92 ALBERT FRANCIS BLAKESLEE. 



DICECIOUS SPECIES. 



No dioecious form of the Mucors is known to be regularly 

 heterogamic. In Rhizopus ('04^, Fig. 15) and in other dioecious 

 species the writer has found that the inequality in size between 

 conjugating gametes has no necessary relation to sex, since the 

 larger gamete is formed sometimes by one sex, and sometimes 

 by the other. In many species, however, the ( + ) race regularly 

 shows a decidedly greater vegetative vigor than its ( ) mate. 

 This greater vigor of growth may be responsible for the fact that 

 the conjugative filaments arising from the (+) mycelium average 

 stouter than those from the ( ), and this difference may be 

 accompanied by a similar difference in the gametes and suspen- 

 sors. One was generally able to recognize the ( + ) from the ( ) 

 sides of the line between the two sexes of Mucor V. by this 

 greater vigor of the (+) conjugative filaments but even in this 

 form where the distinction is most marked, the stouter of the 

 conjugating filaments is not invariably on the ( + ) side. Mucor 

 V. is a form found by the writer ('046) in 1904. It is apparently 

 not specifically distinct from M. hiemalis Wehmer, the sexual 

 races of which were isolated by Hagem ('08) and forms zygo- 

 spores, though in no great abundance, with the strains of this 

 name sent from the "Centralstelle" at Amsterdam. It differed 

 from the Amsterdam material, however, in its much greater 

 sexual vigor. In 1912 when the reactions described in the 

 present paper were investigated, the Amsterdam M. hiemalis 

 failed to show any reactions with other species tested, while 

 Mucor V. was sexually the most active of all the Mucors known. 

 At the present writing, however, Mucor V. has apparently 

 become weakened in sexual activity. Hagem ('08) reports a 

 similar loss of sexual activity in one of his strains of M. hiemalis. 



REACTIONS BETWEEN DICECIOUS SPECIES AND ABSIDIA 



SPINOSA. 



The difficulties in technique involved in following the sexual 

 reactions in a thicket of filaments have been overcome by growing 

 the heterogamic hermaphrodite ( 8 ) in a Petri dish between the 

 (+) and ( ) test strains and cutting channels in the nutrient 

 agar between the different growths. If the Petri dish be 



