214 



FOUNDATIONS OF BOTANY 



226. Fertilization. By fertilization in seed-plants the 

 botanist means the union of a generative cell from a pol- 

 p len grain with that of an egg-cell 

 at the apex of the embryo sac 

 (Fig. 165). This process gives 

 rise to a cell 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 re- 



Fm. 163. Pollen Grains producing , ,, , 



Tubes, on stigma of a Lily. (Much quired for the process to begin 

 magnified.) varies in different kinds of 



g, pollen grains ; t, pollen tubes ; p, -, ... 



of stigma ;c , canal or pas- plants, requiring in many cases 

 twenty-four hours or more. The 

 length of time needed for the 

 pollen tube to make its way 

 through the style to the ovary 

 depends upon the length of the 



FIG. 104. -Pollen Grain of Snow- . le and Qther con( iitionS. In 

 flake (Leucoium) producing a Pol- 

 len Tube with Two Naked Genera- the CrOCUS, which has a Style 



several inches long, the descent 

 takes from one to three days. 



Finally the tube penetrates the opening at the apex of 



a, g e running toward ovary. 



