240 Wisconsin Academy of Sciences, Arts, and Letters. 
however, that although this may be a factor in limiting the para¬ 
sitism of the latter fungus, it was not evidently a factor contribut¬ 
ing to the dilference in resistance between colored and white varie¬ 
ties. Brown (12, 13) has recently pointed out the effect on 
germinating of fungus spores of substance diffusing from the 
host cell into the infection drop; and in this work it was further 
shown that volatile oils from various plants may have in some 
cases detrimental and in other cases stimulative effects upon the 
spores. 
Summary 
For purposes of convenience let us outline the main evidence to 
date as to the nature of disease resistance, remembering that since 
much of the work is fragmentary and not uniformly reliable this, 
is merely a suggestive and tentative summary based wholly on 
statements in the original texts. 
Orton (61) classifies the various known types of resistance into 
three classes: (1) avoidance of disease, (2) endurance of disease, 
and (3) true resistance to disease. We include in the discussion 
the latter class, in which we consider cases where through some 
inherent quality in its composition the plant is capable of success¬ 
fully resisting the attack of a given parasite. In consideration 
of the subject the importance of environment as affecting the ex¬ 
pression of resistant qualities is emphasized. In the few instances 
where the hereditary nature of resistance has been studied it is 
determined in some cases by a single factor, in other cases by 
multiple factors. Vavilov’s (82) separation of the causes of true 
resistance into two groups, mechanical or passive and physiolog¬ 
ical or active is useful. Numerous instances where mechanical 
structures of the host plant are responsible for resistance have 
been noted. These include hairiness of leaves and open habit of 
growth in the case of potatoes resistant to blight, lack of hair on 
terminal end of kernels in case of wheat resistant to bunt, external 
waxy layers in case of raspberries resistant to Coniothyrium and of 
barley resistant to stem rust. Differences in stomatal structure or 
size have been cited in several cases as responsible for the resistant 
quality. The exclusion of parasites by means of thick cuticle or 
the formation of corky layers is quite well established in a few 
eases. 
Physiological or internal factors as causes of resistance have 
been noted in a variety of instances. These internal differences are 
