THE FUNGI WHICH CAUSE PLANT DISEASE 307 



cept the awn and rachis is entirely transformed into smut. In 

 water and in nutrient solutions the spores germinate by a single 

 promycelium, 1 to 3-septate, and often branched, but without 

 sporidia. That infection is floral in loose smut of both wheat and 

 barley was first shown by Maddox ^' and the fact was corrob- 

 orated by Wakagawa,'* Brefeld ^* and Hecke.'^' " The my- 

 celium has been demonstrated in the embryo by Broili.^^' 



The spores falling between the glumes germinate, penetrate 

 the ovary wall, and into the growing point of the embryo. The 

 mycelium here lies dormant until the seed germinates, when it 

 grows, keeping pace with the growing point throughout the season 

 and finally invading the ovaries to produce its spores. 



The infection of the pistil, the penetration of the integuments 

 and the nucellus and embryo sac was followed in microtome sec- 

 tions by Lang.^^^ The embryo was reached by the mycelium some 

 four weeks after infection of the pistil. In resting grains the my- 

 celium is abundant in the scutellum as well as in 

 all embryo parts except the roots. 



Cross inoculation by Freeman and Johnson '' 

 from barley to wheat and the reverse gave 

 negative results. The optimum time for infec- 

 tion has been determined as the period of full 

 bloom. 



U. perennans Rost."*- ^'^ 



Son in spikelets, more or less destroying the 



, , J r ' .. . Fig. 222.— U. tritici, 



basal and mner parts, sometimes even running germination in 

 down on pedicels, oblong, about 3-8 mm. in ^°d^?fd Cohn's 

 length, with dusty, olive-brown spore masses; Keiieiman and 

 myceUum perennial in perennial parts of host; 

 spores chiefly subspherical or spherical, occasionally ovate to el- 

 lipsoidal, usually lighter colored on one side, more or less 

 minutely echinulate, especially on the lighter side, 5-8 n in 

 length. 



On the tall oat grass throughout its range. 



U. rabenhorstiana Kiihn occurs on several species of Panicum. 



U. tritici (Pers.) Rost.^^- "«• i^"- ^^'" ^^ 



Sori in spikelets, forming a dusty olive-brown spore-mass, about 

 8-12 mm. long by half as wide, usually entirely destroying floral 



