Nov. 1906. | THE PRESIDENTIAL ADDRESS. 229 
species, and so on. What is the cause of this? The problem 
here is the old one of immunity and susceptibility. And to 
the solution of this Ward set himself vigorously. 
The suggestion that anatomical considerations, presence of 
hairs, wax, ete., were important factors, was advanced fre- 
quently and received some support, but Ward was able to 
show conclusively that these have nothing to do with it. 
He forces us to recognise that there are two stages, one ot 
application or inoculation by the spore, which germinates and 
sends a tube into the air chamber of the stoma, but that is 
not necessarily followed by the infection, which means the 
entry of the germ tube through the cell wall bounding the 
air chamber. And ultimately he was able to prove that 
the infection depends upon the reciprocal presence of enzymes 
and toxins and antitoxins in parasite and host. Nor was 
this all. Marshall Ward was able to establish his theory of 
inuration and bridging species. That is to say, he found it 
possible to educate a parasite which was harmless to a 
particular host species to attack it successfully through 
cultivation successively upon allied forms. Thus, given 
a parasite growing upon a grass A, but to which grass E is 
immune, it is possible by growing the parasite successively 
upon certain forms B, C, D, to educate it so that it will 
attack EK. These intermediate forms Ward termed bridging 
species. 
All this work on the Uredines brought Ward into con- 
flict with the well-known Swedish agricultural professor 
Eriksson, who had given much attention to the study of 
the epidemics of rust that occur in Sweden. The point 
of conflict concerned the method of perennation of the 
metoxenous Uredines. When it was discovered that plants 
like the barberry, Rhamnus, Anchusa, were hosts of stages 
in the pleomorphous life history, the prophylaxis that 
naturally suggested itself was destruction of these in the 
vicinity of cereal crops. That has been a matter of policy in 
many areas. But now, despite this abolition of the host of 
an essential winter stage of the parasite, it is found that the 
epidemic of rust is as virulent as ever. How is this to be 
explained? All observers, from De Bary and Ward, have 
sought in vain for a perennating mycelium, and it has been 
shown by Ward and others that uredospores may perennate 
