52 PHARMACEUTICAL BOTANY 



Fertilization. About a year after pollination the pollen tubes, 

 lying within the pollen chamber show signs of renewed activity. 

 The tube nucleus passes to the tip of the tube. The generative-cell 

 divides to form a body and a stalk-cell which pass into the tube. 

 The body-cell later forms two sperm nuclei. While these changes are 

 taking place the tube is penetrating the nucellus and growing toward 

 the embryo sac with its contained female gametophyte. It finally 

 enters it, passing between the neck-cells of the archegonium. The 

 tip of the tube then breaks and the entire tube contents is emptied 

 into the egg. One of the sperm nuclei fuses with the egg nucleus 

 and fertilizes it forming an oospore. 



Seed Formation and Distribution. The oopsore undergoes re- 

 peated divisions and forms the embryo or young sporophyte plant 

 and a suspensor to which it is attached. The embryo is nourished 

 by a portion of the prothallus but the greater part of the prothallus 

 forms the endosperm tissue of the seed surrounding the embryo. 

 The thin nucellus persists as an endosperm covering. The integu- 

 ment becomes modified to form the hard protective seed coat. A 

 portion of the scale of the cone directly above and adjacent to the 

 ovule forms a membranous wing which separates from the scale as 

 part of the seed. 



By this time (about two years after pollination) the scales of the 

 cone, now quite woody, separate, the seeds are shaken out, and many 

 are carried for a considerable distance by winds. 



Germination of the Seed. Under favorable conditions, the seeds 

 absorb water and germinate in the spring following their dispersal. 

 The hypocotyl of the embryo appears first, arching upward and 

 downward, and, straightening out, draws the green cotyledons with 

 it which spread out toward the light, while it grows into the soil to 

 form the tap root and in time the remainder of the root system. 

 Thus the seedling sporophyte is formed which in time develops into 

 the mature White Pine tree. 



