WHITE-PINE BLISTER RUST. 37 
Pinus strohus trees with aeciospores, but without producing infection. 
Hennings (53) inoculated P. strohus trees with aeciospores and also 
with teliospores. No infection resulted from either. In the spring 
of 1916 the writer made 100 inoculations with wounds into the bark 
of Pinus strohus trees, out of doors, with fresh aeciospores of Cronar- 
tium ribicola. No infections have resulted to date. In May, 1917, 
the writer (146) inoculated white pines by dipping the tips of young 
twigs in water containing great quantities of newly formed aeciospores 
of C. ribicola. The needles as well as the twigs were covered with 
spores. Glassine bags, containing wet wads of cotton, were then 
tied over the inoculated twigs to keep up the humidity of the air. 
No evidence of infection is yet visible. 
GERMINATION OF THE ^CIOSPORES. 
Experience shows that fresh aeciospores taken from aecia just as 
they are about to break open, or just at the time of breaking, possess 
maximum infective power. Doran (28) confirms this opinion. 
Inoculations made with such spores are sure of results if conditions 
are at all favorable. Older aeciospores are erratic in germination, 
but some of them retain viability to a marked degree. Cooling on 
ice stimulates their germination to a decided degree, as is shown by 
experiments performed by Eriksson (32), Gravatt, and others. 
Each spore produces from one to five (20, 29) germ tubes, which 
branch freely. The viability of fresh aeciospores is generally high, 
as many as 95 per cent germinating under favorable conditions. 
They require 8 to 10 hours to germinate (28). 
Doran (28) determined the minimum, optimum, and maximum 
temperatures for the aeciospores of Cronartium ribicola. Five series 
of tests were made. It was found that germination in distilled-water 
drop cultures occurred through a range of 12° C, but the percentage 
of germination dropped rapidly both above and below the optimum. 
The minimum temperature for the aeciospores was 5° C, the optimum 
was 12° C, and the maximum was 19° C. 
LONGEVITY OF THE AECIOSPORES. 
Klebahn (70, p. 26) seems to have been the only European investi- 
gator who has tested the longevity of aeciospores. He found them 
strongly viable after seven weeks. 
From the beginning of the work of the writer on Cronartium 
ribicola it has been known that the aeciospores retain their viability 
for a relatively long time under favorable conditions. In 1910 a 
single aeciospore which had been kept in the laboratory for more 
than five months in an open aecium on a diseased young tree which 
was dried and kept as a specimen germinated in water (131, p. 30). 
In 1915 McCubbin (85) collected on May 6 a diseased young white 
