FERTILIZATION 



165 



193. Fertilization. — Fertilization in seed-plants means 

 the union of a sperm nucleus derived from a generative 

 p cell of a pollen grain with an 



egg-cell at the apex of the 

 embryo sac (Fig. 124). This 

 process gives rise to a cell 



Fig. 123. Germination of Pollen 

 Grain of an Angiosperm. (JIuch 

 magnified and somewhat dia- 

 grammatic.) 



-1, entire grain, with germination con- 

 siderably advanced. B, tip of pollen 

 tube at a much later stage, after the 

 tube nucleus has disappeared: g, the 

 generative cell beginning to enter the 

 tube ; t, the tube nucleus ; s, sperm 

 cells formed from the generative cell. 



Fig. 122. Pollen Grains producing 

 Tubes, on Stigma of a Lily. 

 (JIuch magnified.) 



g, pollen grains; t, pollen tubes; 

 p, papillse of stigma; c, canal or 

 passage running toward ovary. 



which contains material derived from the pollen and from 

 the egg-cell. In a great many plants the pollen, in order to 

 accomplish the most successful fertilization, must come 

 from another plant of the same kind, not from the indi- 

 vidual which bears the ovules that are being fertilized. 



Pollen tubes begin to form soon after pollen grains lodge 

 on the stigma. The time required for the tube to penetrate 

 to the ovary varies in different kinds of plants. 



Finally the tube enters the opening at the apex of the 



