BLAKESLEE. — SEXUAL REPRODUCTION IX THE MUCORINEAE. 309 



with Mucor v (+) and Mucor v (— ). Broken lines represent areas 

 of hybridization and dotted lines areas of contact between the different 

 mycelia where no sexual response was apparent. It will be seen that 

 lines of hybridization have occurred between the ( + ) and ( — ) strains 

 of the two different species, but no zygospores have formed between 

 those of Mucor iv. The strains of Mucor v seem to be characterized 

 by an unusually marked activity in contrast to those of Mucor iv, which 

 in this instance seem to play a comparatively passive part in the hybrid- 

 ization. In the warm oven and on a different substratum favorable to the 

 interaction of the (+) and (— ) strains of Mucor ly a similar arrangement 

 of the same strains has yielded buth zygospores and hybrids. 



Mucor N will not produce zygospores when grown on such concen- 

 trated nutrients as are frequently used for the cultivation of other forms, 

 and for this reason hybridization does not take place on these nutrients. 

 With the (— ) strain of Mucor v the line of hybrids is better devel- 

 oped than with the (+) strain of this species, and with the (— ) sti-ain of 

 Mucor III a line of hybrids is produced, while with its (+) strain no 

 hybrids occur under the conditions present in the single test made. 



Phycomyces nitens is unique among the heterothallic forms under cul- 

 tivation in that its zygophoric hyphae are not necessarily aerial. For 

 this reason it was thought that hybridization would not occur between 

 it and other species with a different habit, and the flrst tests with Mucor 

 Macedo on potato agar by their negative results seemed to confirm this 

 idea. On the substratum employed the two species will form their zy- 

 gospores, but a higher concentration of nutriment than is afforded by 

 such a substratum is more favorable to the production of those of Phy- 

 comyces. A nutrient agar made up with condensed milk has been 

 found advantageous for the growth of this latter species, and was there- 

 fore used in subsequent tests, which gave positive results. 



The response between the opposite strains of the two species when 

 contrasted is shown by a light line at their areas of contact, but the pro- 

 cess of hybridization has never been observed to go so far as a cutting 

 off of gametes. With the (-f ) strain of M. Macedo the response is more 

 marked than with the (— ) strain. The hyphae shown in Plate III, Fig- . 

 ure 51, were taken from the line between the ( — ) strain of Phycomy- 

 ces and the {-(-) strain of M. Macedo. A filament of the latter is seen to 

 be closely embraced by an irregularly lobed branch of Phycomyces and 

 presents the appearance of being enclosed by the haustoria of some para- 

 site. The hyphae of M. Macedo, thus attacked by those of Phycomyces, 

 seem to be but slightly modified themselves. They are more or less 



