POLLESTATION AND FERTILIZATION 



117 



growth of the pollen tube.i When the tip of the tube reaches 

 the ovary it usually penetrates to the interior of an ovule by 



Fig. 108. Pollen grains produc- 

 ing tubes, on stigma of a lily. 

 Much magnified 



g, poUen grains ; t, pollen tubes ; 



p, papillee of stigma ; c, canal or 



passage running toward ovary 



Fig. 109. Diagram to illustrate course 

 of the pollen tube during fertilization 



p , pollen grains ; t, pollen tube ; n, nucel- 

 lus, or body of the ovule ; a, antipodal 

 cells of embryo sac; en, endosperm nu- 

 cleus of embryo sac ; egg, the egg ap- 

 paratus, consisting of the egg cell and 

 two cooperating cells ; m, the micropyle, 

 or small opening through which, in most 

 ordinary flowering plants, the pollen 

 tube makes its way to the egg at the tip 

 of the embryo sac 



means of the little opening 

 (inicropyle) at one end of the 

 ovule (Fig. 109).2 One of the 

 male cells now unites with 

 the egg nucleus of the ovule. 

 The other male cell in many cases unites with the cen- 

 tral nucleus of the embryo sac to form the endosperm nucleus 



1 See Green, Vegetable Physiology, chap. xxvi. P. Blakiston's Son & Co., 

 Philadelphia. 



2 In some plants the tube makes its way directly through the tissue of 

 the ovule. 



