DISEASE CAUSED BY FUNGI 



203 



grasses are their victims, and in many instances the host 

 can only be infected in the youngest seedling stage, the 



FIG. 53. Ustilago avenae, one of the Ustilagineae causing 

 the disease known as ' smut ' in oats, i, a ' smutted ' ear of oats ; 

 2, spores ; 3, spores germinating and producing minute secondary 

 spores which again reproduce themselves by budding or gemma- 

 tion, after the manner of yeasts ; 4, secondary spores conjugating 

 in pairs. Fig. i, reduced ; the remainder highly mag. 



mycelium of the fungus spreads in the tissues and keeps 

 pace with the growing plant, where it remains for months, 



