ONION SMUT FUNGUS. 113 



aerial mycelium; margin very definite and oven. After about ten days the mycelium 

 shows more and more of a tendency to grow beneath the surface of the agar, and 

 the edge has the appearance of gradually fading away into the surrounding agar. 

 Growth may progress for several weeks, but is gradually checked by the drying 

 out of the agar. Some of the cultures show indistinct zonation. With age the 

 surface of thp felt may become rugose. 



Oat Agar. 



Growth more luxuriant than on potato agar, showing denser zones of white 

 mycelium. No change of color in mycelium or in the medium. Growth not suf- 

 ficiently different from that on potato agar to have any diagnostic value. 



Nutrient Beef Broth Agar. 



The standard agar of bacteriological work. Growth scanty, much less than on 

 potato agar, slimy, and taking on the color of the medium; never dry, very little 

 aerial mycelium. A very poor medium for growing the organism. 



Czapek's Agar. ^ 



This was found to be a very favorable medium, the giowth being more rapid 

 and with a greater abundance of white, cottony aerial mycelium than on potato 

 agar. After about two weeks the agar below the growth, especially in the upper 

 part of the tube, turns maize yellow, ^ due to the suffusion of a pigment. After 

 about four weeks the color becomes more intense — aniline yellow or citrine yel- 

 low. With age this darkens to orange citrine or to various shades of olive. Also 

 the mycelium as seen from above loses its white color after three or four weeks, 

 showing various shades of greenish yellow — citrine drab, olive lake, etc. These 

 color changes on Czapek's agar offer one good diagnostic character. 



Onion Decoction. 



Prepared by boiling a sliced onion in a liter of distilled water and sterilizing the 

 filtered product for one hour at 15 pounds' steam pressure. Growth very slow, 

 resulting in development of little compact balls of mycelium; brown when in the 

 bottom of the tubes or white when on the surface of the liquid. Growth continues 

 for months very slowly, but the little balls of mycelium do not attain a diameter 

 of over 1 cm. 



Onion Agar. 



Prepared exactly like potato agar, but the onion decoction as described above 

 is used instead of potato juice. This was found to be not only the best medium for 

 culturing Urocystis, but also very much better than potato agar for growing many 

 other fungi which the writer had occasion to try on it. It is very easily prepared, 

 has a minimum of sediment even when not filtered, and altogether forms a very 

 superior general purpose agar. Its only objectionable qualities are the obnoxious 

 odor in the laboratory during preparation, and the fact that the growth of certain 

 fungi is too luxuriant for some purposes. The growth starts with a dense white 

 felt much like that on potato agar, but more rank. After about a week wrinkles 

 begin to appear near the center, and these spread and become sharper and the 

 irregular ridges more elevated with age, also at the same time the crests of the 

 ridges become hygrophanous and gray. This appearance spreads until it involves 



' For method of preparation see Soil Science, 2:113. 

 ' All colors according to Ridgway's Color Standards. 



