268 PHYSIOLOGIC SPECIALIZATION AND VARIATION 



Savulescu and Ravss (1930) found minor differences in the 

 sporangia of Albugo Candida and used them as a basis to divide it 

 into 8 form species. Togashi and Shibasaki (1934) were able, by- 

 means of a large series of measurements of sporangia, to divide this 

 species into 2 varieties, microspora and macrospora, and then to 

 separate microspora into 3 form species and macrospora into 2 

 form species. 



Leonian (1925) isolated from Phytophthora parasitica rhei 5 

 tvpes of colonies that were so different no one would regard them 

 as members of the same species. For a long time mycologists have 

 placed great emphasis upon the host species as an aid in identifying 

 rusts, smuts, downy mildews, powdery mildews, and other obli- 

 gate parasites. Undoubtedly some so-called species are in reality 

 only form species. The converse may, of course, be found to be 

 equally true, with changes in concepts of what constitutes the 

 species. 



Physico-chemical differences among specialized races. In 

 the light of statements already made, it would seem possible to 

 isolate races that possessed more marked ability than other isolates 

 to produce a given by-product as the result of their metabolic 

 activity. This is true in the case of the groups, baker's yeasts 

 and brewer's yeasts, that have been selected from the complex 

 known as Sac char omyces cerevisiae. Growth of the baker's 

 yeasts is inhibited in wort in which the alcohol content has ac- 

 cumulated to a concentration of 4 to 5 0/ , and of the brewer's 

 yeast, at an alcohol concentration of 14 to 17%. Similarly races 

 of molds, especially of Penicillium, Rhizopus, and Aspergillus, 

 differing in fermentative ability in the formation of oxalic acid, 

 acetic acid, lactic acid, and other products, have been isolated. It 

 would seem that these races are merely selections within the 

 species. This interpretation has direct bearing on the "species 

 concept." The degree of difference requisite in separating species 

 and varieties, and sometimes genera, of funsri is not fixed. Alor- 

 phology is agreed to be the primary basis of specific distinctions. 

 In some genera, such as Botrytis, Fusarium, and Phytophthora, 

 morphologic differences are either minute or non-existent and 

 hence a source of confusion. Physiologic differences amongr 

 them are, therefore, employed as a convenient basis for specific 

 taxonomic units. If physiologic differences were employed 

 among rusts, precise means of cleaving species exist that are more 



