EXPERIMENTS WITH THE HOP ORGANISM. 85 



Remarks. — The explanation of the failures and of the very slow 

 growth of the successful inoculations is a matter which must be left 

 to the future. Two or three possible <\x])lanations may be offered. 

 No great amount of energy was devoted to attempting to isolate the 

 organism from sugar beets until after wo had read Professor Jensen's 

 paper late in the autumn of 1910. The gall on sugar beet then assumed 

 a new importance in our e3^es, and we made, as we have stated, dili- 

 gent attempts to get out an organism with which the tumor on sugar 

 beets could be reproduced. We began, however, not until the end 

 of the growing season, namely, in November, when the galls were old, 

 and although we plated from those which had no decaj^ed spots on 

 them, it is quite certain that the galls had nearly or quite approached 

 the end of their growth for the current season, and may be assumed 

 to have been several months old. We think, therefore, either that 

 the right organism was dead for the most part in the tissues at this 

 time, or so weakened by its own by-products or by reactions of the 

 plant that it had lost its virulence. There seems to be no good reason, 

 if one thinks about it, why an organism which loses its virulence in 

 culture tubes might not also lose it in the interior of the host plant, 

 if it had ceased or nearly ceased to stimulate growth, and had been 

 subject for some weeks or months to harmful reactions resulting from 

 its own products and those of the host itself. The fact that after 

 3 months of hard work, out of 42 colonies selected from several 

 thousand as the most hopeful we have found only 7 able to produce 

 tumors, shows how difficult it is sometimes to isolate a pathogenic 

 organism from material known or believed to contain it. 



HOP ON DAISY. 



Inoculations op February 8, 1908 (Brown). 



Five plants of the Queen Alexandra variety of daisy were inoculated 

 by needle pricks from a slant agar culture 5 days old. (For origin, 

 see Hop on Hop, p. 90.) Four and 5 shoots were inoculated on each 

 plant. The plants were old and growing slowly. 



Result. — February 18, 1908: There were protuberances at all 

 places of inoculation. 



April 22, 1908: No large knots like those due to the daisy organ- 

 ism were produced, although there was definite evidence of infection 

 in each plant. 



Inoculations of February 10, 1908 (Smith). 



Twelve plants of the Paris daisy were inoculated with the hop 

 organism from cultures of February 3. They were Nos. 1 to 12, 

 inclusive, each made from a separate colony. 



213 



