278 



A TEXTBOOK OP BOTANY 



[Ch. VJ, 3 



enzymes and absorbing the digested products for use in its 

 own further growth ; and commonl^y the tube dies and 

 withers behind as the forward part advances. Thus it 

 reaches and enters the ovary, and, taking a direct path, 

 approaches an ovule which it enters by the microp3'lar 



opening ; thus it reaches the rela- 

 tively large sac, the embryo sac, 

 which every ovule contains (Fig. 

 190), and within which, near the 

 micropyle, lies the egg cell. This 

 mechanism of fertilization is repre- 

 sented in principle in our general- 

 ized picture (Fig. 191). Thus are 

 the sperm cells brought to the im- 

 mediate vicinity of the egg cells. 



The final, and really the essen- 

 tial, stage in this process consists 

 in the fusion of the two sex cells 

 which is thus effected (Fig. 192). 

 One of the two male nuclei (the 

 fate of the other ^vill appear later 

 in connection wdth the fruit), with 

 probably some surrounding C3i:o- 

 plasm, moves out of the pollen 

 tube into the egg cell, presumably 

 opening the way by action of 

 enzymes. For a time the egg cell 

 exhibits two nuclei ; but they move 

 together, touch, and then gradually 

 fuse together into one and fertili- 

 zation is complete. The result is a cell containing a nucleus 

 derived from the union of two nuclei from different parent 

 cells ; and this appears to be the central and essential feature 

 of all fertilization. 



After fertiUzation the sepals, petals, stamens, nectaries, 

 styles, and stigmas, their functions evidently accomplished, 



Fig. 191. — A generalized 

 pistil and ovule, in section, 

 showing the mechanism of fer- 

 tilization described in the text. 



