GENERAL SUMMARY 583 



walls of two aecidiospores at the end of a chain by the osmotic or other 

 forces witliin the spores which tend to round out their walls. 



The peculiar structure of the peridium of an aecidium in Puccinia, 

 Uromyces, etc., is correlated with its function of bursting, ojiening out, 

 spUtting radially into rays, and becoming revolute. The revolution of 

 the peridium prevents the peridium from becoming an obstacle to the 

 spores when these are being shot out violently from the interior of the 

 aecidium. 



A simple calculation shows that uredospores must often be trans- 

 ported many miles by the wind, and supports the view that the simul- 

 taneous outbreak of the Black Stem Rust disease of wheat over large 

 territories in western Canada, where the Barberry is non-existent, is 

 preceded by the settling down from the upper air of uredospores brought 

 a long distance from other regions by the wind. 



