106 Wisconsin Academy of Sciences, Arts, and Letters. 
January 26th, and it seems probable that the mycelium would 
have withstood the remainder of the winter if the host could 
have been kept alive. 
Besides the data recorded in the tables, observations Avere 
made on the spring development of the rust of wheat and rye. 
On March 5th, a number of the winter leaves of rye, spotted in 
the manner described above, were marked. Fifteen days later, 
or March 20th, it was found that many of the spots had devel¬ 
oped to form open pustules. On this last date a general out¬ 
breaking of new pustules was noted, which reached its height 
on about April 3rd, when fresh vigorous spores were abundant. 
The winter leaves nov r began to wither and disappear. After 
April 8th, there Avas a period of about four weeks when it was 
impossible to find a single spore. On May 6th, the new leaves 
began to show a diseased appearance. The plot was visited 
again May 13th, when open pustules were to be found in abund¬ 
ance. 
It will be seen that there were in the spring in question tw r o 
distinct outbreaks: The first occurred on the old wdnter leaves 
in the two weeks following the first warm weather. Uredo- 
spores did not appear again until sometime later when a second 
outbreak occurred on the first of the spring leaves. 
Eriksson and Henning also describe two distinct outbreaks 
and use this fact as evidence for their mycoplasm hypothesis. 
The first appearance of uredospores they admit is often caused 
by mycelium that has survived the winter, but the later out¬ 
break comes from the mycoplasm. In their observations on P. 
ddspersa, they found uredospores April 2, 1892, but think even 
in this case that they have no reason for believing that these 
spores were produced by mycelia living over from the previous 
year. The explanation of the two separate outbreaks is prob¬ 
ably to be sought in another direction. 
I have found by experiment that in the cooler weather of 
spring the incubation period following inoculation with uredo¬ 
spores is usually lengthened to between three and four weeks 
and this explains the existence of a period with no rust after the 
first attack. The winter leaves die in early spring and with 
them the winter mycelium, but not until it has produced uredo- 
