Feb. 9,1924 
Resistance of Wheat to Puccinia graminis tritici 389 
The degree of infection produced by biologic Forms XVIII and XXVII 
on the wheat varieties given in Table V is summarized below. These 
data are based on Table IV, Technical Bulletin No. a 8, Minnesota Agri¬ 
cultural Experiment Station. 
Wheat variety. 
Degree of infection. 
Biologic Form XVIII. 
Biologic Form XXVII. 
Kanred, C. I. C146. 
Very heavy . . . 
No infection. 
Very heavy. 
No infection. 
Weak. 
Do. 
Little Club, C. L 4066. 
.do. 
Kota, C. I. 5878. 
do . . . 
Marquis, C. I. 3641. 
.do. 
Khapli, C. I. 4013. 
Very weak. .. . 
On some leaves observed in the foregoing study, as many as 60 ap- 
pressoria were seen, and where the spore germination was profuse there 
never were fewer than 15. As relatively few uredinia developed, there 
appeared to be but little correlation between the number of appressoria 
formed and the number of subsequent infections. For example, it fre¬ 
quently has been observed that even on the completely susceptible 
Little Club, numerous appressoria may be observed on leaves which fail 
to become infected. It seems improbable that the germ tubes entered, 
because antagonism between the host cells and the parasite, which con¬ 
ceivably could account for lack of infection, never has been observed 
in this variety. 
It is probable that the stimuli for the formation of appressoria and 
for subsequent entrance of the germ tube are entirely different. The 
formation of appressoria may be due to thigmotropism, because they 
often are formed in depressions between epidermal cells and in other 
places where they come in contact with some object. Ward (59), Rob¬ 
inson (44 ), Fromme (15), and Mains (32) noted that the germ tubes of 
certain rust fungi tended to be negatively heliotropic; while Balls (5) 
suggested that a hydrotropic response might account for entrance. 
The germ tubes might grow toward the stomatal opening, either on 
account of a negatively heliotropic or a positively hydrotropic response, 
or both; but the tubes probably can not enter if the stomata are closed 
tightly. 
According to Allen (1), only 67 per cent of the germ tubes which form 
appressoria enter the very susceptible host, Little Club. If only two- 
thirds of the germ tubes forming appressoria enter a very susceptible 
host under favorable conditions, we are not justified in emphasizing 
the relative number of germ tubes which enter from appressoria formed 
on resistant hosts. Before the results of different experiments can be 
compared, it will be necessary to control all the factors which influence 
the movement of stomata during the incubation period. 
It is quite possible that neither the number nor the size of the stomata 
greatly influences the entrance of the germ tubes. It is probable that 
the degree and duration of opening are more important. If the stomata 
are fairly wide open for some time the germ tubes may enter easily, but 
if they remain closed, there is no evidence that the tubes can force their 
way into the substomatal chamber. On the other hand, there is abun¬ 
dant evidence to show that even on the most susceptible varieties spores 
