68 BULLETIN 957, U. S. DEPARTMENT OF AGRICULTURE. 
of 30.5 per cent. In another experiment the sporidia were placed 
on the periderm of white-pine twigs of the same season's growth and 
on living leaves of Pinus strobus and P. rigida. They were then 
exposed dry at 66° F. and with a humidity of 90 per cent. None 
survived for 10 minutes. At 72° F. and a humidity of 69 per cent 
none survived for 10 minutes. They conclude that the sporidia can 
endure very little desiccation and are short lived under seemingly 
optimum conditions. Abundant moisture is necessary for infection 
of pines to occur. 
HETERCECISM OF THE SPORIDIA. 
A number of tests have been made to learn whether Ribes might 
become infected by sporidia of Cronartium rihicola. Jaczewski (59) 
states that experiments have shown that they will not infect Ribes 
leaves. In 1913, Clinton, Stewart, and the writer (151) inoculated 
Ribes nigrum leaves with teliospores overwintered out of doors, but 
there were no infections. In 1912 the writer tested fresh teliospores 
without infection occurring (136). In 1917 Gravatt made several 
tests of fresh, sporidia-producing telia, but no infection resulted. 
OVERWINTERING OF CRONARTIUM RIBICOLA. 
Overwintering on Pines. 
The generally accepted view has been that Cronartium rihicola 
lives over winter by means of the mycelium in the bark of living 
infected pines and by this means only (142). A number of writers 
have mentioned cases where their observations seemed to indicate 
the possible overwintering on infected Ribes, but nothing that could 
be accepted as real evidence was offered until the last few years. 
There is no question that the fungus overwinters chiefly in the 
infected living pine trees and has been carried in the dormant con- 
dition from continent to continent in young infected pines. 
It has been discovered, as has been mentioned earlier, that Cro- 
nartium rihicola may overwinter as mycelium in infected branches 
cut from diseased trees late in the fall, or during the winter, and 
allowed to lie until spring. Then, if these cut-off branches lie close 
to damp soil or with the cut ends in a stream or pool, fresh vigorous 
secia are produced (89). Still another phase of overwintering was 
discovered by Dosdall (29) in Minnesota. On April 19, 1918, a 
dead branch of white pine, bearing an infection which bore aecia in 
1917, was collected. Germination tests in distilled water showed 
that 1 to 2 per cent of the old seciospores were still viable. 
Overwintering on Ribes Plants. 
Investigations of overwintering of Cronartium rihicola on Ribes 
plants, in Europe, seem to be limited to field observations. They 
