314 



SCIENCE 



[N. S. Vol. XXXIX. No. 1000 



eluded, as de Bary had done, that there is 

 no fertilization in these forms. Not until 

 the work of Trow (1904) and Claussen 

 (1908) was it proven that the antheridium 

 ctf these water molds is sometimes func- 

 tional, and not always vestigial as de Bary 

 (1881) and Humphrey had thought. 



The earliest cytological work on the 

 ascomycetes, after the detection of their 

 nuclei by Schmitz, was that of Dangeard 

 (1894). He described and figured a fusion 

 of two nuclei in the ascus of Exoascus, of 

 Peziza, of the truffle and others. The 

 source of the two fusing nuclei Dangeard 

 did not trace back farther than the sub- 

 terminal cell of a hooked hypha from which 

 the ascus arises in Peziza and others. The 

 ascus with its fusion nucleus he regarded 

 AS an oospore. 



In 1895 there was announced from Stras- 

 ^urger's famous laboratory at Bonn a dis- 

 'covery which seemed at one stroke to settle 

 the dispute between de Bary and Brefeld, 

 ■and to definitely demonstrate the occur- 

 rence of a sexual nuclear fusion in the 

 sexual organs seen by de Bary. In that 

 year Harper showed that out of the opened 

 antheridial tube of the hop mildew, Sphw- 

 rotheca, a male nucleus passes into the 

 oogonium and fuses with its nucleus. The 

 whole behavior of antheridium and oogo- 

 nium and their contents had every aspect 

 of a real sexual process, as de Bary had 

 asserted in 1863. What made Harper's 

 discovery still more significant was the 

 determination of the fate of the fusion 

 nucleus in relation to the nuclei of the 

 ascus and spores. Harper found that one 

 of the row of 5 or 6 cells resulting from 

 the division of the fertilized oogonium has 

 two nuclei. These two descendants of the 

 diploid nucleus, formed at fertilization, 

 afterward fuse, and the cell containing 

 them swells to form the single ascus of this 

 species. This, presumably tetraploid, fu- 



sion nucleus of the ascus then grows and 

 divides three times to give the eight spore 

 nuclei. In the following year or two 

 Harper (1896-97) demonstrated a sexual 

 fusion of the same type at the initiation of 

 the fruits of another mildew, Erysibe, 

 and of the saucer fungus, Ascobolus. The 

 numerous asci of these forms all arise from 

 binucleate branches of the binucleate, sub- 

 terminal cell of the fertilized oogonium. 

 Each ascus is at first binucleate, but later, 

 as had been seen by Dangeard (1894), the 

 two fuse and then by division the eight 

 spore nuclei are formed as in Sphcerotheca. 



In the course of the following decade 

 Harper reported the occurrence of two nu- 

 clear fusions, like those of Sphcerotheca, 

 and at the same points in the life cycle, in 

 the mildews Erysibe (1896) and Phyllac- 

 iinia (1905), and in the saucer fungus 

 Pyronema (1900). Moreover, he found in 

 Phyllactinia a synapsis and evidences of a 

 double reduction of the chromosome num- 

 ber in the divisions of the presumably 

 tetraploid, fusion nucleus of the ascus. 

 Pyronema proved interesting also in 

 having multinucleate gametes, such as were 

 at this time being studied by Stevens in the 

 white rust, Albugo. Harper believed that 

 many pairs of male and female nuclei fuse 

 in the oogonium of Pyronema. 



As the outcome of this whole series of 

 studies by Harper it seemed clear that 

 there is in many ascomycetes an alterna- 

 tion of a haploid generation, the vegetative 

 mycelium, with a diploid generation, the 

 fertilized oogonium and the ascus-forming 

 hyphffi arising from it. The second fusion, 

 in the ascus, was regarded as a nutritive 

 phenomenon to provide a nucleus adequate 

 in size for the organization of the relatively 

 huge ascus. 



At the opening of the century the obser- 

 vations of a number of workers on the 

 simpler ascomycetes, e. g., by Juel (1902), 



