FERTILIZATION 



85 



the pollen grain sends out a pollen tube which grows downward 

 through the spongy center of the style, following the path of least re- 

 sistance, to the space within the ovary, and there enters an ovule 

 through a tiny opening in the ovary wall, the micropyle (ml'cro-pllj. 



jS-JTi pollen 



"^£: anther 



pollen. 



tube. 



rcpyte 



ovule 

 nucleus 



Read carefully the paragraph on fertilization and then explain 

 this diagram in your workbook. 



During the growth of the pollen tube the sperm nucleus passes down 

 the elongating tube. When the tube reaches a clear area of proto- 

 plasm, known as the embryo sac, its tip is ruptured and the sperm nu- 

 cleus passes out into this sac. The embryo sac is an ovoid space, 

 microscopic in size, filled with semifluid protoplasm containing sev- 

 eral nuclei. One of the nuclei, with the protoplasm immediately 

 surrounding it, is called the egg cell. It is toward this cell that the 

 sperm nucleus of the pollen tube grows. Ultimately the sperm nu- 

 cleus reaches the egg cell and unites with it. The union of the 

 sperm nucleus with the nucleus of the egg cell in the ovule is known 

 as fertilization. The union of the sperm nucleus and the egg cell re- 

 sults in sl fertilized egg. This egg, by constant divisions of the cells, 



H. BIO 7 



