PLANTS WHICH CAUSE DEC4.T 



403 



a mycelium which spreads rapidly through the leaf and 

 finally forms spores known as cluster- cup spores (or 

 aecidiospores) . 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 

 year — the uredospores in the summer, the 

 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 

 Rust on Wheat was first 



, Section of Barberry leaf, showing the made by praCtlcal farm- 

 cluster-eiip stage of Black Stem Rust ol •, , n i i j 



Wheat. ers, who observed that 



227. 



Autumn spores 

 of the Black 

 Stem Rust of 

 Wheat poduc- 

 ingconidialc). 



