MEIOSIS 



279 



representatives of the first category may be cited species of Spirogyra 

 (Karsten, 1908; Trondle, 1911), Zygnema (Kurssanow, 1911), Coleo- 

 chcete (C. E. Allen, 1905c), Cylindrocystis (H. Kauffmann, 1914), Volvox 

 (W. Zimmermann, 1921), and Ulothrix (I. Gross, 1931; Lind, 1932). 

 These plants have the gametic chromosome number and undergo meiosis 

 in the first two mitoses in the zygote (Fig. 162). In a number of other 

 genera, such as (Edogo7iium, Sphceroplea, and Closterium, in which the 

 chromosomes are not so well known, it is probable that the same condition 

 exists, since the zygote upon germination gives rise with considerable 

 regularity to four cells or nuclei; in some cases {(Edogonium) the four 

 cells are zoospores. 



It has recently been discovered that some of the green algae are 

 diplonts. Here the somatic cells have the zygotic chromosome number, 

 which is reduced at the time of gametogenesis, i.e., meiosis is "gametic." 



Fig. 162. — Behavior of nuclei and plastids in zygospore of Spirogyra. A, second 

 meiotic mitosis in S. calospora. B, four nuclei resulting from meiotic divisions in S. 

 longata. C, degeneration of three of the four nuclei after meiosis in S. longata. D, degenera- 

 tion of plastids contributed by "male" gamete in S. neglecta. (After Trondle, 1911.) 



In this category are the coenocyte genera Codium and Acetabularia 

 (M. Williams, 19256; Schussnig, 19286, 19306). There is also some 

 evidence that Vaucheria may have gametic meiosis (Mundie, 1929). 



Of even greater interest are the forms with two well-marked genera- 

 tions in the life cycle. Here the gametes are borne on plants with the 

 gametic chromosome number and the spores on other morphologically 

 similar plants with the zygotic number; meiosis occurs at zoosporogenesis. 

 Known examples are Enteromorpha and Chcetomorpha (M. Hartmann, 

 1921, 19296, 19306), Cladophora (Schussnig, 19286; Foyn, 1929; Higgins, 

 1930) and Ulva (Foyn). Furthermore, it appears that in these cases the 

 sexual plants are dicEcious. 



Among the Charales, meiosis in the germinating zygote has been 

 described for Char a by Oehlkers (1916). In another form at first thought 

 to be a species of Nitella, Tuttle (1924, 1926) reported haplosis in the 

 apical cells of the oogonia and antheridia, which would make the vegeta- 

 tive body a diplont rather than a haplont. In the oogonium the four 

 cells resulting from meiosis were shown to be strikingly like the egg and 

 polocytes of animals. On the contrary, Karling (1926) and Lindenbein 



