4°4 
Journal of Agricultural Research 
Vol. XXVII, No. 6 
biologic form which has the potential ability to adjust itself to a wide 
range of conditions or reactions might be able to maintain itself in a 
greater number of host varieties than would a biologic form with limited 
range of tolerance to certain physiologic reactions. 
In the same way there may be corresponding differences in the physi¬ 
ology of biologic forms of P . graminis, and this may account even for the 
existence of biologic forms. Then it would naturally follow that forms 
differ in ability to adjust themselves to physiologic environment. 
It is reasonable to expect that all biologic phenomena are ultimately 
to be explained on a physicochemical basis. The action of poisons and 
toxins are not exceptions. A conception of biologic forms based on 
physiological studies, as has just been indicated, would be quite in keep¬ 
ing, therefore, with the accepted ideas of fundamental resistance. 
GENERAL CONCLUSIONS 
It is obvious that the basis for resistance of wheat varieties to biologic 
forms of P. graminis tritici must be either morphological or physiological. 
For a long time it was supposed that the basis of this resistance might 
be morphological. The work of Ward, however, indicated that external 
morphological characters probably were of minor importance in deter¬ 
mining resistance or susceptibility of plants to the attack of rust fungi. 
More recently still, however, the work of Allen and others, together with 
extensive field observations, has called attention to the fact that mor¬ 
phological characters might play some part in determining resistance. 
It has been suggested that the morphology of the plant might affect 
resistance by preventing the entrance of germ tubes. 
It seems reasonable to suppose that certain morphologic characters 
might possibly interfere with the entrance of germ tubes. As the germ 
tubes of the urediniospores always enter the wheat plant through sto¬ 
mata, it would seem especially reasonable to suppose that the number, 
distribution, size, and location of the stomata might be quite important 
in determining how many germ tubes could enter the plant. Further¬ 
more, we may assume that the number of hairs on the plant surface may 
have some effect on the growth of the germ tubes, as an abundance of 
hairs easily may block the growth of the tubes. Extensive observations 
were made on the number of hairs and the number and size of the stomata 
on susceptible and resistant varieties of wheat. While the number of 
hairs in general was greater on the more resistant varieties than on the 
more susceptible ones, it seemed evident that the number of hairs scarcely 
could be a determining factor in the entrance of the germ tubes. The 
number of stomata, as well as the size of the stomatal aperture, differs 
for different varieties. However, some of the most resistant hosts have 
stomata with large apertures, while some very susceptible ones have 
stomata with small apertures. It seems probable that the stomatal 
aperture of all varieties is sufficiently large to permit the entrance of 
urediniospore germ tubes. It seems much more likely that the character 
of the stomatal movements, which may differ in different varieties, would 
have more effect upon entrance. The size of the stomatal opening 
scarcely could account for the resistance of certain varieties to certain 
biologic forms, because some varieties are immune from certain biologic 
forms and completely susceptible to others. It never has been demon¬ 
strated that the size of germ tubes of the different biologic forms differs 
sufficiently to make an explanation of resistance on this basis of entrance 
