' BLAKESLEE. — SEXUAL REPRODUCTION IN THE MUCORINEAE. 283 



"VVhetlier they are also zygotactic, with a mutual attraction influenciug 

 their direction of growth, as in M. Mucedo, is at present uncertain. 



Van Tieghem ('73) was of the opinion that a difference, which is 

 usually apparent in the time of origin of the outgrowths from opposite 

 suspensors, imlicates a sexual ditfereutiation. It has been found, however, 

 that this priority in the development of the spines is in some cases on 

 the (+), and in others on the (— ) side, and is probably, therefore, as- 

 sociated with a purely accidental inequality in the amount of nutriment 

 received from the afferent hyphae. 



Although no differences in structure have been observed, the character 

 of growth of the (+) and (— ) strains is distinctly unlike, and it is there- 

 fore possible to distinguish them when they are grown side by side. 

 The mycelium in the ( — ) strain is slightly less vigorous, and the spor- 

 angiophores which arise from it are fewer and later in appearance 

 (Plate IV, Figure 52). Moreover, while there is a great variation in 

 the size of the sporangia and sporangiophores derived from the same 

 strain, yet in the (— ) the more delicate at first predominate, although 

 in a later period of development no great difference may be discernible 

 between the two strains. On certain nutriments the ( — ) strain is further 

 distinguishable by an abundant production of sterile filaments from its 

 mycelium, giving to the growth a characteristic white felted appearance. 



The effect of external conditions on the activity of conjugation in this 

 species has not been carefully investigated. Zygospores have been 

 secured on all the substrata tested, which fact disproves the generally 

 accepted opinion that oily material is necessary for their production. 

 On nutrients with such low concentrations as are available in potato 

 agar, zygospores are not numerous, but if the concentration is rendered 

 higher by the addition of 4 per cent grape sugar, their formation is con- 

 siderably increased. They have been also obtained on fiour paste, milk 

 agar, and potato agar acidulated with orange juice. 



In the warm oven at 26°-28° C, zygospores are produced much less 

 abundantly than at the room temperature. 



Neither the apparent inequality in vegetative vigor between the (-|-) 

 and (— ) strains, nor their sexual capacity has been affected by cultiva- 

 tion through twenty sporangial generations. 



Since the zygospores of this species were first synthetically obtained, 

 transfers have several times been secured of Phycomyces which has 

 appeared in dung cultures from different sources. Of these strains, two 

 have shown a vigorous (+) character, two a weak (-|-) character, and 

 one is apparently neutral. 



