16 A MANUAL OF THE ASPERGILLI 



in intensity of color, or the presence or absence of color in different sub- 

 strata, make the use of closely defined shades or intensities of color in the 

 substratum an unreliable means of characterizing cultures. 



Observations of colors produced in the substratum remain, however, 

 very conspicuous and exceedingly useful accessory characters which aid in 

 the placing of species. The describer must bear in mind that such color 

 reactions are confirmative, not absolute characters in separating species. 



The final difficulty in dealing with color as a separating character rests 

 in the loose use of color names, which is only partially corrected by the 

 use of color standards. Comparison of the same culture by different 

 individuals introduces very considerable discrepancies which become 

 serious when a specific descriptive name or number from one of these 

 standards is introduced into a technical description. In general, a series 

 of observations giving a range of colors for a species is less liable to introduce 

 errors of subsequent identification. 



MORPHOLOGY 



From the time of Micheli, the name Aspergillus (literally, rough head) 

 has been used for molds with a conidiophore or stalk and spore-bearing 

 head (capitulum). 



The Head 



The first structure observed in a detailed study of the colony is the 

 spore-bearing head. The color, shape, size, and arrangement of such 

 heads are characteristic of the species and to a lesser extent of the groups 

 to which they belong (figs. 4 and 5). Wehmer (1901) roughly grouped 

 his Aspergilli into Microaspergilli and Macroaspergilli on the basis of 

 the size of the fruiting parts. Thus, A. fumigatus, A. nidulans, A. sydowi, 

 and A. versicolor would represent Microaspergilli, while A. niger, A. 

 clavatus, A. ochraceus, A. wentii, and A. tamarii would be readily classed 

 as Macroaspergilli. The distinction breaks down when great numbers 

 of forms are studied, but the comparative size of heads and conidiophores 

 remains a useful adjunct in description. The heads in certain species 

 show a consistent range of measurements and form, as in A. fumigatus 

 and A. nidulans which have heads of small diameter forming columnar 

 masses (fig. 4). Similarly, characteristic heads of A. niger, A. ochraceus, 

 or A. wentii are globose and large (fig. 5). In other species, notably in A. 

 flavus or A. candidus (fig. 60), several sizes and shapes of heads are regularly 

 found in the same colony. The range of size and shape, however, remains 

 characteristic. The observation of many heads in the colony, and pref- 

 erably in many separate cultures, forms a better basis for description of 

 sizes and measurements than limited observation. Further description 



