MOLDS. 21 



are smaller, lack the columella, and produce few to several spores within 

 an outer wall. 



Thamnidium elegans, Link, produces primary and secondary spor- 

 angia on different hyphae, together making white colonies. The fertile 

 side branches are produced in whorls and bear whorls of branchlets 

 from their centers which in turn produce sporangioles from the tips of 

 short straight twigs or branchlets. 



PENICILLIUM. The extremely abundant green molds most fre- 

 quently belong to the genus Penicillium, although some members of 

 other groups may be confused with them at times. 



Characters. Colonies are composed of loosely woven hyphae, branched, 

 septate, colorless, or bright colored. The fertile hyphae (conidiophores) 

 are mostly erect, arising either from submerged hyphae, or as branches 

 of aerial hyphae, septate, usually branched only in the fruiting portion. 

 Conidial fructifications consist of more or less complex systems of 

 branches and branchlets, the ultimate fertile cells each producing a chain of 

 conidia (Fig. 3). The whole system is usually grouped near the end of the 

 conidiophore, giving the appearance of one or more brooms or brushes 

 (whence the name). Very few species are known to produce asci, hence 

 these are rarely encountered. The conidial form continues for an in- 

 definite number of generations, therefore all the activities of the genus 

 are associated with this form. 



Cultural Considerations. Among the numerous species and races, 

 some of the green forms are widely distributed and almost omnivorous 

 in habit. Other species are closely restricted to particular substrata. 

 Starches and sugars appear to be especially favorable components of 

 nutrient media for members of the group. The larger number of the 

 species grows best at temperatures from 15 to 30; a very few of them 

 reach their optimum at 37, but many species are entirely inhibited 

 and some killed at blood-heat. Vegetative mycelium begins to be 

 produced at temperatures very close to freezing, but colored conidia 

 are produced slowly or not at all at low temperatures. The species of 

 Penicillium thrive through a wide range of concentration of culture 

 media, though perhaps the most characteristic growths are produced 

 in media high in water content. The common species of each genus will 

 grow in all the standard bacteriological media. With few exceptions 

 the species grow well in synthetic media composed of assimilable car- 

 bohydrates and inorganic salts. A few species require the presence of 



