SPERMATOPHYTES 



211 



ing), a generative cell, and a tube nucleus. This is the shedding stage, 

 and the subsequent development occurs in the pollen chamber, after 

 the pollen tube has begun to develop. In that position the nucleus 

 of the generative cell divides, but no wall is formed, the stalk nucleus 

 being thrust out to one side of the general cytoplasm, which organizes 

 the body cell in connection with the other nucleus. The events that 

 follow are exactly as in the cycads: two blepharoplasts appearing in 

 the body cell ; the body cell dividing into two sperm mother cells, each 



A 



FIG. 471. Micropylar end of female gametophyte of Ginkgo, showing the endosperm 

 beak developed in the archegonial chamber, and the two archegonia; in the left arche- 

 gonium the sperm and egg nuclei are fusing; in the right archegonium the early free 

 nuclear division of the embryo is occurring. 



with one blepharoplast ; and each mother cell developing a spirally 

 grooved, multiciliate sperm, which later is discharged. 



Fertilization. The details of fertilization also resemble those in the 

 cycads. The pollen tube is a branching and absorptive (haustorial) 

 organ; the pollen chamber and archegonial chamber become continuous 

 by the breaking down of the small amount of intervening tissue; the 

 grain end of the tube is thus brought into position favorable for the 

 discharge of sperms into the archegonial chamber ; and fertilization is 

 accomplished (fig. 471). 



Embryo. The embryo is peculiar among gymnosperms in the 

 absence of a slender, elongated, and tortuous suspensor, but a real sus- 

 pensor is present. Free nuclear division (fig. 471) results in the dis- 

 tribution of 256 nuclei through the cytoplasm of the egg, and then walls 



