152 



MORPHOLOGY OF ANGIOSPERMS 



the nucleus of one synergid moves toward the tube upon its 

 entrance into the sac, and that the nucleus of the other synergid 

 moves toward the nucleus of the egg. 



In case the tube passes between the synergids it advances 

 directly toward the egg-nucleus ; but in case it passes along the 

 wall of the sac the tip of the tube curves toward the egg-nucleus. 

 In any event, the tip of the tube, in which a thin area (pit) 



is developed, is directed toward 

 the egg-nucleus when the dis- 

 charge takes place. Under the 

 pressure developed by the turgor 

 of the end of the tube, and re- 

 sisted by the small caliber of the 

 tube in its passage through the 

 micropyle and sac-wall, the 

 membrane of the pit is ruptured, 

 and a discharge of the contents 

 results. The perforated tip of the 

 pollen-tube, after the discharge, 

 has been demonstrated fre- 

 quently, as seen by Schaffner 4 ^ 

 in Sagittaria (Fig. 68). The 

 discharge seems to be forcible 

 enough to empty the end of the 

 tube of most of its contents, the 

 most important ones being the two male nuclei. Cases have been 

 reported in which only one male nucleus is said to be discharged, 

 as in Alisma (Schaffner 22 ) and Sagittaria (Schaffner 24 ), the 

 other being recognized as degenerating in the tube. However, 

 the frequent presence of disorganizing bodies within the tube 

 after fertilization (Fig. 71), and numerous observations of the 

 discharge of both male nuclei, and especially the rapidly multi- 

 plying illustrations of " double fertilization," incline to the be- 

 lief that the discharge of both male nuclei into the sac is usual. 

 The passage of the male nucleus through the cytoplasm of 

 the egg toward the female nucleus may be attended by an 

 increase in size and change in form, but the changes are not so 

 conspicuous as those that occur in the male nucleus that passes 

 deeper into the sac to fuse with the polar nuclei. For example, 

 in Caltha palustris Miss Thomas 44 found the male nuclei very 



FIG. 68. Sagittaria variabilis. Pollen- 

 tube in the act of discharging; four 

 centrosomes represented; x 900. 



After SCHAFFNER. 2 * 



