BLAKESLEE. — SEXUAL REPROPUCTION IN THE MUCORINEAE. 211 



quently shown to liave no sexual significance. It is true in nearly every 

 heterothaliic species investigated that the strains show certain morpho- 

 logical (lilTerences which correspond to their physiological difEerentiation 

 as sexual strains. In general the difTerence is one in degree rather than 

 in form, and is indicated in several ways. Mucor Mucedo may be taken 

 as an illustration of the simplest typo of such a diflferentiation from the 

 fact that, although the two strains are not distinguished by any apparent 

 morphological difference, the (— ) strain has been shown to lose its sexual 

 capacity sooner than the (+) strain. This difference is further indicated 

 by the fact that, while successive mycelial transfers from young cultures 

 of the (+) strain retain their sexual capacity through indefinite genera- 

 tions, this, with the power of vegetative development, is rapidly lost by 

 the (— ) strain when subjected to a similar treatment. In Phycomyces 

 the differentiation is more distinct in that the sporangiophores are more 

 numerous and less delicate, and develop later in one strain than in the 

 other (on proper nutrients this is more marked than in the culture 

 photographed, Plate IV, Figure 52), while in several species in which a 

 still greater differentiation is exhibited, when the strains are contrasted 

 one will regularly show a lower, less luxuriant, sporangial growth, and 

 often a decided color contrast. The most striking instance of this kind 

 is seen in the species photographed (Plate IV, Figure 58), where the dif- 

 ference in luxuriance of sporangial growth, added to a corresponding dif- 

 ference in the size of the spores (Plate I, Figure 23, 24), might easily 

 lead one in investigating these strains separately to consider them as 

 individual species. 



It is thus evident from even a cursory examination of the known hetero- 

 thaliic species that their sexual strains are with few exceptions character- 

 ized by more or less marked differences, especially in relation to their 

 relative luxuriance of growth, and it was owing to this peculiarity that it 

 was thought appropriate to apply the terms (-f) and (— ) to the more and 

 less luxuriant strains respectively. The assumption that their differen- 

 tiation to (-H) and (— ) strains was indicative of a corresponding sexual 

 difference, and that the strains thus designated sliould be regarded respec- 

 tively as male and female, or vice versa, although naturally suggested by 

 the observed condition, did not at first appear to be warranted. Evi- 

 dence, however, is not lacking which would seem to justify such a con- 

 clusion. Plate IV, Figure 57, for example, shows the strains of Mucor v, 

 forming zygospores at their median line of contact. On either side of 

 these conjugating strains were sown corresponding strains of Mucor N 

 (-I-) at right, (— ) at left. Where the growth of the (-|-) strain of one 



