Gymnosperms : The Conifers 



513 



pollen grains, now in contact with the nucellus, begin developing 

 pollen tubes into the nucellus. Elongation is very slow, and it 

 is not until the following June, or early July, that the tubes 

 pass entirely through the nucellus. Meanwhile two sperm 

 nuclei have been formed from the generative cell. This occurs 

 about the time that the archegonia are formed by the female 

 gametophyte. 



Fertilization and growth of the embryo. When a pollen tube 

 passes between the neck cells and reaches the egg, the sperm 





Fig. 320. Spray of Austrian pine. At the left (above) is a one-year-old 

 ovulate cone and (below) a two-year-old ovulate cone. On the right is a cluster 

 of staminate cones. 



