stained. 



A graying of steamed potato cylinders with subsequent pale brown- 

 ing of the water in which they stand — viz, a change similar to that 

 induced by many different sorts of bacteria — is readily produced by 

 adding to the tubes a few drops of ammonium sulphide. Tannin, in 

 the air, is oxidized readily to deep brown compounds when exposed 

 to ammonia, but this change does not take place in vacuo neither did 

 the potato cultures gray in vacuo. All bacteria or near^- all produce 

 ammonia and hydrogen sulphide, and many vegetable substances con- 

 tain tannins or allied compounds. 



A somewhat different result was obtained with Ps. 2)liaseoli^ which 

 also grays potato cylinders and becomes dulled in color b}^ the forma- 

 tion of a small amount of solu))le brown pigment. My attention was 

 drawn especially to this by the behavior of some potato cultures. 

 Eight of these were alike in their yellow color and Hie substratum 

 was grayed; the ninth, while alike in all other cultural respects, was a 

 much brighter yellow, and there was no distinct stain of the potato. 

 At the time I had in the laboratory two stocks of potato made from 

 different tubers. The question now arose whether the bright chrome 

 yellow culture was specifically different from the wax yellow cul 

 tures, or had been made accidentally on the newer potato stock and 

 was the same species, but different in color on account of some slight 

 chemical difference in the culture medium. To test this latter hypoth- 

 esis a tube from each potato stock was now inoculated from the 

 bright yellow culture. In one of these daughter tubes the growth 

 was dull wax yellow, and the substratum was distinctly grayed withni 

 48 hours. In the other the equally abundant growth was l)right 

 chrome yellow, exactly like the culture from which it was made. 

 There can be no doubt, I think, that the usual dulling of the slime of 

 Ps. phaseoli on potato is to be ascribed to the absorption of a brown 

 pigment formed out of some substance commonly present in the sub- 

 stratum. These two cultures made from the bright yellow culture 

 were tested for H2S on the fifth day by placing strips of lead acetate 

 paper in the top of the tubes under the cotton plug. The paper in 

 the dull yellow culture browned promptly. That in the bright yellow 

 culture did not brown at all at first (24 hours), but finally browned 

 feebly, corresponding to a slowly appearing feeble gray color in the 

 substratum. When the cultures were 9 days old and the paper had 

 been exposed 4 days the conditions were as follows: The tubes were 

 alike in volume of growth and in general appearance, except as given 

 l)elow. In one the color was a dull wax yellow, the lead paper was 

 d irk brown at the lower end, the substratum was distinctly grayed, 



