APOMIXIS AND RELATED PHENOMENA 405 



belong to the nucellus ("nucellar embryony"), but sometimes they are 

 integument cells. 



This process occurs in a number of angiosperms,''' commonly in species 

 exhibiting parthenogenesis or apogamy also (Fig. 227, C). The sacs into 

 which the embryos project may have either reduced or unreduced nuclei. 

 Embryos may arise from the egg, other gametophytic cells, and sporophy- 

 tic cells in the same plant, the condition known as -poly embryony often 

 resulting.* In genetic researches the exact mode of origin of such 

 embryos is a matter of considerable moment. 



Apospory. — The development of a gametophyte from a cell (or 

 cells) other than a spore (in plant groups normally developing gameto- 

 phytes from spores). 



As a rule both the sporophyte and the aposporous gametophyte arising 

 from it have the unreduced chromosome number. In several genera of 

 ferns^ gametophytes may develop as buds on the sporophyte, commonly 

 from the leaf margin or the meristematic tissue at the base of the sorus 

 (Fig. 227, D, E). Such aposporous development of diploid gametophytes 

 can be induced by cultural means, and this may be followed by the 

 production of tetraploid sporophytes.^" Ordinarily, apospory is balanced 

 by apogamy, the chromosome number remaining the same throughout the 

 life cycle. In mosses^^ also the development of diploid gametophytes 

 from portions of the sporophyte can be induced in this way. In some 

 cases these diploid gametophytes produce diploid gametes which fuse 

 and give rise to tetraploid sporophytes bearing diploid spores. In 

 Amhlystegium a tetraploid gametophyte has been obtained. Von 

 Wettstein (19246) reports the production of octoploid gametophytes in 

 certain intergeneric hybrids. 



Apospory may also occur in angiosperms. In such cases^^ an unre- 

 duced female gametophyte develops from a cell of the nucellus or integu- 

 ment, often in addition to the gametophyte arising normally from the 

 megaspore (Fig. 228, D, E). 



False Hybrids. — After cross-pollination individuals sometimes 

 develop which are not true hybrids because they carry nuclear factors 

 from only one of the parents. Such "false hybrids" are metromorphic 



7 E.g., Alchemilla (Murbeck, 1901), Citrus (Osawa, 1912), Rosa (Tackholm, 1922), 

 Spathiphyllum (Schilrhoff and Jiissen, 1925), Nigritella (Afzelius, 1928), Artemisia 

 (Chiarugi, 1926), and Ochna (Francini, 1928). 



8 For cases of polyembryony in angiosperms, see Ernst (1918, p. 436). 

 ^ Pteris, Asplenium, Athyrium, Poly stick urn, Scolopendrium, Lastrcea. 



"Lang (1924) and Manton (1932o) on Osmunda; Lawton (1932) on Aspidium 

 and Woodwardia; Andersson-Kotto (1931, 1932) on Scolopendrium. 



" Mnium, Bryum, Phascum, Amhlystegium (El. and Em. Marchal, 1909, 1912; 

 Schweizer, 1923). 



12 Hieracium flagellare and H. excellens (Rosenberg, 1906, 1907a; see 1930), Arte- 

 misia (Chiarugi, 1927), Oxyria (Edman, 1929). 



