BLAKESLEE. — SEXUAL REPRODUCTION IN THE MUCORINEAE. 287 



been called (— ). The separate (+) and ( — ) strains have now been 

 carried to fourteen sporangial generations, and neither the vegetative 

 difference between them nor their individual sexual character has shown 

 any alteration. 



Plate IV, Figure 58, is a photograph from an agar culture in which 

 the two opposed strains were in the process of forming zygospores at 

 their line of contact. The growth of the (+) strain on the left is 

 much taller, looser, and lighter in general appearance than the other, 

 while that of the (— ) on the right is low and dark. Although, when 

 the two strains are grown side by side, they are always distinguishable 

 by their vegetative appearance, the difference is not equally mai-ked 

 under all conditions. A microscopic examination shows that this diver- 

 sity in aspect between cultures of the two strains is correlated with a 

 corresponding and striking difference in size of the vegetative repro- 

 ductive bodies, the smaller spores being formed by the ( — ) strain. 

 Plate I, Figures 23 and 24, are camera drawings representing the 

 normal variations in size and shape of the spores taken from (-f ) and 

 (— ) potato agar tubes which had been kept under the same conditions 

 of cultivation. It is here seen that the spores produced by the low 

 growth of the (— ) strain are smaller than those produced by the more 

 luxuriant growth of the (+) strain. The difference in vegetative luxuri- 

 ance between the (-|-) and (— ) strains of Mucor iii is greater than 

 between those of any other form whose strains have yet been separated, 

 and coupled with the difference in spore characters would almost cer- 

 tainly have been considered by systematists as of specific value. 



Mucor iv. 



The zygospores of this heterothallic species were found in the fall of 

 1902 in an old tube culture of potato agar into which the pi'evious summer 

 at Cold Spring Harbor, L. I., the writer had made a transfer of one of 

 the Cephalideae together with a weft of Mucor hyphae to furnish a host 

 for its development. 



Separation cultures on potato agar, in the attempt to resolve this 

 species into its sexual strains, failed to produce zygospores in any of 

 the dilutions. The (-|-) and (— ) strains were finally separated, however, 

 by making thirteen contrasts on horse dung agar, from pure sporangial 

 transfers taken from a gross dung culture in which zygospores were 

 abundant. Inoculations taken from either side of the single scanty line 

 of zygospores that resulted formed the starting points for a sporangial 

 series which has already been carried to the fourteenth generations 



