Micropyle 



Ovule 



Pollen tube 



Pollen grain 



Embryo sac 



Stigma 



FERTILIZATION IN A FLOWER 



A thread of protoplasm grows from the pollen grain on the stigma, penetrates through 

 the style and through a little opening in the wall of the ovule. When the tip of the 

 pollen tube reaches the embryo sac, a nucleus of the embryo sac and a nucleus of 

 the pollen tube unite. This is the essential fact in fertilization 



food used by the embryo as it grows to the stage of a ripe seed, other 

 food materials are accumulated in the ripening seed. These reserves are 

 either in the embryo tissues or immediately surrounding the embryo — ^in 

 the so-called endosperm. After the seed sprouts, and before the young plant 

 is ready to supply itself, the new individual lives on this accumulated reserve 

 or surplus. 



FertiHzation brings about changes in other parts of the flower. The petals 

 drop off or shrivel away, and usually the stamens also. The ovary begins to 

 enlarge and at last ripens into the central or the main body of the fruit. In 



THE EMBRYO OF A FLOWERING PLANT 



The fertilized egg cell passes by a series of cell divisions into a mass that gradually 

 takes on a definite form. In most species it becomes possible to distinguish the root, 

 the stem, and the first leaf or leaves 



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