ASCOMYCETES 133 



Vaughan (nee Fraser) the nuclear fusion in the young ascus is not the first 

 and only fusion, but is preceded by another fusion in the ascogonium 

 directly after the sexual act. The ascogenous hyphae, according to this 

 conception, do not contain haploid dicaryons but undivided diploid 

 nuclei which only after the formation of the hooks, come together to 

 dicaryons. Because of this double fertilization, this primary ascus nucleus 

 is tetraploid, and, according to this school, contains 2x double chromo- 

 somes. In the first step in division (heterotypic division or meiosis) each 

 daughter nucleus contains 2x simple chromosomes. The second step 

 is homoeotypic, the 2x simple chromosomes are halved so that every 

 daughter nucleus still contains 2x simple chromosomes. In the third step 

 (brachymeiosis) one-half the undivided chromosomes migrate to each 

 pole, so that the daughter nuclei of the third generation contain x simple 

 chromosomes. Although the cytological findings in part contradict each 

 other (Guillermond, 1913) e.g. Claussen (1912) states the haploid number 

 is 12 while his figures never show more than 6 (Tandy, 1927) — and in 

 part may be interpreted to either conception, at present the author 

 prefers the theory of simple fertilization as developed by Dangeard (1907) 

 and Claussen (1912). According to this conception, the life cycle of the 

 Ascomycetes in the ideal case proceeds according to the following scheme : 



Conidia 



I P C R 



' \scoeonia 



-♦Ascogenous Hyphae— >Asci—>Ascospores 



Mycelium^Ascogonia 

 ^Antheridia 



im \ 



Diagram XIII. 



There arise on the haplont, first imperfect forms, then sexual organs 

 (antheridia and ascogonia). Between these sexual organs plasmogamy 

 takes place, whereby each male and female nucleus pair as a dicaryon. 

 These dicaryons migrate into the ascogenous hyphae and divide con- 

 jugately. The ascogenous hypha, thus, caryologically represents a 

 special phase of the diploid phase, the so-called dicaryophase ; to be sure, 

 this is virtually diploid; cytologically it only produces dicaryons from 

 two sexually different haploid nuclei. 



The dicaryophase, and with it the sexual process altogether, ends with 

 nuclear fusion in the young asci (caryogamy). Caryogamy is followed 

 directly by meiosis, usually producing eight haploid ascospores. 



In the higher Ascomycetes, this scheme of development is further 

 complicated, since the haploid thallus proceeds to form fructifications 

 on or in which the ascogenous hyphae complete their development. As 

 in most Florideae and in the sporophyte of the mosses, the dicaryophyte 

 is to a certain extent parasitic on the haplont and nourished by it. 



