Feb. a8,1916 
Relation of Stomatal Movement to Infection 
1029 
Therefore, it may be concluded that if the relative humidity remains 
above 60 during the hours of daylight the stomata will probably be found 
open, while with a lower humidity the stomatal opening will decrease 
until it becomes greatly reduced and with still lower humidity the sto¬ 
mata may usually be found completely closed, or at least as nearly so as 
ever occurs. In an irrigated area especially, where the humidity is very 
largely controlled by the soil moisture, a high humidity may be directly 
due to a high soil-moisture content and would indicate increased plant 
activity. The beneficial effects of high humidity on increased plant 
growth is generally recognized. Wollny (13), who grew plants of bar¬ 
ley, vetch, alfalfa, flax, and potato under conditions giving three degrees 
of humidity, found that with an increase in the degree of humidity there 
was an increase in the production both of the absolute quantity of fresh 
material and of dry matter. On the other hand, low soil-moisture con¬ 
tent would greatly check such activities, and a low humidity, which would 
be associated with such a condition, would indicate marked differences in 
stomatal movement. Thus, it appears that a low humidity with its as¬ 
sociated causes and effects results in diminished stomatal movement, and 
then the existing percentage of relative humidity becomes an important 
and convenient index to stomatal activities. 
FACTORS INFLUENCING INFECTION 
A consideration of the factors additional to, and somewhat preliminary 
to, stomatal movement that have been found to influence infection in¬ 
cludes some of the conditions that affect both parasite and host in this 
relation. The effect that media, light, and temperature have on the 
rapidity of germ-tube growth becomes important in the relation that the 
fungus bears to leaf penetration. On the other hand, the maturity of 
the leaf, which controls stomatal mobility, plays a comparable part in this 
interrelation. 
RAPIDITY OF GERM-TUBE GROWTH 
No difference has been found to exist in the effect that north light and 
darkness have on the rapidity of germ-tube growth at a constant tem¬ 
perature. From the data given in Table IX it appears that all conidia 
germinated and had approximately the same average germ-tube lengths, 
together with a comparable, average number of germinating cells per 
spore, regardless of the light factor. Consequently, under field condi¬ 
tions conidial germination would be expected to proceed equally fast 
under night or day conditions, except in direct sunlight, where the heat 
factor becomes important in causing rapid evaporation. 
