224 



INTRODUCTION TO CYTOLOGY 



on Hydnobolites, Neotiella, and Laboulbenia, and that of Claussen (1912) 

 on Pyronema. Furthermore, it is becoming increasingly apparent (see p. 

 291) that there is but one fusion in the life cycle — that in the ascus, so 

 that the necessity for a second reduction is removed. 



In the basidiomycetes it has been shown by the researches of Juel 

 (1898), Maire (1905), Guilliermond (1910), Kniep (1911, 1913), Levine 

 (1913), and others on the hymenomycetes, and by those of V. H. Black- 

 man (1904), Dietel (1911), Fitzpatrick (1918), and others on the rusts, 1 

 that reduction occurs in the two mitoses giving rise to the four basidio- 



Fig. 79. — Sexual fusion and maturation divisions in the basidium of 



Nidularia piriformis . 



a, two sexual nuclei about to unite, b, prophase of heterotypic division in fusion 

 nucleus, c, heterotypic mitosis, d, homceotypic mitosis, e, the four basidiospore 

 nuclei. X 1800. (After Fries, 1911.) 



spore nuclei (Fig. 79) . As in the ascomycetes, it thus follows immediately 

 upon the nuclear fusion: in the basidium in hymenomycetes and in the 

 teleutospore in rusts. An exception is reported in the case of Hygro- 

 phorus conicus, in which Fries (1911) finds in the basidium neither a 

 nuclear fusion nor a reduction. 



In the bryophytes reduction, so far as known, is universally brought 

 about by the two mitoses which differentiate the four nuclei of each spore 

 tetrad. It was at one time reported (van Leeuwen-Reijnvaan 1907) 

 that in Polytrickum there is a second reduction at spermatogenesis and 

 oogenesis: the sporophyte was said to have 12 chromosomes, the spore 

 and gametophyte six, and the gametes three. This double reduction 

 was thought to be compensated for by the fusion of the ventral canal 

 cell with the egg, raising the number in the latter to six, in combination 

 with the entrance of two sperms into the egg at fertilization, making the 

 sporophytic number 12. This interpretation has been shown to be 

 false by both Vandendries (1913) and Walker (1913), who find the life 

 cycle normal in every respect: a reduction from 12 to 6 occurs at sporo- 

 genesis but no second reduction follows at gametogenesis. 



1 A summary of researches on rusts is given by Maire (1911). A list of numbers of 

 nuclei in the cells of basidiomycetes is given by Levine (1913). 



