ONION SMUT FUNGUS. 115 



hand, spread filmost as rapidly over the surface for the first few weeks. There can 

 be no question whatever but that soluble elements in the soil furnish sufficient 

 food for the development of the mycelium. 



Dung Decoction Agar. 



Prepared by adding 2 per cent agar to the dung decoction previously mentioned. 

 Growth much thicker than on the soil decoction agar, but not as heavy as on 

 Czapek's, sugar potato, etc. Dense white aerial mycelium. The conclusion seems 

 warranted that horse manure furnishes all the elements necessary for the growth 

 of the fungus, and is more favorable medium than a good soil. Apparently a heavily 

 manured soil would be more favorable for the propagation of smut than one which 

 was not manured. 



Tolerance of Acid. — Four series of cultures were made on onion agar, — 

 the first series without lactic acid; second, with 1 drop of lactic acid per 

 tube; third, with 2 drops per tube; fourth, with 3 drops. All were inocu- 

 lated at the same time. Growth was rank and normal in the series in which 

 no lactic acid was added; no growth whatever in the series in which 3 

 drops were added ; a very sUght growth where 2 drops were added; growth 

 much retarded in the 1-drop series. This series was begun with the pur- 

 pose of finding a method of excluding bacteria from cultures of the smut 

 fungus, but the latter was apparently checked by acid just as much as the 

 bacteria. 



Effect of Freezing the Cultures. 



Cultures on potato agar and on onion agar were kept out of doors for 

 two months during the most severe winter weather of 1919-20. Transfers 

 were then made to fresh agar tubes, and the mycelium grew luxuriantly 

 and rapidly on the surface of the slants. In fact, the growth at first seemed 

 to be even better than when transfers were made from cultures which had 

 not been frozen. Accurate measurements on a second series showed a 

 slight difference in favor of the transfers from frozen mycelium during 

 the first few days, but it was not permanent. We may conclude, then, that 

 freezing not only does not injure the mycelium, but possibly stimulates 

 it to even better growth. 



Microscopic Characters of the Mycelium in Culture. 

 The characters of the mycelium differ somewhat with the age of the 

 culture. Microscopic examination of a culture a week old shows slender 

 hyaline hyphse of rather uniform diameter, about 2/<, with rather indistinct 

 septa and homogeneous contents. Branches arise almost exclusively from 

 the upper ends of the cells and diverge at a wide angle. The characters 

 have not changed from the condition previously described under germina- 

 tion of the spores. Not all of the cells of the mycehum appear to be alive; 

 some of them are empty and apparently dead; others are full of homoge- 

 neous protoplasm with no vacuoles. Under the oil immersion lens one 

 notices certain very refractive granules scattered throughout the dense 

 protoplasm (Fig. 2, A). The cells are easily broken apart, and when a 



