336 GYMNOSPERMS 



Nuclei left at the top of the egg nearly always degenerate prompt- 

 ly. Occasionally a nucleus divides, but no embryo is formed, and 

 there seems to be no influence upon the development of the func- 

 tional embryo, which is at the opposite end of the egg. 



What causes the sperm to move toward the egg nucleus has not 

 yet been determined. In most cases there is a mechanical impulse 

 when the sperm is discharged from the pollen tube, but this is not 

 sufficient to account for much of the movement. Chemotactic ex- 

 periments,which succeed so well with pteridophytes, fail just as they 

 do in cycads. One thing is certain: something brings the nuclei to- 

 gether, and the nuclear membranes are dissolved at the line of con- 

 tact, so that the two nuclei become surrounded by a common mem- 

 brane contributed by both gametes. 



It is a curious fact that the first two cases of fertilization described 

 for conifers were abnormal. StrjVSBurger,^'^ as early as 1878, in his 

 Befruchtung und Zelltheilung, figured two gamete nuclei of about 

 equal size in Picea vulgaris. Nearly 20 years later, Coulter'" 

 described two fusing gametes of about equal size in Finns laricio. 

 There can hardly be any doubt now that the "sperm," in both these 

 cases, was an enlarged ventral canal nucleus, since normal fertiliza- 

 tion has been observed so often in both these species, and the sperm 

 nucleus has been proved to be very small in comparison with that of 

 the egg (fig. 327). 



In the nineties it used to be taken for granted that gamete nuclei 

 fused in the resting condition, the assumption being based upon the 

 appearance of this stage in angiosperms. As technique improved, it 

 became evident that this would not hold for Pinus^°^ (fig. 327 B, F). 

 Even Coulter's"*^ earher account, showing two gamete nuclei of 

 nearly equal size, also showed them in the spireme stage, although 

 this fact was not mentioned in the paper (fig. 328). Later, the in- 

 dependence of the two groups of chromatin was observed in Junip- 

 erus,*^^' ■"" and probably any detailed observation, with modern tech- 

 nique, will show such a behavior to be general, if not universal. In 

 Lilium, upon which the most plausible claim for fusion in the resting 

 condition was based, it is now known that the two gametes form 

 two spiremes and that there is not any fusion of chromatin. 



The most detailed investigation of the behavior of chromatin at 



