[Vol. 



442 ANNALS OF THE MISSOURI BOTANICAL GA11DEN 



cient to be considered of specific importance, except in the 

 case of the form on the rhubarb. In general, the differences 

 referred to consist in a variable amount of the mealy or 



tufted growth, or of the amount of aerial growth; differences 

 in the color of the colony are also observable; and the rapidity 

 with which sclcrotia are formed are all minor distinguishing 

 features. The subject needs further investigation, but in gen- 

 eral it is felt that these differences are such as might be due 

 to permanent differences in the pathological strains, on the 

 one hand, or may be regarded as temporary differences due 

 to the recent environment, on the other. It may be pointed 

 out that the appearance of the mycelium of the beet fungus 

 from the damping off seedlings is not exactly comparable 

 with that of the mycelium derived from the beet rot. When 

 the organisms from both sources are grown in culture they 

 are found to be identical. Strains do occur, however, evidence 

 of which may persist for some time in the general appearance 

 of the cultures. 



The exact conditions under which sclerotia may occur on 

 the various hosts affected have not been determined. It has 

 been noted that affected potato tubers are the main seats of 

 sclerotia formation when the fungus attacks that host. Upon 

 this plant they are typical, and the numerous illustrations 

 published are sufficient evidence that the appearance is much 

 the same under a variety of conditions. Special attention 

 may be called to the illustrations of Duggar and Stewart 

 ('01), Eolfs ('02), Duggar ('09), McAlpine (11), Pethy- 

 bridge ('11), and Morse and Shapovalow ('11-). On the 

 majority of hosts, however, sclerotial formation is relatively 

 rare. 



From the various illustrations referred to it will be seen 

 that the sclerotia vary in size from those so minute as to be 

 scarcely visible, to others which may be a centimeter or two 

 in diameter. They are generally more or less flattened, 

 irregular, deep chestnut-brown, and generally smooth on the 



surface (that is, free from a looser growth of investing 

 hyphae). Smoothness of sclerotia, which has been regarded 

 by Kiihn as of much diagnostic value, should not be considered 



