June 3, 1904.] 



SCIENCE. 



865 



somewhat extended investigation on the meth- 

 od of reproduction in one group of the com- 

 mon molds, and since many of the facts 

 which have been discovered are at variance 

 with the conclusions of other investigators, 

 and since the problems involved have a gen- 

 eral biological interest, it has seemed desirable 

 to publish the following preliminary summary 

 of the more important results obtained. 



Among the Mucorineee, as is well known, 

 the usual form of reproduction is by means 

 of non-sexual spores in sporangia, while the 

 sexual method by means of zygospores is un- 

 known in the gi'eat majority of species, and 

 even where it has been reported our knowledge 

 of its occurrence in about four fifths of the 

 cases is based on the recorded observations of 

 single individuals. For over thirty years the 

 phenomena of reproduction by zygospores in 

 these plants have been an object of consider- 

 able investigation among students of fungi, 

 and as a result a number of conflicting the- 

 ories have arisen as to the significance of the 

 process, and the conditions by which it is 

 induced. Such conclusions as have been 

 reached have in general been based on the as- 

 sumption that external conditions of one kind 

 or another were the essential factors concerned, 

 and, while the process has been generally re- r 

 garded as a primitive type of sexual reproduc- 

 tion, some investigators have denied that any 

 sexuality is involved in zygospore formation. 



In the experimental investigations made by 

 the present writer in order to determine the 

 conditions associated with zygospore produc- 

 tion in more than a dozen different species, 

 results have been obtained which may be sum- 

 marized as follows : 



Zygospore production in the Mucorinese is 

 conditioned by the inherent nature of the in- 

 dividual species and only secondarily or not 

 at all by external factors. 



According to their method of zygospore 

 formation, the various species among the 

 Mucorinese may be divided into two main cate- 

 gories, which may be designated as homo- 

 thallic and heterothallic, and which corre- 

 spond respectively to monoecious and dioecious 

 forms among the higher plants. 



In the bomothallic group, zygospores are 



developed from branches of the same thallus 

 or mycelium and can be obtained from the 

 sowing of a single spore. Although it has 

 been currently assumed that all mucors belong 

 to this class, it comprises but a very small 

 percentage of the species and contains the 

 only forms from which heretofore it has 

 proved possible to obtain a constant produc- 

 tion of zygospores. Sporodinia grandis, the 

 only common species, is very frequent on de- 

 caying agarics, etc., and has served as a basis 

 for experimentation in a majority of the in- 

 vestigations dealing with this subject. 



In the heterothallic group, comprising a 

 large majority of the species, zygospores are 

 developed from branches which necessarily be- 

 long to thalli or mycelia diverse in character, 

 and can never be obtained from the sowing of 

 a single spore. Every heterothallic species is, 

 therefore, an aggregate of two distinct strains 

 through the interaction of which zygosporic 

 reproduction is brought about. If inocula- 

 tions of these two opposite strains of a given 

 species are so disposed that their mycelia can 

 grow together, there will be developed, at the 

 region of contact, a distinct dark line pro- 

 duced by the accumulation of zygospores 

 formed between filaments of the opposite 

 strains. Bhizopus nigricans, the common 

 bread mold which is used by nearly every 

 elementary class in cryptogamic botany, may 

 be taken as the type of this group. An acci- 

 dental mixture of its two strains has been 

 kept under cultivation for nearly ten years 

 and as the ' Harvard strain ' has furnished 

 zygospores for class work to many botanical 

 laboratories in this country. 



In an individual species these sexual strains 

 show in general a more or less marked differen- 

 tiation in vegetative luxuriance, and the more 

 and less luxuriant may be appropriately desig- 

 nated by the use of (-{-) and ( — ) signs re- 

 spectively. In a few forms, no differentiation 

 has been as yet detected; in others, one strain 

 shows a less vegetative vigor when cultivated 

 under unfavorable conditions; in the majority, 

 however, the differentiation is evident from 

 the marked difference in the gross appearance 

 in cultures of the two opposite strains ; and in 

 one form, not only the habit of growth, but 



