ASPERGILLACEAE 



611 



the underside of the slant or Petri dish, it is often described as color of the 

 reverse in taxonomic literature. Coh)r production may be influenced by 

 clianging the nutrient, often witliout apparently nijuring the vigor of the 

 mold and may often be increased by increasing the concentration of some 

 fermentable nutrient or inhibited by tlie absence of that particular substance. 

 Since some of these colors seem to be indicators, they may change with the 

 changes of hydrogen ion concentration in the medium, produced by the 

 metabolism of the organism. Oxidation also seems important in color changes, 

 although it has not been studied carefully. In some cases a considerable 

 pigment is produced in the substratum. The appearance of the mycelium in 

 the colony may be described as velvety, floccose, coremiform, or funiculose. 



Fig. 90. — Diagrammatic radial sections of colonies. 1, velvety type ; 2, coremiform or 

 fasciculate type ; S, //, floccose type ; 5, funiculose type ; AB, surface of substratum ; X, conidlo- 

 phores ; C, conidial heads (X25). (After Thom 1930.) 



In the velvety type, nearly all the hyphae are submerged in the sub- 

 stratum, and the conidiophores branching from the submerged hyphae rise 

 above the surface and produce the conidial masses. Such colonies in section 

 resemble a field of wheat (Fig. 90, 1). 



In the floccose type, a white cottony mass of branching and interlacing 

 hyphae spreads evenly or unevenly over the surface of the nutrient medium. 

 Conidiophores arise from these aerial hyphae, usually beginning in the center 

 and developing toward the margins. Throughout the growing period a sterile 

 margin surrounds the fertile area (Fig. 90, 3, 4). In their extremes, these 

 two types are very distinct, but every intermediate gradation exists. 



In the coremiform type, the colonies consist of a submerged vegetative 

 mycelium and upright columns of hyphae or conidiophores, called coremia, 



