SPERMATOPHYTA 



359 



persistent. The generative nucleus and tube nucleus are differentiated, 

 the former giving rise to the stalk and body nuclei. These are not 

 formed as cells. In this condition the pollen grain is shed. In the pollen 

 tube two male nuclei are formed. The pollen grains of Gnetum and Wel- 



A B 



Fig. 306. Ephedra trifurca. A, longitudinal section of an ovule, showing outer integument 

 (o), inner integument (i) forming a long micropylar tube, pollen chamber (p), and female 

 gametophyte (g) with two archegonia, X42; B, two archegonia just before division of the 

 central-cell nucleus, X112. (After Land.) 



witschia contain an ephemeral prothallial nucleus, a tube cell, and a gen- 

 erative cell. A stalk cell and body cell are not formed, but later the 

 nucleus of the generative cell gives rise directly to two male nuclei, as in 

 angiosperms. 



