BLAKESLEE. SEXUAL REPRODUCTION IN THE MUCORINEAE. 311 



iu regard to the true position of the fungus appears to be thus sub- 

 stantiated.* 



The species which we have been just discussing are either known or 

 supposed to be heterothallic. An extended investigation of the recip- 

 rocal action of different species with the homotliallic forms will be 

 reserved for a later research. It has been determined, however, that, as 

 was to be expected in the light of the present research, one at least from 

 this group responds sexually to the (+) and (— ) strains of heterothallic 

 species. A demonstration of this nature is shown in Plate IV, Figure 56, 

 which has been already described somewhat in detail in the Introduction. 

 It is a photograph of a culture in which a homothallic species, ]\Iucor i, 

 was sown between the (+) and (— ) strains respectively of Mucor v. 

 The growth of Mucor i appears dark iu the photograph on account of 

 the abundance of zygospores produced from its mycelium. At the areas 

 of contact between the growth of the homothallic species and that of 

 either strain of the heterothallic species, a white line is visible, indicat- 

 ing the presence of hybrids. That hybridization thus occurs with both 

 (-h) and ( — ) strains, which may be assumed to be unisexual, shows that 

 the thallus of the homothallic form investigated 's bisexual, inasmuch as 

 it produces hyphae which f-rm progametes as a result of the stimulus of 

 contact with both the sexually opposite strains tested. 



When this same homothallic form is contrasted between the (-f) and 

 (— ) strains of Mucor N, the lines of hybridization are less well marked 

 and unequally distinct, that between Mucor x (+) and the homothallic 

 species being much more evident. A similar but slighter difference 

 between the lines of hybridization with the species previously mentioned 

 can be seen in the photograph. It is also here apparent that the line 

 on the side of the (-f ) strain is better developed. A like difference in 

 the lines of hybridization has been already discussed under the hybridiza- 

 tion of heterothallic forms. The most natural conclusion from these 

 facts seems to be that the (+) and (— ) characters in the bisexual 

 mycelium of the homothallic species are equally active, and such an 

 assumption would force us to regard the (+) and (— ) strains of Mucor v 

 aud Mucor n as unequal in sexual vigor with the (+) the more active- 

 These are the only two species which have been contrasted with a homo- 

 thallic form, and therefore it is at present imprudent to state more than 

 that it seems strongly probable that the difference in question is cor- 



* Since this paper was in press the (— ) strain of tiijs species has also been ob- 

 tained, and contrasts of the two yielded abundant zygospores July 27. 



