ASPERGILLUS. 191 



Certain forms differing somewhat in character from the 

 above were isolated in the author's laboratory. They have 

 a technical value in the manufacture of citric acid, which is 

 produced in a free state as the result of their activity. The 

 fermentation is a form of oxidation, and is dependent upon 

 free access of oxygen. For certain reasons, however, which 

 are as yet unknown, the formation of citric acid is to be re- 

 garded as a progressive but imperfect oxidation. The quantity 

 of acid can rise to 8 per cent, without exercising an appreciable 

 effect on the vegetation, but beyond this point the acid is 

 decomposed by the fungus. If, on the other hand, the acid 

 is neutralised as fast as it is produced by the addition of 

 chalk, the formation of acid continues. The difficulty that 

 is encountered in carrying out this process on a large scale is 

 due to the fact that a comparatively small infection with yeast 

 or with Penicillium glaucum strongly affects the course of 

 fermentation. 



3. Aspergillus. 



The most commonly occurring species is Aspergillus 

 glaucus (Fig. 34), first fully described by de Bary. It forms 

 a fine felty, greyish or greyish-green covering on various 

 materials, and grows with great luxuriance on green malt. 



The mycelium consists, as in the case of Penicillium, of 

 fine transparent and branched threads, provided with trans- 

 verse septa. Some of the hyphal threads grow up perpendicu- 

 larly, are thicker than the rest, and are rarely branched or 

 divided by septa. The upper ends swell to spherical flask- 

 shaped heads (c), and these throw out from their entire upper 

 portion radially divergent papillae of an oblong form ; these 

 sterigmata (s) then develop at their apex small round pro- 

 tuberances, which are attached to the sterigmata by greatly 

 constricted bases, and after some time break off to form 

 independent cells (conidia). Below the base of the first 

 conidium a second begins to form from the crown of the 

 sterigma, and pushes the first upwards ; a third then forms, 

 and so on. Each sterigma thus carries a chain of conidia, the 

 youngest of which lies closest to it. This occurs simultaneously 

 over the whole surface of the enlarged end of the conidiophore, 



