Minnesota Plant Diseases. 



161 



mycelium of the fungus. The cups are at first closed and then 

 resemble small spheres ; the walls later open at the summit, roll 

 back and expose the spores as an orange-yellow dust. The 

 spores are formed in chains which arise in a palisade from the 

 floor of the cup and are formed continuously for some time, 



f 



FIG. 74. More spores of rust fungi. 1. A pycnidium (from wheat rust on barberry), a 

 capsular spore-bearing fruiting body showing dust of spores at r. 2. Spores and 

 spore-bearing threads from 1 greatly enlarged. 3. Same spores germinating. These 

 spores are probably the relics of male reproductive cells which have fallen into disuse. 

 They appear to be functionless since they do not usually germinate and have never 

 been known to cause infection. 4. A cluster-cup of an Anemone rust; s spores, 

 formed in chains; p, threads forming the cup of the cluster-cup. All highly magni- 

 fied. After Tavel. 



those at the summit being the first to mature. These cluster- 

 cup spores are ball-shaped or have flattened sides and their 

 outer wall is frequently provided with small warty roughnesses. 

 The cluster-cup spores are blown about by the wind and are 

 capable of immediate germination. When germinating they 

 send out a germ thread which causes infection. Accompanying 



