SMITH: CROWN GALL AND OTHER OVERGROWTHS 449 



attacked — some species are not subject to this 

 disease; old tissues respond slowly. 

 (4) Individual differences. There are, I believe, indi- 

 vidual differences in susceptibility as well as species 

 differences. 

 B. The host reaction depends also on the activities of the parasite 

 {Bad. tnmefaciens) which are variable. 

 (i) Loss of virulence on culture media. The cause of 

 virulence is not known. The effect of long 

 continued growth on culture media is to 

 reduce the virulence of the organism and 

 finally to destroy it altogether. 



(a) "Old" vs. ''resistant" cultures of Paris daisy 

 organism. Our first isolation from Paris 

 daisy was extremely virulent in the begin- 

 ning but lost all power to produce galls in 

 about three years. Another isolation which 

 we called "Resistant Daisy" is now slowly 

 losing virulence at the end of three and one 

 half years. 



(b) New vs. old cultures of poplar isolation. An 



isolation from a poplar tumor was extremely 

 virulent for some time (several years) but 

 has now lost all power to produce tumors. 

 Along with this loss has come a progressive 

 thickening and toughening of the pellicle 

 on bouillon. This was true also of the daisy 

 isolation which lost its virulence. 



Apparently this loss of virulence is not 

 correlated with loss of power to produce 

 formic acid for, according to the chemist, 

 the non-virulent poplar organism still pro- 

 duces that substance. Loss of power to 

 infect must be related, however, to loss of 

 some chemical or physical property once 

 possessed, and surely we ought to be able to 

 discover the exact nature of this loss. Some 

 strains of Bad. tumefaciens lose virulence 

 much sooner than others. One of our 

 strains (from hop) is still virulent after nine 

 years on culture media. ^ 

 (2) Not every isolation is a distinct strain. I speak of 

 "strains" only when I know that cultural 

 1 Ten years, as this now goes through the press. 



