119 
greatest damage of all grain smuts in the United States. The disease 
makes its appearance at the time of blossoming of the oat-plant, the whole 
head becoming a dusty olive-brown mass. The infected flowers are 
entirely destroyed by the disease and even the awns are affected. 
In the Hidden Smut the spore mass is more or less concealed by an outer 
membrane of the floral parts that remain intact from the disease. The 
dusty mass is made up of great numbers of spores which are blown about 
by the wind, some being caught in the open glumes of flowering oat-plants. 
Since after blossoming the glumes close tightly about the ovary, such spores 
are held imprisoned and remain so until the seed is in a condition to 
germinate. Then the imprisoned spores germinate after the manner of 
several smuts, producing a three or four-septate promycelium, which usu- 
ally bears oval or elliptical sporidia at the apex or laterally at the septa. 
Infection threads are usually produced by these sporidia, but the promy- 
celial threads may also produce them. These infection threads gain en- 
trance to the host by piercing the delicate young cells of the first leaf- 
sheath before the leaf has appeared. Plants are free from infection after 
the growing leaves have pushed themselves as much as one c. m. through 
the leaf-sheath. Brefield found by experimentation that oats germinated 
up to 15° C. gave 3 per cent. smutted heads, but when grown at a higher 
temperature give 1 to 2 per cent. smutted heads or more.! This bears out 
ordinary experience that late sown oats while more liable to rust are 
freer from smut. This immunity is probably due to the short period when 
the plant is open to infection as a result of the rapid germination and 
growth of the seed in the more favorable condition of temperature. 
In from thirty-six to forty-eight hours after infection considerable 
mycelium will be developed which penetrates the first and second leaves 
and gains entrance to the stalk or culm. It grows upward and invades 
the young head in quite the same manner as in case of other smuts. In 
place of a healthy head, a dusty mass of spores appears, which are scat- 
tered to healthy heads by the wind. 
1. Brefield, O., Recent Investigations of Smut Fungi and Smut Diseases. Trans 
by Erwin F. Smith, Jour. Mycol. VI, pp. 1-8; 59-71; 153-164. 1890-91. 
