PROCEEDINGS 
Cttwbnfrgc |)ly(osopbmi( Society. 
On the Question of “Predisposition ” and “Immunity” in Plants. 
By Professor H. Marshall Ward, D.Sc., F.R.S. 
\Eead 20 January 1902.] 
Twenty years ago, in 1881-82, I expressed the conviction that 
Hemileia vastatrix , the fungus of the Coffee-leaf-disease in Ceylon, 
was an extended or adapted form of a native species of this 
Uredine found on another host-plant, allied to Coffea, and be- 
longing to the genus Canthium ; but although so convinced of 
the essential morphological identity of H. Canthii and II. vastatrix , 
all attempts to infect Coffee with H. Canthii and, reciprocally, to 
infect Canthium with H. vastatrix, failed, and the germ of the. idea 
of adaptive parasitism fell to the ground. 
On the other hand, I was able to show that Hemileia vastatrix , 
commonly supposed to be confined to Coffea Arahica, was capable 
of infecting Coffea Liberica, thus proving that the latter species 
was not immune to the parasite. 
The questions which arose — but which were not then ripe for 
such clear enunciation as they have since become — were, Have we 
here a case, or cases, of differences of predisposition (susceptibility) 
or immunity (resistance) on the part of the host-plants ; or is it 
a matter of adaptability, or of failure to adapt itself to the new 
host, on the part of the fungus ? 
Clearly both possibilities are theoretically to be expected, but 
very little advance was to be made by speculation. 
The opinion has gained ground among planters, horticulturists 
and others, however, that different races of cultivated plants differ 
much in their susceptibility or predisposition to various fungi 
which induce diseases, and in view of the facts brought forward 
by those who grow such plants as Chrysanthemums , for instance, it 
is impossible to overlook the significance of the case as one for 
scientific enquiry. 
VOL. XI. pt. v. 
23 
