620 



WEED FLORA OF IOWA 



Fig. 483. Diagrammatic representation of fertilization of an ovule, i, inner 

 coating of ovule ; o, outer coating of ovule ; p, pollen tube proceeding from 

 one of the pollen grains on the stigma ; c, the place where the two coats 

 of the ovule bend. (The kind of ovule here shown is inverted, its opening 

 m being at the bottom, and the stalk f adhering along one side of the 

 ovule.) a to e, embryo sac, full of protoplasm; a, so-called antipodal 

 cells of embryo sac ; n, central nucleus of the embryo sac ; e, nucleated 

 cells, one of which, the egg cell, receives the male nucleus of the pollen 

 tube ; f, funiculus or stalk of ovule ; m, micropyle or opening into the 

 ovule. 



(After Luerssen.) 



The egg is in the end of the sac nearest the micropyle, in the 

 most convenient position for the entering pollen tube. The en- 

 dosperm cell is near the center of the embryo sac. The embryo 

 sac is now mature and awaits the entrance of the pollen tube. 



Fertilization. — The pollen tube traverses the tissues of the 

 stigma and style and finds its way to the micropyle. It passes 

 through the micropyle, penetrates the tissues of the nucellus, and 

 pierces the membrane of the embryo sac. The two sperms, which 

 have had a rather long journey through the pollen tube, now enter 

 the embryo sac. One 'finds its way to the egg and soon fuses with 

 the egg nucleus. The other fuses with the nucleus of the endosperm 



