14 A MANUAL OF THE ASPERGILLI 



as in A. niger; or the heads may be uncolored or nearly so, while the outer 

 layers of the conidiophore wall may be colored as in A. flavipes and A. 

 ochraceus. In some species of the A. glaucus group, the colony color is 

 at first green with the development of conidia, then predominantly yellow 

 to ferrugineous from ripening penthecia. Again, in other species of the 

 same group, the walls of the conidiophores and, more particularly, aerial 

 hyphae become encrusted with granules that are characteristically yellow 

 in the young colony, but become reddish or ferrugineous in age. Such 

 colonies are at first predominantly yellow with green heads inconspicuous 

 on a yellow background and latei become rusty red or brown. 



In the A.flavus group, Saito (ly07) followed by Thorn and Church (1921, 

 p. 115) found that cultures with brighter shades of green when subjected 

 to a vapor of ammonia would lose the green color and assume the somber 

 yellow shades of variant members of the same group, and that this reaction 

 was reversible, since the vapor of acetic acid would restore or even intensify 

 an original green shade. The experiments pointed to the hypothesis that 

 the wide range of shades produced by mixtures of yellow and green in the 

 A. flavus-oryzae group' may be attributed to racial limitations, strain by 

 strain, in the range of hydrogen-ion concentration produced by metabolism. 

 When a carbohydrate fermentable by the particular species is present, an 

 acid reaction is promptly produced in the growing colony; as growth pro- 

 gresses alkaline products are also produced. The colony color in this series, 

 therefore, reflects first the intensity of the initial acidity as shown by the 

 intensity of the green color reached. The persistence of this shade or its 

 subsequent reduction or entire disappearance to leave a somber yellow or 

 finally brown colony, represents the balancing of the two activities. As 

 a result, certain strains of this group, if grown on Czapek's solution agar, 

 are quickly and very persistently deep green, others become particular 

 shades of green, which fade to yellow and finally some of them to brown, 

 and a few forms produce no true green color but assume a somber yellow 

 with the first development of conidia. 



Color changes in the conidia have not been satisfactorily worked out. 

 In the A. niger group, the shade of yellow to purple-brown or black seems 

 to be a strain or race character little influenced by handling which is not 

 destructive of the racial entity. Each race seems to reach a fixed quanti- 

 tative limit in the secretion of the coloring substance, thus reducing the 

 most conspicuous diagnostic character among closely related forms to a 

 quantitative rather than a qualitative basis of separation. 



Color in Conidial Walls 



The conidial walls may be smooth but carry sufficient coloring matter 

 in diffused form to give the characteristic colony color. Within such 



