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4 







HOW THE POLLEN TUBES GROW 



Vertical section of ovule of a pine, shortly before fertilization; 

 magnified about 100 diameters. Three pollen tubes can 

 be seen growing down from the micropyle, the narrow 

 channel at the top by which they entered, s.n. is the 

 sperm nucleus, t.n. the tube nucleus, 5/. c. a stalk-ceU. 

 There are two archegonia (arch.) or egg cells, each with its 

 egg nucleus (e.n.). When a sperm nucleus from one ot 

 the pollen tubes finally reaches one of these egg nuclei, 

 it will unite with and fertilize it, thus initiating the 

 development of the seed. After Ferguson, Life History 

 of Finns, Washington, 1904. (Fig. 10.) 



