THE QUESTION OF IMMUNITY. 181 



On February 9 all of the resistant cuttings had small galls, except 

 4, and 2 of these showed indications of swelling. This was seventy 

 days after inoculating, and nothing comparable with this has been 

 known to follow the inoculations of a daisy plant which had never 

 before been inoculated with the gall organism. The beginnings of 

 gall formation have been seen on daisy as early as the fifth day after 

 inoculating, but the usual time for decided evidence is ten days or 

 two weeks and always within three weeks. 



On March 10, 1910, galls were forming on the 4 resistant cuttings 

 which were still free from galls on February 9. 



In July, 1910, all of the resistant plants bore large galls, i, e., 

 growths H to 2 inches in diameter." 



(7) Cuttings were made from these plants in August, 1910 (sixth 

 set), and inoculations were made on these in November, December, 

 and January, after they Avere well rooted and growing rapidl}^ The 

 results are not yet ready to be reported upon. 



So far as we have gone, loss of virulence ma}^ account for some of 

 our failures to infect, but not, it would seem, for all, since in some of 

 the experiments already described the check plants contracted the 

 disease promptly, while the others did not. The results now under 

 way ought to settle the question. 



The following results are believed to be due, in part at least, to loss 

 of virulence, but in part also to increased resistance. The weak point 

 in the reinoculations is the almost complete failure of the checks. 



In September, 1909, about 200 rapidly growling young daisy plants 

 (rooted cuttings from old plants) were inoculated in the top of the 

 shoot with young slant agar cultures of the old daisy gall organism 

 (strain B). 



No galls resulted. Thinking this complete failure might equally 

 well be attributed to increased resistance on the part of the plants, 

 since all of the cuttings had been taken from plants already twice 

 and thrice successfully inoculated, the plants were repotted, top 

 pruned, forced into rapid growth, and reinoculated. 



The first reinoculations were on December 6, using young agar sub- 

 cultures from several typical-looking colonies recently derived from a 

 daisy gall by Miss Lucia McCulloch. The bacteria were pricked in. 

 A. small part only of the plants were inoculated. Checks were kept. 

 All failed. 



On December 13 to 17 the entire 200 plants were reinoculated by 

 needle pricks, rather more than 400 groups of punctures being made 

 on young branches. For this purpose young agar subcultures were 

 used. They were derived from a colony recently isolated from a 



o A comparison of No. G with earlier results seems to indicate that even when first isolated from a gall 

 some colonies are more virulent than others. 

 213 



