FusARiA OF Potatoes 103 



from the bottom of the plate with comparative ease. When a single 

 spore at some distance from the others was located, it was marked with a 

 circle of india ink. In this manner several spores were marked and the 

 plate was left for another day, after which the colonies were usually large 

 enough to be observed with the unaided eye. Transfers could then be 

 made easily and surely. These transfers have been kept as the stock 

 cultures. The dilution plates from which stock cultures were obtained 

 were kept for several weeks longer, in order to see whether all the colonies 

 were alike. 



In only three cases, after the first dilution, did there appear to be different 

 colonies present in the same separation plates. In one of these cases this 

 was the result of a mixed growth of two distinct fungi. It is of some 

 interest to note here that the mixed culture appeared to be a fine specimen 

 of a pink fungus. On dilution it gave rise to a brick red fungus, F. meta- 

 croum, and a white one, F. diversisporum. In the other two cases the 

 fungi obtained from the differently appearing colonies represented most 

 close!}'' related organisms which never were isolated again, but which 

 nevertheless remain distinct from each other. Whether they represent 

 mutations, fluctuations, or mere chance coexistence of the two actually 

 different organiems, is a matter yet undecided. They are tentatively 

 designated as two different varieties — F. hullatum var. roseum and F. 

 arthrosporioides var. asporotrichius. 



The stock cultures were subsequently repeatedly rediluted, and, as 

 was to be expected, almost invariably'^ every dilution plate showed all 

 the colonies from the same stock culture to be alike. 



As it is not possible to use a high-power objective with the ordinary 

 petri dish, and as single spores of some of the species are veiy difficult 

 to locate because of their size and lack of color, there was always the 

 possibihty that some of the cultures which seemed to be pure were really 

 mixtures, for it is a well-known fact that the spores of many fungi often 

 cling together tenaciously. In order to make absolutely sure that pure 

 cultures were being dealt v/ith, the further precaution was observed in 

 1913 of locating single isolated spores with a high-power objective and 



' Only two exceptions were observed: (1) In the case of F. metacroum var. minus among characteristic 

 red colonies, there appeared one colorless colony. For a while the absence of color was a constant char- 

 acter, but later the color was produced again. (2) In the case of F. diversisporum among colonies with 

 high aerial mycelium there was a colony almost without aerial mycelium. In subsequent generations the 

 aerial myceUum reappeared almost to the same extent as originally. 



