Chapter IX 

 ASYMMETRICA 



Sub-section: VELUTINA 



This section includes species of Penicillium with asymmetric penicilli 

 that are characteristically developed in a dense and even stand to produce 

 a velvety effect. Strictly speaking, it is typified by species in which the 

 vegetative mycelium is largely or wholly submerged, and the aerial portion 

 of the colony consists of conidiophores standing like a field of wheat — the 

 conidiophores being comparable to the wheat culms and the conidial heads 

 representing the heads of grain. Actually, many gradations are encoun- 

 tered, and in practice, the term velvety is broadened to include species that 

 are obviously related but in which conidiophores arise from a basal net- 

 work of interwoven aerial hyphae. The section does not include any forms 

 in which colonies are definitely floccose or lanose ; or in which aerial growth 

 is collected into ropes or funicles; or in which conidiophores are aggregated 

 into tufts or fascicles. Species developing these characteristics are con- 

 sidered in other sub-sections which follow, namely: Lanata, Funiculosa, 

 and Fasciculata. Evidence of cross-relationship between these different 

 sub-sections has been observed in a limited number of cases, but on the 

 whole lines of separation are fairly definite and easily determinable. 



Six series are recognized within the ^^elutina. These include some of the 

 most abundant and the most important of all the Penicillia from an agri- 

 cultural and industrial point of view. The series, which are centered 

 around well kno\^^l and easily recognized species, may be briefly differen- 

 tiated as follows: PenicUlium citrinum series, characterized by asymmetric 

 penicilli that usually consist of terminal verticils of metulae, bearing sterig- 

 mata with conidial chains typically adherent in divergent columns. Peni- 

 cillium chrysogenum series, characterized by penicilli that are often re- 

 branched below the level of the metulae. Colonies are usually marked by 

 bright to pale yellow pigmentation in exudate and reverse. Conidial 

 chains commonly adhere into well-defined columns. Members of the 

 series produce the antibiotic penicillin in varying amount. Penicillium 

 oxalicum series, common in soil, is characterized by strongly elliptical 

 conidia which in newly isolated strains often form heavy crusts. Peni- 

 cillium digitatum series, represented by the single species of the same name, 

 is responsible for the so-called "green rot" of citrus fruit. Penicil- 

 lium roqueforti series, including the molds used for the production of 

 Roquefort-type cheeses, is characterized in culture by colonies with arach- 

 noid margins and by conidiophores with walls conspicuously roughened. 



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