78 



PLANT BIOLOGY 



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FIG. 28. Diagram of a 

 longitudinal section of a 

 pistil showing germination 

 of pollen grains. 



a sperm cell with the 

 nucleus of an egg-cell. 

 Only one pollen grain or 

 sperm cell can be used in 

 fertilizing each egg-cell. 

 Usually, however, far larger 

 numbers of pollen grains be- 

 come attached to the stigma 

 than can be used by the 

 ovules in the ovary. All the 

 pollen grains that germinate 



produce pollen tubes which FlG> 2 9. ^Fertilization of an ovule and 

 may be Said to begin a tn e early stages in the development of 



race down the stigma and 

 style. The tubes that first enter ovules are the ones that 

 carry on the process of fertilization. Those that are beaten 

 in the race are of no further use and therefore die. 



