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 
(Bact. tumefaciens) which are variable. 
(1) 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 Bact. 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. 
