n] 



ASCOMYCETES 



47 



four. Further the adhesion of chromosomes already described for Phyllac- 

 tinia must not be forgotten. 



The occurrence of paired nuclei in the ascogenous hyphae was thus the 

 most important evidence in favour of Claussen's view until in 1916 Welsford 

 showed that the nuclei even of gametophytic, multinucleate hyphae are 

 habitually paired if rapid growth and division are taking place ; this is due 

 to the fact that mitoses follow 

 one another so rapidly that the 

 daughter nuclei of any parti- 

 cular division have not time to 

 move apart, before they them- 

 selves divide. Such paired nu- 

 clei have often previously been 

 figured though without attract- 

 ing special attention ; excellent 

 examples are to be seen in 

 Nichols' paper on the Pyreno- 

 mycetes' or in Ramlow's more 

 recent work on Ascophanus car- 

 neus"^. It seems hardly possible 

 to place a different interpreta- 

 tion on the nuclei which lie 

 close together in the ascoge- 

 nous hyphae. 



According to our present 

 knowledge of the cytology of 

 the Ascomycetes there are two 

 nuclear fusions in the life- 

 history of these plants. 



The Significance of the Fusion in the Ascus. If this be the case it 

 remains to consider the significance of the fusion in the ascus. The presence 

 of more than one nucleus in this cell, destined to be one of the largest in 

 the life-cycle of the fungus, is hardly surprising especially in coenocytic 

 forms. In uninucleate species it forms part, as Harper pointed out in 1905, 

 ■of the quantitative adjustment frequently observed between cytoplasm and 

 nuclear material. But this nucleo-cytoplasmic relation does not explain why 

 fusion should take place between the nuclei concerned or why they should 

 be regularly two in number^ It is possible that, crowded as they are in the 

 ■newly constituted ascus, the nuclei merely flow together as they make ready 

 for the prophases of division. Whatever may have been the determining 



1 Bot. Gas. 1896, xxii, pi. xv, fig. 32. ^ Myc. Centralbl. 1915, pi. i, fig. 12. 



^ In Humaria rutilans, however, and doubtless in other forms, young trinucleate, and quadri- 

 mucleate asci are found. 



Fig. 16. Ascophanus carneus PeTS,; germinating spores 

 with paired nuclei in the germ-tubes, x 450 ; after 

 Ramlow. 



