516 FROMME: THE CULTURE OF CEREAL RUSTS 
Effect of light 
To determine the effect of light exclusion on spore germination 
and rate of development, four culture pots of the same age, seven 
days, were inoculated simultaneously. Immediately afterwards 
one of the four was transferred to a physiological dark room which 
joins the greenhouse. A continuous circulation of air between the 
two rooms is maintained by an electric fan. Thus the average 
degree of humidity of the dark room, which is about 80 per cent., 
does not fall below that of the greenhouse although the range of 
fluctuation, which is from 60 per cent. to 95 per cent., is somewhat 
greater. The other three cultures were placed in the culture box 
as controls. The culture was exposed in the dark room for three 
days and at the end of this time was returned to the culture box. 
The plants at this time were quite as green and fresh as those of the 
control cultures and could not be distinguished from them. The 
incubation period for the three controls was eight days, while that 
of the culture left three days in the dark room was eleven days. 
At the time of sporulation some of the leaves of this culture 
showed signs of yellowing at their tips. No pustules were pro- 
duced on these discolored areas but on the normal green parts they 
were as numerous as on the controls. The difference of three days 
in the incubation periods is exactly equal to the period of light 
exclusion and indicates a complete arrest of the development of the 
fungus in the dark room. 
The effect of light exclusion during the latter part of the 
incubation period was also tested. Four cultures were inoculated 
and placed in the culture box. Four days later one of them was 
transferred to the dark room, where it was left four days, and then 
returned to the culture box. No signs of infection were visible 
on it at this time, while the unripe pustules on the controls were 
plainly visible. The pustules on the controls ripened on the ninth 
day, while three additional days, twelve days in all, were necessary 
for a similar development of the culture that had been in the dark 
room. By excluding light four days in the latter part of the normal 
incubation period, the maturation of the rust had been delayed 
three days. This shows that even after the fungus has become 
well established in the host its development is strongly retarded in 
complete darkness. 
