450 BROOKLYN BOTANIC GARDEN MEMOIRS 



and other dififerences exist; otherwise, I 

 speak only of "isolations." There are cer- 

 tainly two of these crown gall strains, and 

 probably many. 

 (3) Feeble and virulent strains exist in nature, i. e., there 

 is variation in virulence of colonies from the 

 same source — hop, carnation, rose, sugar beet, 

 etc. The author believes that the crown 

 gall bacteria not only lose virulence on cul- 

 ture media but also in the tissue of the gall. 

 Examples are : (a) of three colonies selected 

 as typical from plates poured from a hop 

 tumor in 1910 only one proved infectious; 



(b) of six colonies plated from a witch broom 

 on carnation, all of which looked alike and 



« typical for crown gall, only one was found 



to be able to cause tumors when inoculated; 



(c) of seven colonies selected as typical from 

 a plate poured from a rose gall only three 

 proved infectious, and of these two were 

 actively infectious, while the other was only 

 feebly so; (d) of five colonies selected as 

 typical from a plate poured from a tumor on 

 Pelargonium none proved infectious, al- 

 though in advance we felt quite sure of all 

 of these colonies; (e) of thirty colonies 

 selected from plates poured from natural 

 tumors on sugar beets only five were infec- 

 tious and all feebly so (Bui. 213, pp. 192-194 

 and Plate XXXVI). 



The first and natural supposition when a 

 culture has lost virulence is that some in- 

 truder has displaced the right organism; and 

 when only certain colonies on a plate are in- 

 fectious, that the others are intruders how- 

 ever much they may resemble the right 

 organism. I cannot say that we have abso- 

 lutely excluded this hypothesis, to which I 

 held strongly in the beginning, but we are 

 gradually coming to believe that it does not 

 explain all the facts. 

 II. Some other types of plant galls. 



(i) Nonparasitic intumescences. These can be produced in 

 several ways: by overwatering and exposure to 



