223 
aud others. The greatest of these values is that reported by Pritchard, 
whose statement is here quoted: “The rust was abundant within 25 yards 
of the barberry bushes, but practically disappeared at a distance of 60 
yards. The most persistent searching was required to discover a single 
pustule beyond S80 yards.” Another observation by Pritchard” indicated 
that urediniospores of P. graminis are carried only short distances in suffi- 
cient number to cause an epidemic. We have the published record, how- 
ever, of rust spores having been blown as much as a mile and producing 
infection; and, strange as it may seem, the spores in question are the 
smallest, most delicate ones in the life cycle of the rusts if we omit the 
non-functional pycniospores, namely, the basidiospores. FE. T. Bartholo- 
mew gives a table which shows that 59.1% of the leaves on apple trees 
near cedars were infected with rust. A quarter of a mile away it was 
55.4% and a mile away it was 6.5%. All this does not show, of course, that 
rust spores are not carried by the wind for long distances in a vigorous 
condition, but it does show that the distance for abundant infection from 
any spore producing center is not great. 
With this as a basis it should be possible to obtain an idea of the 
maximum distance a rust might be expected to progress in a season. The 
greatest distance recorded above is one mile, but those spores would doubt- 
less travel a mile and a half farther (or two and a half miles) and pro- 
duce infection. As a factor of safety, let us double this value; and as a 
further factor of safety, let us double this latter value. This gives us ten 
miles. A rust generation, according to Freeman and Johnson,” takes eight 
to twelve days, and more in cold, bad weather; and our own results at the 
Purdue Experiment Station agree very well with those figures. Assuming, 
then, ten days for a rust generation, ten miles of migration per generation, 
2 growing season from the middle of April to the middle of October, ap- 
proximately 180 days, and good weather with no interruption to the growth 
of the fungus, we should expect it to migrate for a distance of 180 miles. 
This value will be used presently in comparing the telial distribution of 
some of the rusts with their possible vecial distribution. 
PROPAGATION OF INDIVIDUAL SPECIES. 
The black rust of grasses was among the first to be observed living, 
and apparently thriving, at long distances from any of its wcia. There 
71Bot. Gaz. 52:184 1911. 
Phytopathology 2:255-6. 1912. 
Bur. Plant Ind. Bull. 216:45. 1911. 
