232 PROCEEDINGS OF THE AMERICAN ACADEMY. 



peiisors is not obscure, and in consequence the figures of any author, 

 even were they not confirmed by so many others, would be more trust- 

 worthy than in most cases. From a few experiments on this form van 

 Tieghem ('75) thought that he had established the principle that lack 

 of oxygen is the controlling factor in the production of the zygospores. 

 The papers of Klebs and Falck have already been referred to (p. 218). 

 The species is very common in this country, and cultures have been 

 kept running in the laboratory for several years. 

 The species is homothallic. 



Rhizopus nigricans Elirenbcrg. 



In de Bary's ('66) paper on Rhiznj)us nigricans is the first mention o4' 

 the zygospores of a heterothallic form. They were discovered in this 

 species by A. Janowitsch in the Botanical Laboratory at Freiburg. 

 De Bary found zygospore formation occurring during warm weather in 

 May, June, and July, in cultures on fleshy fruits and on bread. On 

 other substrata and in winter only sporangia were observed. In Plate 

 VII, Figure 2, we have the apparently impossible case represented of 

 two conjugating zygophores arising from the same hyplia. 



Van Tieghem ('75) obtained the zygospores by employing de Bary's 

 suffocation theory. A cylindrical vase, previously washed with boiling 

 water, is filled one-half or two-thirds full with fresh bread crumbs. Upon 

 this are scattered a few drops of boiled water into which a sporangium 

 ("un sporange") has been transferred, and the vase is closed. After a 

 dozen days, when the mycelium is sufficiently suffocated by the first vege- 

 tation, zygospores appear in the interior of the bread, and especially 

 between the bread and the walls. Our present knowledge of this species 

 indicates that spores from more than a single sporangium must have 

 gained access to the culture, since the form is typically heterothallic. 



Eidam ('83) found zygospores of this species in winter on " Erdnuss- 

 kuchen." 



Leger ('96, p. 74) tried van Tieghem's method for obtaining zygo- 

 spores, but found only a small number in his cultures. 



It is of interest to know that de Wevre ('92), in working on this 

 species, attempted to get the zygospores by following the method given 

 by de Bary and van Tieghem, and in addition subjected the fungus to all 

 the unfavorable conditions he could devise, but without obtaining other 

 than sporangial spores. He concludes that either the form with which 

 he worked had lost its sexual capacity or else was a dift'erent variety 

 from that which produces zygospores. 



