USTILAGO. 



285 



very frequently on Avena sativa, also on AveTm orientalis, 

 A. fatua, and A. strigosa in Europe and North America. So 

 common is it that one seldom sees a field of oats free from the 

 black smutted ears (Fig. 156). 



All parts of the flower are attacked, the ovary, stamens, 

 glumes, and even the awns. The grains become filled with 

 the black spore-powder, which shows through the transparent 



Fig. 156. — Ustilago avenae. The Oat-smut on Avena sativa. (v. Tubeuf phot.) 



membrane of the ovary wall. The diseased ears emerge from 

 their enclosing leaf-sheaths, and become exposed to wind and 

 rain, under the effects of which the delicate membrane soon 

 becomes ruptured and the spores are blown or washed away, 

 till only the axes of the spikelet are left with a few ragged 

 remains of the flower. As a rule every shoot of a plant and 

 all the grains of an ear are attacked ; if single grains do escape, 

 they remain poorly developed. 



The spores (5-8,u) have a smooth or slightly granular coat, and 



