FEKTILIZATION IN PLANTS 



217 



plants. In Albugo bliti (one of the Peronosporeae), for example, as 

 shown by the recent work of Stevens ('99), the mature ovum contains 

 about a hundred nuclei, and is fertilized by a multinucleate proto- 

 plasmic mass derived from the antheridium, each nucleus of the latter 

 conjugating with one of the egg-nuclei. But although the conjugat- 

 ing bodies are here multinucleate, the germ-nuclei conjugate two and 

 two (as is also the case in the multinucleate cysts of Actinosph(Erium, 

 p. 279); and the case therefore forms no real exception to the 

 general rule that one paternal nucleus unites with one maternal. 



D 



Fig. 106. — Formation of the ovum and penetration of the pollen-tube in flowering plants. 

 [Strasburger.] 



A. Embryo-sac of Monotropa, showing the division that follow's the two maturation-divisions 

 and produces the upper and lower " tetrads." B. The same, ready for fertilization, showing ovum 

 ((?), synergidas (j), upper and lower polar cells (/), and antipodal cells {a). C. Penetration of 

 the pollen-tube (/./.) in Orchis ; o. ovum, with synergidas at either side, g.n. generative nuclei in 

 the pollen-tube. D. Slightly later stage with generative nuclei entering the micropyle. 



Whether a union of more than two germ-nuclei occurs in any of the 

 lower plants is a question still disputed by botanists. ^ Such plural 

 fusion is rendered a priori improbable by the observations thus far 

 made upon the one-celled forms both in plants and in animals ; and 

 the known facts are sufficient to show that it must be, to say the 

 least, an exceptional process. 



In cases where the paternal germ-cell is a ciliated spermatozoid, as 

 in Fiiais, Pilularia, and the ferns and cycads, the germ-nuclei differ 



1 Cf. Hartog, '91, '96, Trow, '95, Stevens, '99, Zimmerman, '96, and literature there cited. 



