GENERAL MORPHOLOGY OF FUNGI 23 



pairing of nuclei within a single multinucleate cell of the ascogonium. This 

 automictic fertilization within a cell is called autogamy. 



From these forms in which the sexual organs (or in any case the female 

 organ) are apparently typical in form but no longer functional and serving 

 only as a site of automictic processes, there is a series of intermediate stages 

 to the other extreme where the sexual organs disappear entirely, the sexual 

 processes occurring in the mycelium between any two sexually differentiated 

 cells. The latter process is called pseudomixis. Since the copulating cells 

 are not morphologically distinguishable from other vegetative cells and since 

 only the release of specific developmental stimuli marks this anastomosis of 

 two vegetative cells as a sexual process, pseudogamy is often difficult to dis- 

 tinguish from the usual pseudosexual anastomoses which are brought about 

 by food relations. Its true character is recognizable cytologically only in the 

 pairing of nuclei. If pseudogamy occurs between two sprout cells, tliej^ are 

 sometimes wrongly called gametes. In order to avoid misunderstanding, this 

 term should be reserved for merogamous gametes. The ambiguous term 

 pedogfamy, often employed in other senses, should be used to indicate pseu- 

 dogamy between adult and young cells. The special case of pseudogamy 

 between mother and daughter cell is called adelphogamy. 



Apomixis, the entire loss of fertilization, represents the last step in this 

 series of reduction of sexuality where growth from reproductive cells occurs 

 vegetatively without cell or nuclear fusion, or any external stimulus of de- 

 velopment. If the new individual (in the absence of fertilization) arises from 

 haploid sexual cells, the process is called parthenog'enesis ; if they arise (in 

 the absence of meiosis) from diploid sexual cells, the process is called apogamy. 



In the study of fungi, there is the further difficulty that the original 

 processes of fertilization are replaced by all sorts of substitutes. Among the 

 lower fungi, there is simple fertilization when a fusion of the cytoplasm of 

 two sexual cells (plasmogamy) is immediately followed by a fusion of both 

 haploid nuclei into a diploid zygote nucleus, a syncaryon (caryog-amy). In 

 most fungi, however, caryogamy is delayed and is only completed just before 

 meiosis. Thus the sexual haploid nuclei, while remaining spatially separate, 

 unite only to form a dicaryon, where the paired nuclei divide synchronously 

 (conjugately) while retaining the same ability to activate somatic develop- 

 ment as after complete caryog*amy. This phenomenon is analogous to that 

 in Cyclops, in which the parent chromosomes remain distinct up to the time 

 of egg formation (synapsis) although they are surrounded by the same nuclear 

 membrane, whereas in the fungi they remain Avithin their original nuclear 

 membranes. 



In this retardation of caryogamy, the binucleate "zygote" continues its 

 growth without completing nuclear fusion, developing a new mycelium whose 

 cells, morphologically virtually diploid, contain two sexually differentiated 

 haploid nuclei. This new phase, intruded between plasmogamy and caryog- 

 amy, is called the binucleate phase. In the Ascomycetes, this phase is mostly 

 limited to the ascogenous hyphae and the hymenium of the fructification, but 

 in the Basidiomycetes, it is usually the most conspicuous phase of the life cycle. 



