INTRODUCTION 13 



Further investigation is necessary to establish their value. Within the 

 Pyrenomycetes, it has proved desirable to take account of the tissue-like 

 bands upon which the Pseudosphaeriaceae were based, and to employ this 

 character under the term paraphysoid as a generic criterion in Sphaeriales 

 especially. However, as Theissen and Sydow have pointed out, there is 

 every possible gradation between these and true paraphyses. 



While the presence or absence of paraphyses had been employed for a 

 considerable number of genera by Saccardo and others, it remained for 

 Theissen and Sydow to apply it consistently in their several monographs 

 (1915, 1917). This widespread application has been criticized by one or 

 two mycologists, but it seems to be justified by the earlier practice and has 

 been adopted here. The objection that its real significance is unknown may 

 be raised against most criteria. 



The branching of the paraphysis has usually been regarded as a 

 character of generic significance, as well as special modifications of note, 

 and the formation of a definite epithecium by the tips has likewise been 

 employed. 



Genera 



There is a difference of opinion among taxonomists as to whether a 

 genus is an objective entity consisting of a group of species of living organ- 

 isms differing from other groups of species by distinctive and more or less 

 fixed morphological characters, or whether it is primarily a mental concept 

 of the taxonomist which has no real objective existence as a separate group. 

 In the present state of our knowledge, most fungus genera are to be regarded 

 as tentative concepts, still to be verified or modified by further study and 

 comparison of the species involved, in conformity with accepted practice. It 

 has been fairly well demonstrated in some cases, however, that there are 

 groups of species which differ from other related groups by distinct morpho- 

 logical characters. Such groups may vary greatly in the number of their 

 constituent species and in the number and importance of the different char- 

 acters involved. In many cases from lack of adequate material and our 

 imperfect knowledge of the species known, and of those perhaps not yet dis- 

 covered, our generic concepts can not be verified at present. The segrega- 

 tion of genera should therefore in the present state of our knowledge be 

 done with conservatism and caution and serious consideration should be 

 given to the practical as well as the scientific aspects of the subject. The 

 publication of new genera based upon inadequate study, on scanty or imper- 

 fect specimens or cultures or characters of unknown value or stability can 

 only add to the present confusion and result in impeding the progress of 

 systematic mycology. 



It is manifestly impossible for any individual to become critically 

 familiar with the thousands of genera that have been proposed, even if 

 authentic material of all were available. It should be clearly recognized 

 therefore that the present book is largely a compilation based upon a careful 



