y-IO ADDITIONAL EXERCISES 



the removal of the material for study is more easily accomplished, and because the 

 whole growth can be examined readily by placing the Petri dish on the stage of the 

 microscope and examining with the low power. In mounting such fungi for study 

 beneath a coyer-glass lo per cent, alcohol should be used to wet the spores and 

 hyphs, otherwise difficulty will be encountered with spores flowing together in mass 

 and the hyphs becoming knotted together. Thorn, in his paper on the " Cultural 

 Studies of Species of PeniciUium," published as Bull. ii8 of the U. S. Bureau of 

 Animal Industry in 1910, recommends that the following media be prepared for the 

 study of the species as his key for the identification of the species given below is 

 based on their behavior upon the recommended culture media. For this purpose 

 prepare the following media: (i) 15 per cent, gelatin ("gold label") in distilled water; 

 (2) IS per cent, gelatin in distilled water plus 3 per cent, cane sugar; (3) either bean 

 or potato decoction plus 1.5 per cent, cane sugar; (4) bean or potato agar plus 3 

 per cent, cane sugar. Litmus solution may be added, if desired, when cultures are 



Fig. 249. — PenicilUum clamforme. a, Coremium grown upon sugar media; b, 

 coremium on gelatin free from sugar. {After Thorn.) 



made. Prepare Petri dishes with 10 c.c. of each of the media used and aUow them to 

 cool. Inoculate two or more Petri dishes of each medium with spores of the species 

 to be distributed to the class. Incubate at 2o°C. (the laboratory temperature is 

 usually satisfactory). Have the members of the class examine at intervals of three 

 days, or less, making naked-eye observations from above and below also with a 

 hand lens and with the low power of the compound microscope. A drop of litmus 

 solution at the margin of a colony can be used to test acidity, or alkalinity. 



Have the class examine i and 2 for liquefaction; 2 and 4 for coremium amd sclero- 

 tium formation which will call for continued examination for at least two weeks. 



Below will be found two separate keys. One, after Thom, is a general key 

 of species of PenicilUum grown upon the above-recommended agar and gelatin 

 media. The second key, after Buchanan, which includes the species of most eco- 

 nomic importance, is based on the character of the substratum on which the 

 fungi are found growing in a state of nature. 



