386 INTRODUCTION TO CYTOLOGY 



develop into male and female plants, respectively. ^^ In these gameto- 

 phytes, which may be reproduced by regeneration, the sexes are very 

 firmly fixed, but gametophytes produced by regeneration from diploid 

 tissue of the sporophyte are bisexual. In pteridophytes the sexual tend- 

 encies of ordinarily heterothallic gametophytes are not so firmly fixed 

 and can often be readily altered by varying the cultural conditions. ^^ 

 In these forms very little is known about the chromosomes or factor 

 segregation. 



Algae and Fungi. — A series of experiments performed by Klebs 

 and others many years ago showed that in certain green algse the environ- 

 mental conditions very largely determine whether the plants shall 

 grow vegetatively, produce zoospores, or reproduce sexually. The factors 

 influencing the kind of sex developed have, however, been very little 

 investigated in the algffi. A great many species are homothallic. In Vau- 

 cheria it has been found that bisexual plants may be formed by regenera- 

 tion from the contents of either the antheridium or the oogonium; this 

 indicates that the differentiation of the sex organs can hardly involve a 

 segregation of genetical sex factors. In the heterothallic Cladophora 

 Suhriana it has been reported that the diploid sporangium-bearing plant 

 has 13 chromosomes, including an unpaired X. In meiosis both halves of 

 the split X pass to the same pole in the first mitosis and separate in the 

 second, so that half of the spores and dioecious gametophytes carry six 

 chromosomes while the other half carry 6 -{- X. In certain red algae, for 

 example Polysiphonia violacea, the development of distinct male and 

 female plants from different tetraspores suggests a segregation of sex 

 factors in meiosis. Studies on diatoms have yielded evidence that such 

 factors segregate at I in Nitzschia subtilis and at II in Anomoeoneis 

 sculpta."^ 



In the fungi comparatively little is known about the chromosomes 

 aside from their number and the occurrence of meiosis at sporogenesis 

 in certain groups (p. 281). In many forms there is, however, strong 

 evidence for a meiotic segregation of factors influencing the course 

 of sexual reproduction. In Coprinus, Aleurodiscus, Ustilago, Puccinia, 

 and other genera of basidiomycetes the spores produced by a single 

 basidium may be of two "strains," as indicated by the behavior of the 

 mycelia developing from them. Hyphal unions initiating the binucleate 

 phase of the life cycle take place only when mycelia of opposite strain 

 are brought together. ^^ In some cases the mycelia from the spores of a 



21 El. and Em. Marchal (1906, 1907, 1909), Schweizer (1923), Fleischer (1920). 



22 Wuist (1913), Czaja (1921), Mottier (1927, 1931), Nagai (1914, 1915). 



23 Von Wettstein (1920) on Vaucheria; Schussnig (1930a6) on Cladophora; Geitler 

 (19286, 19296) and von Cholnoky (1928) on diatoms. 



24 Kniep (1919 et seq.), Vandendries (1923 et seq.), Brunswik (1924, 1926), Hanna 

 (1925, 1928, 19296), D. Newton (1926), Sass (1928, 1929), Zattler (1924), Craigie 



