FROMME: THE CULTURE OF CEREAL RUSTS 517 
The average difference in degree of humidity, of approximately 
13 per cent, between the culture box and the dark room could 
not have been an important modifying factor in these results, as 
I have found that the incubation period is not modified by relative 
degrees of humidity after the first twenty-four hours. Thus 
parallel sets of cultures, when grown in the culture box and in the 
greenhouse under similar conditions of light and temperature 
but with the same difference in humidity that is found between 
the culture box and the dark room, had the same incubation period. 
Likewise when a culture was kept under a bell jar during the entire 
incubation period, with occasional removal for change of air, 
there was no difference between its incubation period and that of 
the control in the greenhouse that was covered for the first twenty- 
four hours only. 
The question naturally arises how this retardation of the growth 
of the fungus is brought about. It may be the direct effect of total 
absence of light on the fungus itself. Then, again, it is possible 
that the fungus simply suffers from lack of food, since the host is 
incapable of assimilation in the darkness. It seems hardly pos- 
sible, however, that such a complete inhibition in the growth of 
the fungus should have resulted in the brief time involved unless 
it is dependent on the transition products in photosynthesis. This 
latter possibility is by no means inconceivable and should this 
explanation prove the correct one it could be made the basis for 
an explanation of the obligate parasitism of the rusts and their 
inability to develop on any form of artificial medium. 
VIABLE PERIOD OF UREDOSPORES 
Two series of tests were made to determine the period through 
which the uredospores of P. coronifera would retain their vitality. 
Ripe spores were removed from pustules on the cultures and placed 
in small gelatine capsules which were then stored in the laboratory 
at room temperature. The first set of spores was stored on March 
13. A drop culture made on this date gave a high per cent of 
germination in twelve hours. Drop cultures of the stored spores 
were made on March 109, 23, and 28 and April 12 and 30. From 9 
to 16 per cent. of the spores sown germinated in each of these 
tests. No further trials were made until June 1 and at this time 
