236 INTRODUCTION TO CYTOLOGY 



end, discharging the two male gametes, together with a certain amount 

 of pollen-tube cytoplasm and sometimes the tube nucleus. Frequently 

 the tube appears to pass directly through a synergid. It has been 

 suggested that the synergids produce enzymes which swell and rupture 

 the membrane of the pollen tube (Goebel; Haberlandt, 1927). As they 

 appear in the embryo sac, the male gametes may be complete cells or 

 only nuclei with no specially delimited cytoplasm (see p. 210). Fre- 

 quently the nuclei are markedly vermiform. Double fertilization, a 

 process peculiar to angiosperms, now takes place: one male gamete fuses 

 with the egg, while the other unites with the two polar nuclei or their 

 fusion product^^ (Fig. 144). 



The Gametic Fusion. — The nucleus of one of the male gametes enters 

 the egg — through a sort of rift, according to Heimans. In cases where 

 the male gamete is a complete cell in the pollen tube, its nucleus may 

 become free from its cytoplasm before reaching the egg.^^ In other cases, 

 however, the male cytoplasm remains distinct from that of the pollen 

 tube and embryo sac until the surface of the egg is reached. 2"* It has not 

 yet been demonstrated cytologically in any case that male cytoplasm 

 enters the egg along with the nucleus. Wylie thinks it probable that 

 it does so in Vallisneria, and there is genetic evidence that it may enter 

 in certain other cases (Chapter XXV). It would obviously be difficult 

 to identify male cytoplasm during the passage of the nucleus through the 

 egg membrane, or to show that the nucleus is absolutely freed of all 

 adhering matter previously associated with it. It is expected that 

 refinements in technique will shed light on this question. Occasional 

 differences in chromaticity between male and female nuclei are reported,-^ 

 so that one may hope for similar evidence for the cytoplasm. 



The fusion of the two gamete nuclei probably occurs in most cases very 

 soon after they come in contact, though in certain forms it is known to be 

 considerably delayed. The chromatic matter of the two nuclei at the 

 time they unite may be in the reticular condition, the male and female 

 elements becoming indistinguishable in the fusion nucleus. In other 

 cases^^ it has already reached the condition characteristic of the prophase, 

 the paternal and maternal elements often being distinguishable in the 

 spindle in the ensuing division of the zygote. That the same species may 

 show considerable variation in this respect is indicated by the situation in 



22 For reviews of the features of fertilization in angiosperms, see Dahlgren (1927a), 

 Schurhoflf (1926), Schnarf (1929), and Coulter and Chamberlain (1903). 



" E.g., Lilium (S. Nawaschin, 1910; Welsford, 1914), Viola (Madge, 1929), and 

 Melandrium (Breslawetz, 19306). See Tischler (1921-1922, p. 486). 



2^ Vallisneria (Wylie, 1922, 1923), (Enothera (Ishikawa, 1918), Asclejjias (Finn, 

 1925). 



2^ E.g., Heimans (1928; on Lilium) and Kuwada (1925a, 19266; on Cycas). 



^^ E.g., Lilium (Guignard, 1891; Weniger, 1918), Calopogon (Pace, 1909), and 

 Trillium (Nothnagel, 1918). 



