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- 



Fi(i. 163. —Pollen Grains producing • j (■ i i . 



Tubes, on Stigma of a Lily. (Much quiTed lor the process to begin 

 ""«"'"«*•) varies in different kinds of 



g, pollen grains ; t, pollen tubes \ P, i , 



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



sage running toward ovary. tWCUty-f OUr 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 

 ";Ve*!.;;.^r"ptrci::f pI «^1« ^^^ ^^^er conditions. In 



len Tube wltb 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 



