382 REPORT OF THE CONFERENCE ON GENETICS. 
distinctive physiological (or constitutional) characters—that is to say, 
with regard to the forms of the fungus in question, an immune and a 
susceptible race. 
When we seek to discover the nature of immunity—that is, to 
ascertain exactly in what manner the constitution of a plant enables it to 
resist disease—we find, by using certain cultural methods which I have 
lately described,* that it is possible to demonstrate that immunity in 
no way depends on any structural or anatomical peculiarities, such as 
hairs, ribs, thickness of the cell-wall or cuticle, or the chemical nature of 
the cell-wall. The immunity shown is not to be ascribed to the failure of 
the germ-tube of the spores of the fungus to penetrate the leaf-cells, but 
to the inability of the fungus to develop further the incipient haustorium 
which is formed, or if, as occasionally happens, the haustorium is 
completely developed, to its incapacity to adapt itself to the intracellular 
conditions obtaining in the plant. That is to say, immunity in these 
cases depends on the power possessed by the plant of preventing, by 
means of certain physiological processes, the attainment of that balance 
whereby working-relations between the haustorium and the host-cell are 
brought about and maintained. 
In those cases, as, for example, in the mildews, the rusts, and probably 
many other diseases, where the place of the decisive conflict between 
parasite and host is intracellular, and the issue (which gives immunity 
or susceptibility to the plant) determined by the “constitution ” of the 
plant, the most important question in connection with the breeding of 
disease-resistant plants becomes the one as to whether such “ con- 
stitutional’’ characters are Mendelian as regards the laws of their 
transmission. Mr. Biffen’s work t has now answered this question; we 
know, for instance, that with regard to rust, susceptibility and im- 
munity are definite Mendelian characters, the former being the dominant 
one. Remembering the all-important position which “ constitutional” 
characters occupy in connection with the breeding of disease-resistant 
plants, it seems to me impossible to overestimate the importance of 
Mr. Biffen’s discovery. 
What I have wished to do in these few remarks was to point out 
that our present varieties of cultivated plants show very different “ con- 
stitutions ’’ with respect to fungus diseases; that such constitutional 
characters, where they have been tested, have been found to be fixed for the 
species, variety, or race, and confer immunity or susceptibility on the plant 
in question; and that such characters appear unchanged in “ hybrid” 
offspring in the definite manner following Mendel’s law. If the scientific 
experimenter can make use of the practical knowledge of the horticulturist 
with respect to the “constitutions ” of the plants he has bred, there is every 
reason to hope that considerable success will soon attend the efforts of 
the plant-breeder to breed, by crossing and selecting, strains of plants 
more and more resistant to fungus diseases. 
* «Qultural Experiments with ‘Biologic Forms’ of the Erysiphacee.” [Phil. 
Trans. Roy. Soc. vol. evii. 107 (1904).] 
+ A detailed account of these phenomena will be found in my paper “ On the 
Stages of Development reached by certain ‘ Biologic Forms’ of Hrysiphe in cases of 
Non-infection.”” [The New Phytologist, iv. 217, plate 5 (1905).] 
t See Jowrn. Agric. Science, i. 40 (1905). 
Py 
