A critical study of certain species of Mucor* 
ALFRED H. W. PovaH 
(WITH PLATES 17-20) . 
I. INTRODUCTION 
4 
The Mucorales have been widely investigated in Europe for the 
past half century; consequently much has been written about 
them and many have been described. In this country, however, 
such has not been the case; for, with the exception of several 
papers on some of the rarer genera, Blakeslee’s (1904) important 
publications on the sexual reproduction of the group comprise 
the only American contributions to a knowledge of these most 
interesting fungi. The Mucorales include a large number of 
forms, exhibiting considerable variation; in fact, the individual 
plants are quite plastic, so that we may obtain striking differences 
within the species under different external conditions, such as 
medium, light, temperature and moisture. It is, therefore, ex- 
tremely desirable, when dealing with descriptions of species in this 
group, to have full data on the most important growth conditions. 
Thus far this fundamental fact has been under-emphasized in the 
work on the mucors, especially on the taxonomic side. 
The object of the present work is an attempt to standardize 
and unify the genus Mucor with reference to the morphology of 
its individual species and their cultural reactions. The lack of a 
general knowledge of the physiological attributes of the many 
species which have hitherto been described makes it desirable to 
give these forms a detailed comparative study in order that some 
fundamental standard for their future classification may be ob- 
tained. 
The investigations, upon which this paper is based, have been 
carried on during the years 1913, 1914, and 1915 in the Cryptogamic 
Laboratory of the University of Michigan under the direction of 
* Contribution No. 159 from the Botanical Department of the University of 
Michigan. 
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