Inheritance of Disease-Resistance in Plants. I. T. Brooks. 77 
resistance in plants affected by wound parasites. As regards 
plant diseases caused by obligate parasites such as wart disease 
of potatoes and rust in wheat, it seems necessary to distinguish 
between what is apparently absolute immunity and what is on 
the other hand a very high degree of resistance to disease. Now 
in the case of wart disease some varieties appear to be absolutely 
immune to this disease, and as far as I am aware, this immunity 
appears never to have broken down, though it would be haz- 
ardous to say that this will not be accomplished under experi- 
mental conditions. Rust-resistance in cereals on the other hand 
does not seem to be absolute in character, but to be influenced 
in part at any rate by the conditions of environment, including 
the physiological constitution of the host. Thus, Einkorn wheat, 
which under ordinary conditions is practically immune to attacks 
of rust fungi, has been shown by Howard (2) to lose its character 
of resistance to Black Rust under certain extreme climatic 
conditions (great heat) at Pusa in India. Again, the variety of 
wheat “‘Little Joss,’ which in the actively growing state is 
practically free from Yellow Rust during most years is some- 
times found to be considerably rusted during the early part of 
the year. The results given in the earlier part of this paper 
illustrate the same or an allied phenomenon, for it was often 
found that plants—described as “genetically immune’’—were 
affected to a slight extent by rust. The offspring of these slightly 
rusted plants might remain completely rust-free another year, 
the environmental factor which operated to reduce the resistance 
during the previous year being thus eliminated. It is clear 
therefore that although there is an hereditary factor as regards 
resistance to rust in some wheats, it is not impossible that the 
conditions of environment have the power of modifying the 
expression of the genetic factor. There is some evidence too 
that in wheats of hybrid origin, other factors such as the nature 
of the root system may themselves influence the general physio- 
logical constitution of the plant so as to modify somewhat its 
powers of resistance to disease. The plant breeder must, 
therefore, take into account the possibility that changed con- 
ditions of environment may break down to some extent the 
resistance-powers of the host as regards certain diseases, al- 
though it is likely that under average conditions of environ- 
ment, the “genetic immunity” of the resistant variety will be 
able to hold its own. As is well known, excessive nitrogenous 
manuring sometimes increases the susceptibility to fungoid 
disease, and in this connection it is interesting to note that on 
the Cambridge University farm this year—May and June being 
very dry, nitrogenous manuring of the highly resistant wheat 
“ American Club” and of the practically immune derivatives of 
