1446 A TEXTBOOK OF THEORETICAL BOTANY 



and is easily ruptured. Occasionally terminal or sub-terminal openings in 

 the wall of the tube have been seen. A noteworthy point is that the tube has 

 never been seen to enter the oosphere, even when there are no synergids, 

 although fertilization of the oosphere is the primary object of the whole 

 process. 



Quite frequently more than one pollen tube may enter the embryo sac. 

 As many as twenty have been known to do so. Sometimes the accessory 

 tubes do not open, but they may do so and discharge their sperm nuclei in 

 various parts of the sac. Normally these superfluous nuclei simply degene- 

 rate and disappear, but this is not always so. 



Two sperm nuclei may fertilize the oosphere giving rise to a triploid 

 embryo. Fertilization of an antipodal cell has already been referrred to and 

 fertilization of synergids has been inferred from the occurrence of super- 

 numerary embryos, which may, however, have arisen apomictically. In 

 Myricaria each polar nucleus has been observed to fuse separately with a 

 male nucleus. Lastly, abnormal embryo sacs may contain more than one 

 oosphere, each of which may be fertilized. All these processes are abnor- 

 malities and polyspermy, as it is called, is undoubtedly rare. When two 

 sperm nuclei from different pollen tubes take part in fertilization they may 

 be genetically different and give rise to anomalous inheritance. This is 

 spoken of as heterofertilization. 



The two normal sperm nuclei move from their point of liberation, one 

 to the oosphere and one to the primary endosperm nucleus or to the pair of 

 still unfused polar nuclei, as the case may be. This movement must take 

 place very rapidly since intermediate stages are rarely to be seen in fixed 

 material. They move in the cytoplasm of the embryo sac, but their move- 

 ment seems to be active, not passively due to cytoplasmic movement. This 

 is evidenced by the divergent paths taken by the two nuclei, which are nor- 

 mally close together at first, and by the fact that neither the tube nucleus, nor 

 the remains of the synergid nucleus, if they are also extruded into the sac, 

 show any movement of their own. 



The union of the second male nucleus with the polar nuclei constitutes 

 the double fertilization (Fig. 1327), which was established as a normal 

 occurrence by Navaschin in 1898 and Guignard in 1899 (Fig. 1328). The 

 actual union is also called triple fusion (Fig. 1329), from the three nuclei 

 involved, though in some embryo sacs, the number of polar nuclei may be 

 much more than two. The only parallel happening in other groups of 

 plants is the occasional fertilization of the ventral canal nucleus in some 

 Gymnosperms, which we have already referred to. 



Although the oosphere is usually reached first by a male nucleus, their 

 fusion takes longer than that of the second male nucleus with the primary 

 endosperm nuclei, although the two processes appear to follow the same 

 lines. There is very seldom any difference in appearance or size between 

 the two male nuclei, but it has been fairly established, especially by the very 

 careful observations of Steffen on Impatiens glandiiligera, that the male 

 nucleus going to the oosphere may retain its cytoplasmic sheath until fusion 



