MORPHOLOGY AND DESCRIPTION 17 



of the structure of the head is given in the more logical order, following 

 the discussion of the stalk or conidiophore. 



The Foot-Cell 



The first step toward conidium formation in the Aspergilli is the differ- 

 entiation of certain cells (the foot-cells) in the mycelium for propagative 

 purposes. These cells become larger, thick-walled, and each usually bears 

 a single conidiophore as a branch, perpendicular to the long axis of the 

 cell (fig. 2 A) and usually about midway between the ends of the cell. In 

 age these cells frequently become fantastically curved and twisted with 

 their connection to vegetative hyphae inconspicuous but usually still 

 determinable. These foot-cells are commonly submerged in the sub- 

 stratum, although there are a number of strains of the A. glaucus, A. 

 fumigatus, A. versicolor, and especially of the A. flavus-oryzae groups in 

 which the conidiophores arise in this way from aerial hyphae. In A . effusus 

 the foot-cells are frequently long and several of them connected together 

 to form whole hyphae bearing considerable numbers of very short conidio- 

 phores (fig. 71 B 3 ), hence their differentiation from the sterile or vegetative 

 cells is less easily determined. Failure to recognize the foot-cells as 

 present in the Aspergilli led Ferdinandsen and Winge (1920) to describe 

 S. dipus, using the foot-cell as the principal diagnostic character of the 

 species. The presence of such a differentiated foot-cell is proposed as an 

 arbitrary character to be used in separating certain depauperate forms of 

 Aspergilli, which approach the structure and appearance of the mono verticil- 

 late Penicillia (Citromyces), from the Penicillia. Organisms which lack 

 the typical Aspergillus head with its conidiophore and especially its foot- 

 cell may be best classified elsewhere. 



The Conidiophore or Stalk 



The erect, perpendicular branch from the foot-cell constituting the 



conidiophore usually enlarges upwards toward the apex at which it dilates 



more or less definitely to form the vesicle (fig. 2 C-E). The section of 



the conidiophores from the foot-cell to the base of the conidial head is 



measured and reported in describing species. 



The conidiophore in some groups is not only septate, but each cell is 

 sufficiently distinct to justify the term articulate which is frequently 

 encountered in the older descriptions. In our experience in examining 

 specimens, articulate conidiophores are found only in the A. glaucus group. 

 In most Aspergilli the unity of the whole conidiophore is fairly accentuated. 

 Septa, if present, are thin, fragile, and inconspicuous; the whole conidiophore 

 is enclosed by continuous characteristically thickened walls without 

 conspicuous nodes as an evidence of septation. 



