20 STRUCTURE AND CLASSIFICATION OF FUNGI 



and DeBary. It was not until Theobald Smith, twenty-eight years 

 later, had worked out the cycle of the protozoan parasite of Texas 

 tick fever that such a life cycle was known in animal pathology. 

 The alternate method of fertilization, that is, the pairing of the 

 nuclei from the receptive hyphae and from pycniospores of opposite 

 mating type, was discovered only fairly recently by Craigie in 

 Canada. 



Physiologic Races. Although the smuts may be cultivated on 

 artificial media, growth in a host plant is usually necessary for the 

 development of the diploid smut spores. The rusts are obligate para- 

 sites, which have not been grown in artificial cultures. In both of 

 these groups, host specificity has developed to a very high degree, 

 most species of smuts being capable of infecting but a single kind of 

 plant, whereas with the rusts, each species of the parasite requires 

 one or two unrelated host plants. But it is known that there occurs 

 a still higher degree of host specificity, for within the species of the 

 parasite there are found different races or strains, morphologically 

 identical, but capable of infecting only certain varieties or "lines" 

 of the host species. These races or strains of parasitic fungi ap- 

 parently arise by mutation and hybridization. Such divisions of a 

 species, capable of infecting only certain lines of the host species, 

 are known as physiologic races, and the phenomenon is referred to 

 as physiologic specialization. 



Ascomycetes. The Ascomycetes are the largest and perhaps the 

 most important class of the fungi, including such widely different 

 types as the large fleshy morels and the minute one-celled yeasts. 

 Many important plant pathogens are Ascomycetes, and some of the 

 molds important to the bacteriologist also belong to this class. It is 

 characterized by the formation of spores, called ascospores, con- 

 tained within a membrane or sac called an ascus. There are gen- 

 erally eight ascospores in an ascus, but many asci may be formed by 

 one thallus. Although there is a wide diversity in gross characters 

 and external appearances between the various groups of Ascomycetes, 

 the mode of formation of the ascospores is fairly uniform and indi- 

 cates the homogeneity of the class. 



Ascospores. The formation of the ascospores is a result of sexual 

 fusion of nuclei. The mechanism of their production is simplest in 

 the yeasts. Different stages in the formation of the ascospores of 

 a yeast, Schizosaccharomyces, are shown in Fig. 12. Two contiguous 

 yeasts cells send out minute tube-like processes which meet and fuse ; 

 their nuclei come together and unite, the single nucleus resulting 

 undergoes division three times to form eight daughter nuclei. Each 



