402 INTRODUCTION TO CYTOLOGY 



1. Reduced Parthenogenesis. — The developing gamete has the reduced 

 (gametic) number of chromosomes. 



[Called haploid parthenogenesis by Hartmann (1909) and Renner (1916), generative 

 parthenogenesis by Winkler (1904, 1908, 1920), and true parthenogenesis by Stras- 

 burger (1907a).] 



For many years no angiosperm sporophytes with the reduced chromo- 

 some number were known, but during the last decade several cases have 

 been discovered. They occur in Datura, Nicotiana, Triticum, Crepis, 

 Zea, and other genera (see p. 356). In some instances it is clear that the 

 chromosome set is truly monoploid and not merely the reduced comple- 

 ment of a polyploid race. The origin of such monoploid sporophytes has 

 not been traced in detail, but the genetic evidence leaves little doubt that 

 reduced eggs have developed. Mitosis in such eggs has been observed. - 

 In none of these cases has a regularly apomictic race been established. 



This mode of development is known to occur also in thallophytes. 

 In Fucus the development of the unfertilized egg has been induced by 

 artificial means (J. B. Overton, 1913), but the cytological features here 

 are unknown. Motile gametes of certain other algse have been observed 

 to develop without conjugation, as in Edocarpus (Kylin, 1918). In 

 Vaucheria (von Wettstein, 1920) the contents of either the oogonium or 

 the antheridium may be made to regenerate a new individual; what is 

 virtually "male parthenogenesis" occurs in the latter case. An example 

 of reduced parthenogenesis in the fungi is afforded by Saprolegnia 

 (Mackel, 1928; Schlosser, 1929). 



2. Unreduced Parthenogenesis. — The developing gamete has the 



unreduced (zygotic) number of chromosomes. 



[Called diploid parthenogenesis by Hartmann and Renner, somatic parthenogenesis 

 by Winkler, ooapogamy by Strasburger, Juel, and Ernst, and parthenapogamy by 

 Farmer and Digby (1907).] 



This form of parthenogenesis occurs very frequently in vascular plants 

 as a regularly recurring reproductive phenomenon. A few representative 

 cases are listed in the footnote.^ The unreduced state of the egg is due 

 to a previous failure of meiosis, the sporophytic number of chromosomes 

 being carried through to the gametophyte. The two sporocyte divisions 



2 Kusano (1915) on Gastrodia, Haberlandt (1921, 1922) on (Enothera. 



^ Marsilia Drummondii (Strasburger, 1907a); Athyrium filix-foemina, var. claris- 

 sima and Scolopendrin?n vulgare (Farmer and Digby, 1907); Alchemilla (Murbeck, 

 1901; Strasburger, 1904c; Boos, 1917, 1920); Antennaria alpina (Juel, 1898?>, 1900); 

 Archieracium (Rosenberg, 1906, 1917; Ostenfeld, 1910, 1912); Atamosco texana (Pace, 

 1913); Erigeron arniuus (Tahara, 1915a, 1921; I. Holmgren, 1919); Eujmtornim 

 glandidosum (Holmgren, 1919); Calycanthus (Schiirhoff, 1923); Taraxacmn (Murbeck, 

 1904; Juel, 1904, 1905; Osawa, 1913; Sears, 1917, 1922; Stork, 1920a); Chondrilla 

 (Rosenberg, 1912); Wikstrcemia (Winkler, 1906; Strasl)urger, 1909a); Allium odorum 

 (Haberlandt, 1923); Humulus lu-pulus (von Wettstein, 1925); Ochna serrulata (Chiarugi 

 and Francini, 1930). An example among lower plants is Chara crinita (Ernst, 1918, 

 1921o). 



