BLAKESLEE. — SEXUAL REPRODUCTION IN THE MUCORINEAE. 213 



and tlie liyphae of these forms as in many other species differ siiiRciently 

 to be distinguished even without tracin;^ them to their sporangia. Perfect 

 hybrids have never been found, and thougli for convenience the term 

 hybridization will be used hereafter in the present paper, it should be 

 remembered that these attempted imperfect conjugations have never been 

 observed to go further than, at most, a cutting off of the firmly united 

 gametes. 



In other organisms, where the sexes are segregated on separate indi- 

 viduals, the female is generally characterized by a greater vegetative 

 development, but the assumption that the same condition prevails here, 

 and (+) and (— ) strains are equivalent to female and male, respectively, 

 would from our present knowledge hardly seem justifiable. It is hoped, 

 however, that further investigation may demonstrate the correctness of 

 such a conclusion. 



A further indication of the strictly sexual nature of these processes in 

 general is found in the interaction between homothallic forms which may 

 be assumed to be bisexual [(+) and (— )], and (+) and (— ) strains of hete- 

 rothallic types. Plate IV, Figure 56, for example is photographed from 

 a culture in which the (+) and (— ) strains of a heterothallic species, 

 iMucor V, have been sown on opposite sides of a homothallic species, 

 Mucor I. Had a (+) or (— ) strain of a heterothallic species been sown 

 in place of Mucor i, conjugation would have taken place on one side only, 

 namely, on that side where (+) and (— ) strains were opposed. In the 

 present instance, however, the middle strain having both (+) and (— ) 

 qualities, conjugates with the ( + ) heterothallic strain on the one hand, 

 and with the (— ) strain on the other, as is indicated by the evident white 

 lines of demarcation between them. The response here demonstrated 

 between a homothallic form and both (+) and (— ) heterothallic strains 

 which have been shown to be unisexual, can thus be explained only on 

 the assumption that the homothallic Mucor i possesses both sex qualities. 



It may be mentioned in this connection also that a number of species 

 the zygospores of which are unknown have been made to produce im- 

 perfect hybrids with the (+) and (— ) strains under cultivation. Thus 

 individual strains of Syncephalastrum, a new species of Chaetocladium, 

 and a number of other forms, have been found to be (— ) from the fact 

 that they interact only with (+) strains, while a culture of Cunningfiam- 

 ella echinulala Thaxter (C. Africana Matruchot), which was formerly 

 classed as a Hyphomycete, has been shown to be (+). From our present 

 knowledge one is justified in assuming that a culture which responds sexu- 

 ally to only one of the test strains must belong to the heterothallic group, 



