58 A MANUAL OF THE ASPERGILLI 



resulting may assume the character of either organism with the other 



present only as an inconspicuous, even unrecognizable, mycelium with 



dwarfed heads, yet continue present for a long period. In this way the 



retarded species may suddenly reappear in some later transfer upon media 



favorable to it. In other cases, the dominant species may grow and fruit 



in an erratic manner that is deceptive in suggesting a reaction to the medium 



r other physical factors. Again, in less common cases, the two may grow 



jid fruit together without apparent inhibiting effects. This is the most 



lifficult form of mixture. 



Mixtures are sometimes encountered in which mycelia, sterile under all 

 conditions tested, become so intimately mixed with the mycelium of an 

 Aspergillus or Penicillium as to persist through many generations without 

 apparent effect in the earlier states of growth of the Aspergillus, but develop 

 as overgrowths of sterile hyphae in very old cultures. Many of these forms 

 can be isolated, but they defy identification because of an absence of diag- 

 nostic characters. Such sterile mycelia may arise from the species studied, 

 but unless such origin can be definitely proved, they must be regarded as 

 contaminants. Great care is necessary in interpreting cultures producing 

 sterile overgrowths, since the contaminating organism, or non-sporulating 

 variant of the same strain, may induce marked changes in the physiological 

 activity of the species supposedly pure. 



Secondary Growth 



In many species, part or all of the conidia produced by a colony germinate 

 and cover the primary mycelium with more or less abortive hyphal growths, 

 many of them unrecognizable unless traced by their origin. Again spores 

 floating on the surface of a globule of transpired fluid may germinate, their 

 hyphae interlace and a hollow ball surrounded by felted mycelium produce 

 structures which probably account for reported perithecia without asco- 

 spores. Such overgrowths, being irregularly produced, interfere with one's 

 judgment as to the whole character of the colony. 



Replacement by Other Species 



Other species of fungi invade mold cultures and some of them become so 

 intimately associated with the mycelium and conidia of particular species 

 that it is difficult to eliminate them by the ordinary method of transferring 

 spores with a loop or wire to streaks or stabs. Proper dilution culture 

 presents the possibility of elimination but demands careful examination 

 and selection from the resulting colonies. The progressive replacement of 

 a particular species, by invading organisms, is constantly encountered in 

 examining cultures passed from laboratory to laboratory. The original 

 organism may disappear without detection if the displacing species bears 



