5IO THE ASPERGILLI 



CONIDIA OR SPORES AND THE CLASSIFICATION OF ASPERGILLI 



Such heads thus produce enormous numbers of spores or conidia, varying accord- 

 ing to the species from 2 to 10 micra (roughly 1/12,000-1/2,000 in.) in diameter. 

 They are so light that they float readily in every breeze or breath. Some of them are 

 so waterproofed that they will float upon the surface of water a considerable distance 

 before they sink, absorb water and grow. Others are variously sticky, roughened, 

 eroded, or chiseled to favor adherence to moving objects. Thus the widest diflusion 

 by wind, water, and animals is provided for. The molds produc'ng their spores in 

 the form of heads as already described are grouped into the hyphomycete genus 

 Aspergillus (literally "rough head"). Some of them produce perithecia and ascospores 

 as a sexual form of fruit, hence are properly transferred to the Ascomycetes and com- 

 monly allocated to the Perisporiales. The vast majority of these molds as encountered 

 in nature are known only as producing heads of conidia or asexual spores, hence must 

 be kept for practical convenience in the old Hyphomycete or "form genus" Aspergil- 

 lus. In this generic group there is a large number of species, races, and strains; for 

 more than three hundred names have been proposed for the materials studied by 

 various authors during the past two hundred years. Thom and Church split this 

 whole number into sixteen species aggregates which are fairly easily separated where 

 pure cultures are studied with the aid of a compound microscope. This division 

 is shown on page 511. 



Within these species aggregates, the task of separation is much more difflcult, but 

 they name sixty-six species already described and probably identifiable upon morpho- 

 logical diagnoses. 



The problem of identifying species among these molds is complicated by the 

 changes induced by environment upon the structure and appearance of the mold 

 colony. As they appear in nature, moldy masses usually contain more than one species 

 and the individual species may be so far influenced by its associates as to render it 

 scarcely recognizable by gross or even microscopic examination. Before these effects 

 were understood, the students of fungi described a great many species, giving the 

 colors and measurements found in particular collections upon more or less vaguely 

 described substrata. Two tendencies ate encountered in such discussions, the de- 

 scription of new species for every change in size of head or length of stalk or shade of 

 color, and, on the other hand, lumping all forms with even a superficial resemblance 

 into heterogeneous aggregates so complex that the names used ultimately came to be 

 meaningless. 



BIOCHEMICAL CHARACTERISTICS 



With the development of improved methods of isolation and culture, Aspergilli 

 have been found to grow readily upon a wide range of laboratory media. It has, there- 

 fore, been possible to isolate them and work out their life-history and reactions to 

 selected media. In their metabolism they have proved able to utilize inorganic salts 

 of afl the usual elements required except carbon. Solutions made by the formulae 

 proposed by Raulin, Cohn, Czapek, and many others have been used as a basis to 

 test the availability of particular nutrients to individual species. The Aspergilli thus 

 furnish favorable species for many forms of physiological and biochemical investiga- 

 tion. 



