278 



INTRODUCTION TO CYTOLOGY 



1916c). Meiosis with spore formation is reported in Paradinium (Chat- 

 ton, 1927). 



Turning to the myxomycetes, it seems that meiosis accompanies the 

 formation of the spore nuclei, a fusion of nuclei in the Plasmodium having 

 preceded the process. ^^ In Plasmodiophora brassicce, according to 

 Prowazek (1905), the Plasmodium breaks up into uninucleate masses 











Fig. 161. — Meiosis and syngamy in Actinophrys sol. In each of the copulating indi- 

 viduals meiosis occurs, after which one of the resulting nuclei unites with a similar one from 

 the other individual. (From Minchin, after Schaudinn.) 



which fuse in pairs and become spores with two nuclei each. These 

 nuclei in two mitoses give off "reduction nuclei," leaving two sexual 

 nuclei which then fuse. In this species, therefore, the sexual nuclei are 

 the only reduced ones in the cycle. 



The green algse, formerly supposed to show "zygotic" meiosis exclu- 

 sively, are now known to have other types also. Three general categories 

 may be distinguished: haplonts, diplonts, and diplohaplonts.*^ As 



^"^Ceratiomyxa fOlive, 1907; Jahn, 1908), Trichia and Arcyria (Kranzlin, 1907), 

 Didymium (Skupiehski, 1928; genetic evidence). 



"Terms introduced by Svedelius (1915, 1931). Haplonts are sexual plants in 

 which only the zygote has the zygotic chromosome number. Diplonts are sexual 

 plants in which only the gametes have the gametic number. Diplohaplonts are those 

 with an alternation of two generations having the gametic and zygotic numbers respec- 

 tively. For general reviews of reproduction and alternation of generations in the 

 algse, see Bonnet (1914), Janet (1914), Oltmanns (1922-1923), West (1927), and 

 G. M. Smith (1933). Davis (1916) gives a convenient summary of the life histories of 

 the red alga}. Svedehus (1931 ) and Knight (1931 ) discuss and diagram the various life 

 cycles in Rhodophyceae and Pha;ophyceae, respectively. Dodge (1914) compares the 

 cycles of red algse and ascomycetes. For further discussions, see Tansley (1912), Buder 

 (1916a), Kylin (19166), Renner (1916), Fritsch (1916), and Svedelius (1921, 19276, 

 1931). 



