THE FORMATION OF HYPHAL FUSIONS 47 



influence on growth. Thus Gerassimoff, 1 on artificially increasing 

 the size of the nucleus in Spirogyra filaments, observed that the 

 cytosome increased in size correspondingly, so that the normal 

 filaments developed into thicker ' giant ' filaments. These experi- 

 ments, as Wilson 2 has remarked, " seem to afford decisive proof that 

 the nucleus is the primary agent in the constructive processes of 

 cytoplasmic growth." It has also been shown that enucleated 

 fragments of Amoeba, Polystomella, Stylonychia, and other 

 Protista, as well as enucleated fragments of cells of Higher Plants 

 (Cucurbita, etc.) are incapable of further growth and live only for 

 a relatively short space of time. 3 Haberlandt 4 emphasised the 

 fact that local growth in a cell-wall of a Higher Plant is always 

 preceded by a movement of the nucleus to the point of growth. 

 It appears that growth is associated with " an extensive interchange 

 of materials between nucleus and cytoplasm, so that a certain 

 proportion of nuclear and cytoplasmic substances (the ' nucleo- 

 plasm^ ratio ') must be maintained " if growth is to continue ; 

 and that a single spherical nucleus, through the surface of which 

 the interchanges occur, has a strictly limited ' sphere of influence ' 

 so far as the cytoplasm is concerned. 5 



From our general knowledge of the effect of the nucleus on growth, 

 in part based on the facts just set forth, we should be justified in 

 assuming that, in the Hymenomycetes generally, just as in the 

 Higher Plants and in Animals, the nucleus interacts with the 

 cytosome and thus affects the growth of the hyphae which make up 

 the thallus. In one particular Hymenomycete, Pholiota mutabilis. 



be seen as just having been formed across the main hypha. A minute later, the 

 second septum — that across the base of the hook — came into view. Six minutes 

 later a peg was pushed out from the main hypha opposite the end of the hook, and 

 after about two minutes more the hook and peg fused completely. The hook- 

 septum is always formed a little later than the septum across the main hypha. 



1 J. J. Gerassimoff, " Ueber den Einfluss des Kerns auf das Wachsthum der 

 Zelle," Bull. Soc. Imp. Natur., Moscow, 1901, pp. 185-220. For Gerassimoff's other 

 papers, vide E. B. Wilson, The Cell, p. 1164. 



2 E. B. Wilson, The Cell in Development and Heredity, New York, third edition, 

 1 925, p. 655. 



3 For the literature on this subject vide K. B. Wilson, he. cit., pp. 657-662. 



4 ( !. Haberlandt, Pftysiologische Pflanzenanatom ie, Aufl. 6, 1924, Leipzig, pp. 24-25. 

 ■' L. W. Sharp, An Introduction to Cytology, New York, 1926, pp. 53-54. 



