PENICILLIUM 103 



branches (metulae) given off, usually also in verticils, from the tip 

 of the conidiophore. The appearance of a low-power view of the 

 spore head is somewhat like that of a brush; and the spore head is 

 called a penicillus, which is Latin for a brush. Some species pro- 

 duce ascospores after homothallic conjugation. These have been 

 extensively studied.*' 



Our knowledge of the Penicillia has been greatly extended in re- 

 cent years by the extensive work of Biourge ^ and the still more 

 exhaustive treatise of Thom.^^ The latter authority describes (in- 

 cluding three closely related genera) over six hundred species! The 

 differentiation of these is more difficult than that of Aspergilli. In 

 such a condition it is obvious that the precise determination of par- 

 ticular strains must remain a task for specialists. 



In the earlier literature (and all too frequently even today) papers 

 were published on the ecology, physiology, morphology, etc., of vari- 

 ous "species" of Penicillium without an accurate determination of 

 identity. For instance, any and all green forms were often referred 

 to as Penicillium glaucum. This term has been used so indiscrimi- 

 nately for a variety of species that the name is worthless and should 

 be avoided. The problem of identifying species of Penicillium (and 

 to a less extent other genera as well) is not one that is easily solved 

 by the bacteriologist who works on the biochemistry of molds. If 

 he cannot identify them accurately (and few mycologists even at- 

 tempt to identify species of Penicillium) or if he cannot get them 

 identified by specialists, he had better not give them Latin binomials 

 at all, but deposit them in a culture museum if possible, or at least 

 keep his cultures so that others may verify or extend his work. 

 There are very few of us who have done biochemical work on molds 

 who have failed to make such insufficient identification or to let cul- 

 tures die out. There is thus no way to find the identity of the molds 

 that were studied. One might as well make a physiological study 

 of a "grasshopper," a "wild sunflower," or a "spore-forming bacillus." 



The Penicillia are subdivided by Thom into four sections, and 

 these are further subdivided into subsections. The basis for the 

 primary subdivision is the nature of the branching of the spore 

 heads, whether this is symmetrical about the axis of the conidio- 

 phore or asymmetrical. The symmetrical types are separated into 

 three groups: the Monoverticillata, with a single whorl of sterigmata 

 at the tip of the conidiophore; the Biverticillata-symmetrica, in 

 which the verticils of sterigmata arise from short branches or metulae, 

 which themselves form a verticil on the end of the conidiophore ; and 

 the Polyverticillata-symmetrica, in which three or more stages of 



