HOMOTHALLISM AND HETEROTHALLISM 9 



only in some particular habitat, or in the presence of some environ- 

 mental factor which is not at all necessary for asexual reproduction. 



Sexual spores result from the fusion of two nuclei containing chro- 

 mosomes in the haploid, or Ix number. The fusion of the gametes, 

 followed by a fusion of the nuclei, leads to the diploid, or 2x chromo- 

 some number. Immediately after fusion of cells or at some later 

 stage in the life history of the organism, a reduction division leads 

 to a segregation of the "sexes" and to the haploid state of the nucleus. 

 In all this the fungi behave in many respects like other plants and 

 animals that reproduce sexually. Some of the fungi, however, ex- 

 hibit certain special features. 



In some cases fusion of the gametangia, that is, the structures 

 which contain the gametes, is not followed by an immediate fusion 

 of the nuclei. From the fused cells there may arise an extensive 

 mycelium wdth binucleate cells. The paired nuclei divide simulta- 

 neously and one nucleus from each parent remains in the old cell 

 and one of each goes into the new cell. This is known as conjugate 

 nuclear division. In some cases the pairs of nuclei are not separated 

 by septa until just before maturity. The nuclei, however, continue 

 to divide separately and are found in pairs, each nucleus of the pair 

 being of opposite "sex" to the other. The mycelium with paired 

 nuclei of opposite "sex" which divides conjugately is known as the 

 dikaryon or is described as dikaryotic. A mycelium such as fused to 

 produce this dikaryon is often called the haploid mycelium. A hap- 

 loid mycelium consists of uninucleate or multinucleate cells, all nuclei 

 in the mycelium being haploid and identical genetically. All the 

 nuclei were derived ultimately from the same uninucleate haploid 

 spore. This dikaryon may give rise to binucleate spores which, when 

 they germinate on the proper substrate, give rise to mycelium wdth 

 paired nuclei again. Eventually the nuclei fuse in special organs 

 and this fusion is followed immediately or after a period of dormancy 

 by a reduction division. In many yeasts this diploid cell proliferates 

 for a time before reduction division. Tracing the nuclear fusions 

 and segregations, the diploid or the dikaryotic and the haploid stages 

 of the fungus through its life history, has become an important part 

 of modern mycology. It would take us too far afield to discuss this 

 in detail. The reader is referred to monographs of Gaumann and 

 of Kniep and to current botanical journals for further information. 



Homothallism and Heterothallism. In some fungi the gametes 

 may arise from the same thallus or plant mass and are said to be 

 homothallic. In many cases the cells which fuse must be derived 

 from separate thalli, usually not of the same sex, and the process is 



