93 
are produced from this infection. The connection between 
the fungus on the barberry and that on wheat was thus 
established, and it was shown that, although the a pear- 
ances of the fungus on the two host plants differ so 
materially, each represents a stage in the life history of 
one fungus. 
Since De Bary’s remarkable discovery very many 
rust fungi, showing this type of life history, have been 
studied. The rust which frequently appears on the leaves 
of the pear, for example, is the aecidiospore stage of 
Gymnosporangium which produces swellings on the 
branches of juniper, where the teleutospores arise. The 
Blister-Rust of the Weymouth or white pine, a disease 
which is very prevalent on the continent of Europe, is the 
aecidiospore stage of a fungus which forms its uredospores 
and teleutospores on the leaves of currants. Similarly, 
the Gooseberry rust is the cluster-cup stage of a fungus 
which forms the uredospores and teleutospores on the 
leaves of sedges. Many other similar cases occur as 
diseases of cultivated plants and trees. 
When the facts became known for the wheat rust it 
was thought that in order to eliminate the disease from a 
given district all that was necessary was to destroy the 
barberry plants in the district. It was soon found, how- 
ever, that in some countries, for example, Australia, where 
the rusts of wheat are most destructive, the barberry is 
almost unknown. In such cases it seems likely that the 
rust fungus is able to maintain itself without passing 
through the barberry. It probably does this by means 
of the uredospores, which have been shown to be capable 
of resisting a mild winter. 
Apart from the destruction of the barberry other means 
of combatting the wheat rust had to be devised. Up to 
the present this has proved a very difficult problem. The 
most satisfactory progress has been attained along the 
lines of breeding disease-resistant wheats. It has long 
been known that certain wheats are much more resistant 
to rust diseases than others. The chief difficultv lay in 
the fact that the most resistant forms, e.g., certain sem1- 
wild wheats, were almost valueless as crops. In recent 
years, however, much has been done, especially by Pro- 
fessor Biffen, of Cambridge, to produce by crossing, wheats 
which combine qualities of rust-resistance with good 
cropping and milling capacities. The subject is neverthe- 
