VARIATION 71 



often shows longer and fewer conidiophores and larger heads than in the 

 central area. 



Within the group with the usual structures still recognizable, many 

 variants with contrasting features appear. Heads with very long primary 

 sterigmata, which are sometimes septate, appear in A. carbonarius (Bainier) 

 Thom, A. pulchella Speggazini, or A. tubingensis Mosseray. The primary 

 sterigmata may grow out into sterile filaments as in Mosseray 's figure for 

 A.ficuum (Reich.) Henn.; much more commonly, some primary sterigmata 

 grow out as tiny conidiophores and produce little heads, often consisting 

 only of a cluster of simple sterigmata and conidial chains. Thus, sterig- 

 matic changes may run from the simple sterigmata of A. japonicus Saito, 

 A. luchuensis Inui, or A. malvaceus Mosseray, where only some are double, 

 to other species showing the widest range in length and arrangement. 



Another group of variants show marked suppression of the ordinary 

 structures expected. Strains in which conidiophore formation has been 

 reduced or almost suppressed have been studied in continuous culture. 

 Such colonies showed an occasional long conidiophore and large head, con- 

 forming to the A. niger pattern, produced at the end of the colony growth 

 period. Meanwhile the mycelium was fully covered with irregularly 

 branching hyphal elements bearing single sterigmata variously placed, 

 groups of sterigmata, or penicilloid clusters of sterigmata each bearing a 

 short chain of conidia showing the characteristic markings of the group. 

 Transfers from the simplest form developed the complex or A. niger ele- 

 ments. Transfers from the large heads brought a recurrence of the re- 

 duced type of fruiting. No method of selection tried brought back the 

 typical A. niger aspect, and cultures of this type appear to represent de- 

 generate forms. 



It would appear, then, that a general type or morphological picture when 

 found dominant in large numbers of natural isolates can be regarded as 

 typical for a species of Aspergillus. It is recognized that marked diver- 

 gences from such types occur under the unrecorded stimuli of nature. 

 Some of these forms succeed in establishing themselves as permanent 

 elements of the microflora and thus become successful as species, or va- 

 rieties. Others do not digress quite so markedly and thus constitute 

 intermediate or bridging forms. At the same time marked changes can 

 be induced by the application of artificial stimuli. Where the origin of 

 such altered strains is known, they are commonly regarded as mutants. 

 Were they isolated directly from nature and their previous history not 

 known, it is probable that they would be considered as separate varieties 

 or even species. 



Natural Mutation 



While most of the mutants which we recognize as such have originated 

 in the laboratory as the result of certain artificially imposed stimuli (or 



