2o8 



MORPHOLOGY OF GYMNOSPERMS 



tubes, as in cycads, seems to function only as a haustorial system. 

 At this stage the generative cell has the appearance of being pushed 

 away from contact with the persistent vegetative cell, on account of 

 the enlargement and development of vacuoles in the latter. The 

 generative cell increases very much in size, its nucleus keeping pace 

 with it; and these relative positions' are maintained until just before 

 fertilization, which occurs about ten weeks later. About the middle 

 of July the generative nucleus divides to form the stalk and body 

 nuclei, lying side by side and with no separating wall (figs. 237, 238). 



HiRASE says that the non-func- 

 tioning stalk nucleus is gradually 

 thrust out of the general cyto- 

 plasm by the growth of the 

 functioning body nucleus and the 

 organization of the body cell, and 

 functions no further (see fig. 

 163). At the end of July, the 

 much enlarged body cell is rich 

 in contents, and its nucleus is 

 very large. At this stage the two 

 blepharoplasts appear and as- 

 sume their polar positions, the 

 stage persisting for three weeks, that is, until the third week before 

 fertilization. 



At the beginning of August the tube nucleus begins to pass back 

 to the pollen grain end of the tube, which it reaches in about two 

 weeks, consorting with the body cell, and later with the sperms, until 

 fertilization. During the return of the tube nucleus, the endosperm 

 beak has developed between the two archegonia as described above, 

 pushing up the megaspore membrane farther, and permitting the 

 grain end of the pollen tube to penetrate to the region of the arche- 

 gonial chamber, where, freed from pressure, it becomes very turgid 

 (fig. 241). 



During the third week before fertilization (about the last of August) 

 the body cell begins to divide, the division occurring in the axial plane 

 of the spore, resulting in two sperm mother cells lying side by side and 

 each containing a blepharoplast (fig. 239) . In each mother cell the 



Fig. 241. — Ginkgo biloba: upper part 

 of female gametophyte, showing two 

 archegonia, the beaklike process of the 

 endosperm, and the swollen tips of two 

 pollen tubes; September 9; X24. — After 

 HiEAsA (16). 



