532 



SPECIAL PLANT PATHOLOGY 



central one of which contains cytoplasm, being surrounded by an 

 envelope of sterile cells. Such spore balls are 17 to 25^ in diameter 



Fig. 191. — Smut of oats. A, Uslilago avenee; B, Ustilago leeis. {After Jackson, 

 H. S., Bull. 83, Del. Coll. Agric. Exper. Slat., December, 1908.) 



and may retain their capacity for germination in the soil for a period of 

 12 years. 



