392 



MORPHOLOGY OF GYMNOSPERMS 



Fig. 444. — Ephedra trifurca: fer- 

 tUization; pt, pollen tube; v, ventral 

 nucleus; »Si, 0, fusing nuclei; Wa^second 

 male nucleus; X21S. — After Land (23). 



functioning male nucleus slips 

 out of its cytoplasmic sheath in 

 the periphery of the egg, and 

 probably this is true of the species 

 of Ephedra in general. 



In Welwitschia the pollen 

 tubes and prothallial tubes come 

 in contact in the nucellar cap. 

 How the transfer of male cells 

 or nuclei is effected has not been 

 observed, but several eggs are 

 fertilized in each ovule, and 

 Pearson says (26) that the cyto- 

 plasm of the fusion nucleus is 

 mainly that of the male cell, 

 which would practically repeat 

 the behavior observed in the 

 sexual fusion of the Taxaceae and 

 Taxodium. 



In Gnetum (15) one or more 

 pollen tubes enter the embryo 

 sac, and their tips are observed 

 among the free nuclei (fig. 419, /) . 

 The tip of the tube becomes 

 distended and discharges its con- 

 tents (usually four nuclei), both 

 male cells being free in the sac 

 and functioning. Approximately 

 as many eggs are fertilized as 

 there are male cells discharged 

 into the' sac, that is, twice as 

 many as there are entering pollen 

 tubes. About the fusion nucleus 

 a dense layer of cytoplasm 

 appears and soon a cell mem- 

 brane is formed. The discharge 

 of male cells into an embryo sac 



