278 



A TEXTBOOK OF BOTANY 



[Ch VI, 3 



enzymes and absorbing the digested products for use in its 

 own further growth; and commonly 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 micropylar 



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 will appear later 

 in connection with the fruit), with 

 probably some surrounding cyto- 

 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 fertilization 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. 



