Minnesota Plant Diseases. 



389 



The fungus threads, which are found in the swollen parts of 

 the host plant and which cause the stimulation, whereby the 

 increase of growth or swelling of the stem takes place, form 

 their winter rust-spores inside of the cells of the epidermis of 



FIG. 206. Sunflower rust. Winter and summer spore clusters on 

 the leaf of a sunflower. Original. 



the host. This epidermis is much altered on account of the 

 action of the fungus. Its cells have thin, instead of thick, 

 outer walls, and are much increased in size. These spores 

 germinate in the spring in the usual way, except that they 



