THALLOPHYTES FUNGI 



285 



Their function is uncertain. Although the barberry is an im- 

 portant factor, it is not necessary for the continuation of this 

 fungus on the wheat, the uredospores from one plant can infect 

 another plant and the fungus can be perpetuated indefinitely in 

 this manner. Many of the rusts have only one host; some of 

 them have only two stages and others only one. They are said 



FIG. 135. Cedar rust. 



to be the most highly specialized parasites in the plant kingdom. 

 Other very important species are the apple rust (Gymnosporan- 

 gium juniperi-virginiana), which has its aecial stage on the 

 apple (Fig. 134) and its telial stage on the red cedar (Fig. 135) ; 

 and the blister rust of the white pine (Cronartium ribicola}, 

 which has its secial stage on the white pine and the other two 

 stages on the currants and gooseberry. The rusts take an annual 

 toll of many millions of dollars from the wealth of the country. 



