MYCOLOGY DURING THE PAST FIFTY YEARS 575 



considered. The pioneer work of Blakeslee fifty years ago gave recognition to 

 the existence of two distinct and separate sexes in unisexual individuals of 

 certain members of the bread-mold group. His discovery of this situation, 

 unexpected but readily understandable and acceptable to human beings of 

 similar pattern, had its stimulating impact on Botany and Biology. The reali- 

 zation of the potentialities of such simple, easily manipulatable material 

 instigated a rapid extension of similar investigations. Continued studies by 

 Blakeslee and his associates in this country and similar research in Europe 

 by such workers as Burgeff and his associates yielded additional and signifi- 

 cant results, but also brought out the fact that the suitability of this material 

 was severely handicapped by the drawback of the multinucleate condition of 

 the sporangial spores and of the zygospores. Therefore, while the impetus 

 continued, the trend of investigation shifted to other groups as more suitable 

 material. 



In the case of other Phycomycetes, but of the biflagellate aquatic line, the 

 pioneer work of Couch, followed later by the work of Bishop, showed that 

 the separate-sexed condition occurred also in some members of this series, 

 and the work of J. R. Raper revealed that this was in reality a condition of 

 relative sexuality with a range showing pure male and pure female strains as 

 the extremes between which occurred intermediate forms of mixed sexuality, 

 with one sex predominantly expressed in various degrees, the other suppressed 

 and latent but capable of expression in appropriate matings. Also, continued 

 work by J. R. Raper convincingly demonstrated that here the four major steps 

 in the sexual process were under the control of distinct, successively produced 

 hormones, the first recognition of such a situation in any of the lower plants. 

 Similarly, in the uni-flagellate series of the aquatic Phycomycetes, the pioneer 

 work of Kniep, and especially the later and still continuing work of Emerson, 

 together with some additional contributions by later investigators such as 

 Harder, Sorgel, and others, established the now classic, full-cycle, diplobiontic 

 alternation of generations in Allomyces and gave fitting prominence to this 

 genus and related forms as highly favorable material for the investigation of 

 such significant features as the variant life cycles, the parthenogenetic develop- 

 ment of unfertilized female gametes, and the significant variations in sex ex- 

 pression. 



In the case of the Ascomycetes, a similar sequence of investigations finally 

 settled the long-continued controversy between Harper's contention that 

 double fertilization occurred and Clausen's that a single fertilization took 

 place, the evidence being preponderantly in favor of the latter interpretation. 

 In this group also, the heterothallic, separate-sexed condition has been found 

 to be a frequent occurrence, and the advantageous arrangement of the eight 

 spores in the ascus has facilitated establishing in which of the three successive 

 nuclear divisions the segregation of sex potentialities occurs, while in other 

 forms has been reported the interesting condition of hermaphroditism simulat- 



