228 Wisconsin Academy of Sciences, Arts, and Letters. 
avenae) (34) indicates that resistance to a given biologic strain is 
determined by a single dominant factor. Crosses between varieties 
of bean resistant and susceptible to anthracnose (Colletotrichum 
lindemuthianum) show that there is only a single factor difference 
between the two when a single strain of the parasite is considered 
(14, 49). Where two strains are involved a two factor difference 
is indicated (50). In both cases resistance is dominant over 
susceptibility. With two other diseases of bean, mosaic and root 
rot (Fusarium martii A. & W. var. phaseoli Burk.), susceptibility 
is at least partially dominant and resistance must be explained 
on the basis of more than one factor (50). Vavilov (81) in study¬ 
ing resistance of wheat to mildew {Erysiphe graminis DC.) found 
that from crosses between the immune Persian wheat (Triticum 
vulgare var. fuliginosum Al.) and common susceptible bread wheat 
varieties, P^ hybrids were secured which were immune to the disease. 
Klaphaak and Bartlett (46) find that immunity to powdery 
mildew {Erysiphe polygoni DC.) in certain species of Oenothera is 
determined by a single factor, which is dominant. In the case 
of flax wilt (Fusarium Uni) Tisdale (79) finds the behavior of re¬ 
sistance more readily explained on the basis of multiple factors. 
Gaines (33) reports there are different types of resistance to bunt 
(Tilletia tritici (Beij) Wint.) in different varieties of wheat and 
that in any particular case the resistant quality is composed of 
multiple factors. Johnson (40) finds that inheritance of resistance 
in tobacco to Thielavia hasicola (B. & Br.) Zopf is best explained 
by a multiple-factor hypothesis. 
Classes of True Resistance 
The causes of true resistance in plants have been grouped by 
Vavilov (82) into two classes: (1) mechanical or passive im¬ 
munity and (2) physiological or active immunity. The first class 
includes those cases in which resistance depends upon certain 
mechanical differences in structure or habit of growth of the 
plant. The second class includes those cases in which resistance 
is due to inherent physiological qualities of the host cells which are 
sufficiently antagonistic to check the parasite. While this classi¬ 
fication is convenient, it is obviously unwise at our present state 
of meagre knowledge of the subject to attempt to apply it too 
rigidly. 
