[Chap. XLIV THE FUNGI 559 



and produce short hyphae that bear "sporidia." These fuse m pairs and 

 then germinate. The resulting hyphae penetrate the young wheat seed- 

 hng. The growing internal mvcelium reaches the stem tip of the seedling, 

 keeps pace with the growing wheat plant, and finally enters the ovularies 

 of the inflorescence. The floral parts are greatly distended by harvest 

 time. Masses of smut spores have replaced the embryo and endosperm 

 of the seeds. During the threshing of the wheat these spores are released 

 and scattered among the other grains of wheat. 



Loose smut of wheat and barley. The life cycle of the loose smut of 

 wheat is in one important respect very diflFerent from the bunt of wheat 

 just described. The spores produced in the dark brown or black masses 

 in the inflorescences of wheat at flowering time may be blown to the 

 stigmas of other wheat plants. There thev mav germinate, producing 

 hvphae that penetrate the ovularies. The mycelium remains dormant 

 within the embryo until the seed is planted and starts to grow. It is not 

 possible bv superficial examination of the seed to ascertain the presence 

 or absence of the internal mvcelium. Following the germination of these 

 seeds the mycelium grows within the developing host plant, and masses 

 of spores appear just before flowering time, replacing the destroved 

 flower parts. Thus the life cycle is complete. The methods of control of 

 this fundus must evidentlv be different from those used in the elimina- 

 tion of the stinking smut of wheat. The loose smut of barley is similar 

 to that of wheat. 



Smut of corn. Smut masses mav be seen on anv part of an infected 

 mature corn plant. The infection, however, alwavs takes place when the 

 tissue is young. The corn smut fungus does not spread throughout the 

 host from the point of infection but remains more or less localized. The 

 smut galls and their masses of spores appear one to three weeks after 

 infection. The spores may remain alive in the soil or in plant litter for 

 many months, and consequentlv the fungus is difficult to eliminate. 



Stem rust of wheat. One of the most complex life histories in the plant 

 kingdom is that of the black stem rust of cereals and grasses.^ The stem 

 rust of wheat is important commercialh' because during some years it 

 decreases the yield of the American wheat crop bv an estimated 120 



^ There are at least 8 varieties of the black stem rust fungus, Puccinia graminis, but only 

 3 infect cereals: P. graminis tritici, infecting wheat, barley, and many wild grasses; P. 

 graminis avenae, on oats and some wild grasses; and P. graminis secalis, on rye, barley, 

 and wild grasses. 



