10 MISCELLANEOUS PUBLICATION 955 ; U.S. DEPT. OF AGRICULTURE 



corales have been used in steroid transformations, for the assay of 

 vitamins and antifungals, and for microbiological study because of 

 the many forms readily available. They are especially useful, since 

 large amounts of mycelium are produced in a few hours on chemically 

 denned media. Asexual spores develop readily and rapidly for 

 inoculum. 



Phycomyces has been studied extensively because of its extremely 

 long sporangiophores, which respond to light. They are often 20 cm. 

 long and yet exist as single cells. This genus has been used in many 

 studies on the biosynthesis of carotenes. Under certain conditions 

 some species, especially in Mucor, may take on a yeastlike phase. 



As far as is known, all genera of the Mucorales produce sexual spores 

 known as zygospores. Some species are homothallic (zygospores are 

 produced from a colony derived from a single sporangiospore) or 

 heterothallic (zygospores are formed only when two opposite mating 

 types are brought together). One disadvantage to genetic studies of 

 this order of fungi is the fact that spores are multinucleate. Also, 

 the zygospores are very difficult to germinate. 



The Mucorales grow almost everywhere and are easily isolated by 

 removing some of the hyphal tips or by isolating a few spores from a 

 fruiting stalk. Mucorales often occur among the first organisms 

 during incubation of various substrates. Because they grow rapidly 

 and utilize simple sugars, they are sometimes referred to as the sugar 

 fungi. 



The Entomophthorales have a coarse mycelium made up of hyphae 

 of reduced length. Typically, conidia are formed in abundance and 

 these are forcibly discharged. In spite of the fact that a number of 

 species are parasitic on insects, growth of these forms is relatively 

 easy both on solid and in liquid media. 



The following orders in the Phycomycetes seem to offer the best 

 opportunities for biochemical and physiological studies: Mucorales, 

 Entomophthorales, Blastocladiales, and Saprolegniales. In addition, 

 the family Pythiaceae of the order Peronosporales should be consid- 

 ered. The first two orders have nonmotile spores and are typically 

 terrestrial. The other groups have soil or aquatic forms and motile 

 spores. The family Pythiaceae has several important parasites of 

 higher plants, with infection beginning usually in the root system of 

 the host. A great many Phycomycetes offer practically a virgin field 

 for the study of nutritional and growth- factor requirements. 



Ascomycetes 



The Ascomycetes comprise perhaps the largest class of true fungi, 

 or Eumycetes. Ainsworth (1) conservatively estimates the number of 

 valid species to be 15,000, or one-third of the 45,000 so far described. 

 According to him, these 15,000 species are contained in 1,700 genera, 

 of which 850 are monotypic ; i.e., containing only 1 species. 



Although the Ascomycetes are a large class and have many diverse 

 organisms, they all have one structure in common, the ascus. It is a 

 membranous, saclike structure produced by sexual processes (i.e., proc- 

 esses involving nuclear fusion and reduction), in which ascospores 

 are formed. The ascus may lie single and naked, as in such simple 





