July 27, 1906.] 



SCIENCE. 



119 



(+) and ( — ). The rarity of the zygospores 

 of heterothallic species therefore may be ex- 

 plained by assuming that when zygospores 

 fail to form under conditions which of them- 

 selves are not unfavorable to zygospore forma- 

 tion, only one of the two sexual strains is 

 present. 



Ever since de Bary' discovered the zygo- 

 spores of Rhizopus in 1865 — now forty years 

 ago — various and conflicting theories, based 

 many of them upon the character of the 

 substratum upon which the zygospores were 

 accidentally found, have been brought for- 

 ward to account for the rarity of their occur- 

 rence. The writer has attempted to show 

 the insufficiency of the assumption that ex- 

 ternal conditions are of more than secondary 

 importance, and has given at the same time 

 a rather full account of cultural experiments 

 with the strains of Bhizopus.^ It is not the 

 purpose of the . present article to repeat the 

 details there given. It seems not inappro- 

 priate on this occasion, however, to say a few 

 words regarding the occurrence in nature of 

 the strains of this species based upon recent 

 investigations as well as upon those already 

 published. For the sake of comparison, a 

 list of those species the thallic condition of 

 which has been determined, will be given ar- 

 ranged according to the type of their sexual 

 reproduction, and something will be said con- 

 cerning the cultivation of Rhizopus for class 

 purposes. 



In Table I. are listed the zygosporic cul- 

 tures from which the sexual strains have been 

 isolated by the writer. In making the separa- 

 tions, a large number of cultures were neces- 

 sary as indicated in the table. Tests with 

 both the standard (+) and ( — ) strains were 

 not made for all the cultures listed, but no 

 strains were found which alone in pure cul- 

 tures could be brought to zygospore formation. 



In the preparation of Table II., the indi- 

 vidual strains obtained through the kindness 

 of the writer's correspondents were tested in 



^ ' Beitrage zur Morphologie und Physiologic der 

 Pilze,' II., 1866. 



^ ' Sexual Reproduction in Mucorinese,' Proc. Am. 

 Acad., 40: 205-319, pis. 1-4. 1904. 



separate cultures against standard (+) and 

 ( — ) strains. Each strain therefore listed as 

 (+) or ( — ) produced zygospores in contrast 

 with the test strain of opposite sign and failed 

 to produce them in cultures with the test 

 strain of the same sign. Those strains which 

 would form zygospores with neither the (-[-) 

 nor the ( — ) test strain on the substratum 

 used were provisionally listed as neutral. 

 Judging from the possibility of inducing tem- 

 porary neutrality in the strains of Mucor 

 Mucedo by cultivation under unfavorable 

 temperatures, one might suppose the neutrality 

 in these strains of Rhizopus to be likewise 

 but a temporary condition. With their ex- 

 ception, all of the 43 individual strains tested 

 are either (+) or (— ). to the 11(+) and 

 19 ( — ) strains in Table II., should be added 

 the 8(^) and 8( — ) strains isolated from the 

 zygosporic cultures of Table I. We shall 

 then have from the strains tested 19(-|-), 

 27 ( — ) and 13 apparently neutral. Out of 

 this total of 58 individual strains, none have 

 been found which will yield zygospores when 

 grown alone in pure cultures. As may be 

 seen by a glance at the tables, strains have 

 been collected from various parts of the world, 

 and the conclusion seems irresistible that the 

 common bread mould, Rhizopus nigricans, is 

 heterothallic and that the interaction of the 

 two sexually opposite strains is necessary to 

 the formation of their zygospores. 



TABLE I. STRAINS IN SPONTANEOUS ZTGOSPOEIC 

 CULTUBES. 



