LIFE HISTORY OF A GYMNOSPERM 
2 9 
large number of nuclei which are scattered throughout the cyto- 
plasm. Each nucleus accumulates around itself a portion of the 
cytoplasm and ultimately cell walls are laid down and the entire 
embryo sac contains endosperm (prothallial) tissue. Toward the 
micropylar end of the endosperm { proihallus ) originate several 
archegonia. 
Each archegonium consists of a much-reduced neck of four cells 
and an egg {ovum) which lies embedded in the prothallus which 
forms a narrow layer of cells around it called the jacket. The con- 
tents of the mature embryo sac constitutes the female gametophyte. 
Fig. 11. — White pine (Pinus Strobus). At left, megasporangium with mega- 
spore in the center; above, pollen grains in the micropyle and pollen chamber. At 
right, pollen grains beginning to germinate; the cells of the integument have 
X enlarged and closed the micropyle. {Gager, after Margaret C. Ferguson.) 
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. 
