Accessory Organs of Flowers Surrounding the stamens and pistils in 

 all the familiar flowers is a ring of colored or white leaflike structures that make 

 up the corolla, or "crown", of the flower. The separate parts are called petals. 



The Ovule as a Female Organ As the ovule develops, two layers of 

 tissue grow around the large cell on the inside and finally enclose it, leaving a 

 small opening at the end. In the meantime, the nucleus of the central cell 

 undergoes two divisions, the second division leaving the number of chromo- 

 somes reduced by half (see page 385). One of the nuclei enlarges and crowds 

 the three others to one end, where they eventually die. The enlarged cell 

 with its haploid nucleus is called the eynbryo sac. 



We saw that the reduction in the number of chromosomes is characteristic 

 of the formation of sexual reproductive cells. The embryo sac, however, 

 despite its haploid nucleus, is not a germ cell: it corresponds to a spore. Now 

 the embryo sac nucleus divides, and the new nuclei divide further several 

 times. The haploid nuclei resulting rearrange themselves, but no cell walls 

 are formed. One of these nuclei becomes the female gamete and moves 

 toward the end of the embryo sac near the opening in the ovule. Other nuclei 

 later take part in the complex processes that accompany fertilization and the 

 early stages of development. They seem to be related to the nourishment of 

 the fertilized egg and the young embryo. 



Two layers of tissue grow around large cell inside ovule 



while nucleus of central cell divides twice 



Spore mother 

 cell dividing 



Second division 

 (reducing) 



Mature ovule 



Spore 



Micropyle 



THE OVULE AS A FEMALE ORGAN 



402 



