PLANTS WHICH CAUSE DECAY 



403 



a mycelium which spreads rapidly through the leaf and 

 finally forms spores known as cluster- cup spores (or 

 secidiospores) . These spores, as their name 

 implies, are arranged in clusters in cup- 

 like cavities of the leaf (Fig. 228), which 

 are produced by their growth. On exam- 

 ining thin sections of the leaf carefully, we 

 see that the cluster -cup spores are in long 

 chains borne on short stalks (Fig. 229) . 

 On the upper surface of the leaf occur 

 smaller cavities filled with smaller slender 

 spore -bearing stalks; their function is not 

 understood. We see, then, that the Black 

 Stem Rust of the Wheat occurs in three 

 different forms, at. different times of the Autunfn 7 ' spores 



., of the Black 



year the uredospores m the summer, the stem Rust of 



J Wheat poduc- 



teleutospores in the autumn, and the clus- 

 ter-cup spores on Barberry in spring. Formerly, when 



it was not known that 

 these were all forms of 

 the same fungus, they 

 were described as sepa- 

 rate genera. The discov- 

 ery that the Barberry had 

 something to do with the 

 Eust on Wheat was first 



228. Section of Barberry leaf, showing the made by practical farm- 

 cluster-cup stage of Black Stem Rust of -, -, j ,-i , 



ers who observed that 



