280 INTRODUCTION TO CYTOLOGY 



(1927) find no haplosis in the antheridia of Nitella gracilis, Chara aspera, 

 and other representatives of the group. 



In the diatoms^^ the bilaterally symmetrical Pennatse and the radially 

 symmetrical Centricae have been said to be characterized respectively by 

 gametic and zygotic meiosis, but observations in the latter group are 

 scanty and at least one exception to the rule is known. In Surirella 

 saxonica, a pennate form, meiosis occurs at the time of conjugation. In 

 each individual the nucleus divides to four, reducing the chromosome 

 number, after which one of the four unites with one from the other indi- 

 vidual as the cells fuse. The fusion product is an auxospore (Karsten). 

 In Corethron valdivice, a centric type, the behavior of the nuclei suggests 

 the occurrence of chromosome reduction in the germination of the zygote, 

 which in this case is formed by the union of motile gametes (Karsten). 

 Biddulphia sinensis, however, resembles pennate forms in having gametic 

 meiosis (Schmidt). 



Among the brown algae, many of which exhibit a well-marked alterna- 

 tion of generations, meiosis occurs in most cases in sporangia on plants 

 with the zygotic chromosome number. In some groups the spores are 

 motile,'*^ whereas in others they are non-motile tetraspores.^^ In Fucus 

 the vegetative body has the zygotic chromosome number, but it produces 

 gametes instead of spores. Meiosis occurs in the early divisions in the 

 oogonia and antheridia. "^^ Hence Fucus is a diplont with a life cycle closely 

 similar to that of animals. It is the opinion of some algologists {e.g., 

 Svedelius) that spores have been replaced by gametes in Fucus through a 

 progressive abbreviation of the gametophytic phase. 



Among the red algae^^ two principal conditions are known. In 

 Scinaia and Nemalion, meiosis occurs in the zygote. There are then 

 developed gonimoblast filaments and carpospores with the reduced 

 chromosome number, these spores in turn developing into new plants 

 which retain this number. In Polysiphonia and several other forms 

 meiosis does not take place in the zygote, but the carpospores develop into 

 plants with the zygotic number. Meiosis eventually occurs in tetra- 

 sporangia on these plants, the resulting tetraspores producing plants 

 with the reduced number. 



^^ See especially Klebahn (1896), Karsten (1900, 1904, 1912), P. Schmidt (1923, 

 1927a6), Geitler (1927a6c, 1928a6c, 19296, 1931), and von Cholnoky (1928, 1929). 



*^ Cutleria-Aglaozonia (Yamanouchi, 1912), Zanardinia (Yamanouchi, 19136), 

 Ectocarpus (Kylin, 1918), Pylaiella (Knight, 1923), Egregia (Myers, 1928), Stypocaulon 

 (Higgins, 1931). 



« Dictyota (Mottier, 1900; J. Williams, 1904a), Padina (Williams; P. Carter, 1927). 

 Zonaria (Haupt, 1932) has more than four spores. 



*' Yamanouchi (1909) on Fucus; Kunieda (1926) and Okabe (1929) on Sargassum. 



^ Yamanouchi (1906) on Polysiphonia; I. F. Lewis (1909) on Griffithsia; Svedelius 

 (1914a6c, 1915) on Nitophyllum, Delesseria, and Scinaia; Cleland (1919) on Nemalion; 

 Yamanouchi (1913c, 1921) on Corallina; Mathias (1928) on Callithamnion. For a 

 summary, see Svedelius (1931). 



