172 A MANUAL OF THE PENICILLIA 



tion in culture of more than 50 strains, including the types of several species 

 described originally by Biourge (1923), Zaleski (1927), Stapp and Bortels 

 (1935), and others. The establishment of new species by these and other 

 workers in many cases apparently resulted from the unavailability of type 

 material of earlier authors and from the inadequacy of published descrip- 

 tions. It is not surprising, therefoie, that much duplication occurred. 

 While some investigators may wish to recognize additional species in this 

 general series, it is our belief that most workers will benefit by emphasizing 

 broad relationships and trends rather than by the recognition of multiple 

 species based upon minor and often variable strain characteristics. 



Historically, the series is of considerable interest since it, in all prob- 

 ability, includes forms similar to those upon which Wehmer based his genus 

 Citromyces and the two species, C. glaber and C. pfefferianus that he em- 

 ployed for the production of citric acid (1893b). No one can now say with 

 certainty just what type of molds Wehmer used as the bases for his tw^o 

 species, although we believe C. glaber must have represented some form 

 approximating P.frequentans Westling (see p. 176) and C. pfefferianus prob- 

 ably represented some form such as P. spinulosum Thom (see p. 184). 

 Many additional investigators since Wehmer 's time have attempted to 

 utilize species of monoverticillate Penicillia (or Citromyces of Wehmer) for 

 the production of citric acid but without notable success. Insofar as we 

 know, selected strains of black Aspergilli, belonging to the A. niger group, 

 are now regularly employed for the commercial production of this acid by 

 fermentation methods. 



Members of this series, especially Penicillium frequentans Westling and 

 P. spinulosum Thom, are unusually common in soil and upon organic 

 materials undergoing slow decomposition. Xo other species of the Mono- 

 verticillata, and few outside this group, are encountered more frequently. 

 How important these organisms are in the decomposition of vegetable 

 residue in nature has not been widely studied, but their common occurrence 

 there is believed to indicate an active role in such processes. 



Penicillium frequentans Westling, in Arkiv for Botanik 11: 58, 133-134, 

 figs. 39, 78. 1911. See also Biourge, Monograph, La Cellule 33: fasc. 

 1, pp. 292-293, Col. PI. X and PI. XVII, fig. 99. 1923; and Thom, The 

 Penicillia, pp. 216-217. 1930. 



Colonies on Czapek's solution agar (Col. PI. Ill) spreading rapidly, at- 

 taining a diameter of 5.0 to 6.0 cm. in 12 to 14 days at room temperature, 

 broadly zonate, radiately wrinkled in most strains with central area com- 

 monly sulcate, thinning at the margin (fig. 49A), consisting of a closely 

 woven felt of coarse hyphae either above or below the surface from which 

 abundant crowded conidiophores arise, velvety, but commonly showing 



