CHAPTER XXXVIII 



THE ASPERGILLI 



A TYPICAL GROUP OF MOLDS 

 CHARLES THOM 



Microbiological Laboratory, Bureau of Chemistry, U.S. Department of Agriculture 



The genus Aspergillus' is selected as a group of common molds which may be used 

 to illustrate the place of these organisms in nature, their position in a scheme of plant 

 classification, and a perennial interest as individual organisms. This natural group 

 includes a great series of species and physiological varieties, many of which accom- 

 pany man closely in all his activities. They are omnivorous, resistant to climatic heat 

 and cold, drought and flood. As producers of a wide range of enzymic activities their 

 growth becomes a help and a menace in many fields of human endeavor. 



These molds are mycelial fungi; their bodies consist of branching filaments com- 

 posed of cells placed end to end, growing mostly at their tips, and extending slowly 

 or rapidly in the substratum. Some ramify deeply throughout great masses of ma- 

 terial; others more closely dependent upon immediate access to the free oxy^gen of 

 the air grow widely over the surface, but penetrate only the outer few millimeters of 

 unbroken surfaces. Even these secrete enzymes which may diffuse widely and cause 

 great changes in large quantities of material. 



One spore, lodging favorably in any decomposable product, may germinate and 

 produce a mass of vegetative mycelium which will transform a large volume of organic 

 matter. This mycelium, under favorable conditions of heat and moisture, throws up 

 from the surface within a few days erect branches known as "conidiophores" (stalks). 

 The tips of these stalks swell to form little vesicles from which bud out large numbers 

 of spore-producing cells known as "sterigmata," in some species simple and un- 

 branched, in others again branching to produce a secondary cluster or verticil, and 

 from each ultimate tip spores or conidia are cut off rapidly. These spores or conidia 

 are at first cylindrical segments cut from the tip of a narrow tube. Each is quickly 

 succeeded by a newer cell cut from the same tube. There is thus formed a chain of 

 spores like a string of beads, actively lengthening at the point of origin. Each spore 

 or conidium, growing quickly, swells and assumes the size, shape, and markings 

 characteristic of its species. The whole mass at the tip of the stalk thus forms a head 

 which may vary from globose if its elements are radially directed from the central 

 vesicle, hemispherical when only half the area is covered, to columnar if only a cluster 

 of sterigmata are borne upon its very tip and the chains adhere to form a solid mass. 

 The form assumed is more or less definite for each race. 



' A selected series of references is appended to this paper. More extensive bibliographies are 

 given in Wehmer for work antedating 1901, and by Thom and Church for the literature up to 1925. 



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