FLOWERS: POLLENATION AND FERTILIZATION 117 



Pollen protection. 



From rain by closing or drooping of flower. 



From unwelcome insects by sticky stems or hairy flowers. 



Essential organs. 

 What are they? 

 Why so called? 



Fertilization. 



Definition: union of sperm nucleus of pollen with egg nucleus of the ovule. 



Pollen. 



1. Produced by stamen (anther). 



2. Structure: one-cell stage. 

 Three cell stage. 



Pollen tube (use ?). 

 Sperm cells (use ?). 

 Ovule. 



1. Produced in the ovary: undeveloped seed. 



2. Structure: coverings (seed coats later). 



One-cell stage. 

 Eight-cell stage. 



Two cells from endosperm. * 



One forms egg cell, proper. 



Fertilization. 



1. Pollen tube penetrates micropyle. 



2. Sperm cell passes down pollen tube. 



3. Nuclei of sperm and egg unite (fertilization proper). 



4. Embryo begins to develop. 



Origin of seed parts. 



1. Ovule walls become seed coats. 



2. Opening for pollen tube is the micropyle.' 



3. Fertilized egg becomes the embryo. 



4. Endosperm nuclei become the endosperm. 



Endosperm may be used by developing embryo. 

 Endosperm may remain to be used in germination. 



