Observations and Experiments on Cereal Rusts. 175 
a comparatively long incubation, the exact duration of which 
is dependent upon conditions of weather—temperature being 
the chief regulating influence. 
(9) The length of the incubation period is not only not fixed 
at eight to ten days but is very variable. Although the last two 
seasons were comparatively mild the incubation period in some 
of the experiments was prolonged to five to six weeks. 
(10) These facts also suggest a satisfactory explanation of 
the phenomenon of spontaneous outbreaks (so characteristic of 
yellow rust) over large areas within the space of a few days. 
(11) Cultures of all the three rusts at comparatively low 
temperatures show the shortest incubation period for the yellow 
rust and the longest for the black one. A clear difference be- 
tween the duration of the incubation periods of yellow and brown 
rusts suggests the cause of the usually earlier appearance of the 
former. 
(d) Influence of temperature on the viability of uredospores and 
the growth of the mycelium. 
(12) The uredospores of black rust retain their viability at 
comparatively high temperatures. Those of yellow rust suffer 
most from heat and are more viable when kept at lower tem- 
peratures. The uredospores of brown rust however combine the 
powers to resist cold as well as heat. 
(13) The uredospores of yellow rust are short lived and are 
impaired at temperatures which both black and brown rusts 
can withstand. 
(14) Higher temperatures are inhibitory to the growth of the 
mycelium of yellow rust, and therefore prolong the duration 
of the incubation period, and may even kill the mycelium. 
(15) Some of the most susceptible varieties of wheat and 
barley even when exposed in the open for over ten months do 
not become affected by yellow rust if the viability of the uredo- 
spores (the only source of infection) is impaired. 
(16) The observations and experiments recorded in this paper 
deny all possibility of an hereditary source of infection and offer 
an adequate explanation of the annual recurrence of these rusts. 
Part II. 
Inoculation experiments with pure cultures of the following 
biologic forms showed that: 
(1) The black rust of wheat can infect only wheat and barley, 
but does not infect rye and oats. 
(2) The black rust of Couch grass readily infects Couch grass, 
rye and barley, and contrary to what has been observed at 
other places can also infect one variety of wheat (Red Sudan). 
