ASYMMETRICA-VELUTINA 357 



that hardly resemble the original, yet retain sufficient of their characteristics 

 to counter-indicate any possibility of contamination or replacement by 

 foreign strains. 



In the morphology of the penicillus, members of the series vary from 

 strains in which the separateness of individual verticils gives the effect of a 

 fertile hypha beai-ing a series of branches with each producing a mono- 

 verticillate conidial mass, to a complex and typically asymmetrical conidial 

 apparatus (fig. 94). The conidia vary from globose to subglobose, and from 

 (luite small, as in typical strains of PenicilUimi notatum with diameters 

 mostly less than 3.5)u, to strains of P. chrysogenum with conidia 4.0 to 4.5m 

 in long axis and definitely elliptical. 



It is quite impossible to draw any sharp lines of separation within the 

 Penicillium chrysogenum series due to the prevalence of intergrading strains. 

 Satisfactory definition of species, therefore, becomes extremely difficult, and 

 recognition of a particular culture as representing this species or that, must 

 rest ultimately upon the judgment and experience of the individual worker. 

 Thom (1930) listed nine species as belonging to this series, which he termed 

 the Radiata, after a prior usage by Biourge (1923), and cited individual 

 strains as more or less adequately satisfying the description of each. Most 

 of these had come recently from Biourge and represented the type material 

 for species published by him or his predecessor, Dierckx. Many of them 

 are still in our possession and in general continue to exhibit the character- 

 istics noted for them two decades ago. In the meantime, however, hun- 

 dreds of new isolates have been examined, including many intergrading 

 forms. We are compelled to conclude that Biourge (1923) and Dierckx 

 (1901) described their species largely upon individual strains which repre- 

 sented contrasting forms in a series for which intergrading individuals were 

 not then known. 



AVhile it is extremely difficult to define a species in this series, it is obvious 

 that all strains belonging to it cannot be regarded as representing a single 

 species, for many of them differ greatly from one another in almost any 

 character selected as a basis for comparison. Based upon careful, and in 

 some cases exhaustive, study of hundreds of strains belonging to this series 

 during the past few j^ears, we believe a separation into four species will 

 prove reasonably practicable. The species so recognized, together with 

 their salient characteristics, are as follows: 



Penicillium chrysogenum Thom, marked by heavy sporing, velvety, often 

 rather loose-textured colonies in blue-green to yellow-green shades; com- 

 paratively large and often rebranched penicilli, with the conidial chains 

 arising from individual metulae usually adhering into fairly well-defined 

 columns, and with conidia regularly elliptical and fairly large, up to 4.0 to 

 4.5/i in long axis. 



