292 INTRODUCTION TO CYTOLOGY 



In certain yeasts it has been shown (see Guilliermond 1920) that the 

 production of ascospores is preceded by a copulation of two cells with a 

 fusion of their nuclei, the fusion nucleus dividing to form the spore nuclei. 

 A somewhat similar copulation of the ascospores themselves has also 

 been observed in a few cases. 



Among the Basidiomycetes the nuclear phenomena are best known 

 in the case of the rusts, owing to the researches of Blackman (1904), 

 Christman (1905), and a number of later writers. In the typical rust 

 life cycle there is a fusion of uninucleate cells at the base of the aecial 

 sorus (Fig. 116, B). The binucleate cells thus arising produce the binu- 

 cleate aeciospores; and these upon germination form a mycelium with 

 binucleate cells, the two nuclei dividing in unison ("conjugately") at 

 each cell-division. After producing a series of crops of binucleate 

 uredospores this mycelium eventually bears teliospores which may con- 

 sist of one or more cells. In each cell of the teliospore the two nuclei 

 delivered to it as the result of the conjugate divisions throughout the 

 binucleate mycelium finally unite, initiating the uninucleate phase of 

 the life cycle. Here the fusion of sexual cells and the fusion of their 

 nnclei — two events which in most organisms occur very near each other 

 in time — are widely separated in the cycle. The two nuclei dividing 

 conjugately constitute together a synkaryon in many respects equivaleng 

 to a diploid nucleus. Since there is as yet no evidence to show in what 

 degree the two effects of fertilization (the stimulus to development and the 

 mixing of hereditary lines) are brought about in the rusts by the fusion 

 of the sexual cells on the one hand and by the final union of their nuclei 

 on the other, it seems best to regard the two fusions as two phases of the 

 fertilization process in spite of their wide separation in the life history. 



In the Hymenomycetes it has been known for some time that a fusion 

 of two nuclei occurs in the basidium, itself the terminal cell of a binucleate 

 hypha, prior to the formation of the four basidiospore nuclei (Fig. 79). 

 The origin of the binucleate condition in the mycelium which has ap- 

 parently arisen from a uninucleate spore has long been an obscure point. 

 It has recently been shown by Miss Bensaude (1918) in the case of 

 Coprinus fimetarius that the binucleate hyphse arise as the result of cell 

 fusions ("plasmogamy;" "pseudogamy") between uninucleate hyphse 

 arising from different spores, and that no carpophores are produced upon 

 a uninucleate mycelium arising from a single spore. Thus it appears 

 that in at least some hymenomycetes the sexual process is initiated by a 

 fusion of two cells of different strains ("plus" and "minus"), as in the 

 heterothallic molds. 



Bryophytes and Pteridophytes. — In bryophytes and pteridophytes 

 the details of the union of the motile spermatozoid with the egg in the 

 archegonium have been described in very few cases. In the former 

 group may be cited the works of Garber (1904) and Black (1913) on 



