50 PHARMACEUTICAL BOTANY 



of three nuclear divisions takes place which cut off two small prothal- 

 lial cells (traces of one of which may be seen pushed up against the 

 wall of the fertile cell of the pollen grain), a tube nucleus and a 

 generative cell. At this stage the pollen is shed and some of it is 

 carried by air currents to the carpellate cones where it sifts in be- 

 tween the ovule-bearing scales and accumulates at the scale bases. 

 A number of the pollen grains are drawn close to the nucellus of the 

 ovule by the drying up of the viscid fluid which fills the pollen 

 chamber. In this fluid they germinate forming pollen tubes. The 



PIG. 24. The white pine (Pinus Strobus). Sections through mature pollen 

 grains; at the left the remnants of two prothallial cells can be seen, while at the 

 right all signs- of the first cell have disappeared. Pollen collected June 9, 1898. 

 X about 600. (Gager, after Margaret C. Ferguson.) 



transfer of pollen grains from the pollen sac to the pollen chamber 

 and consequent germination therein is called pollination. The con- 

 tents of a mature pollen-grain constitutes the male gametophyte. 



The Female Gametophyte. If the embryo sac be examined at 

 about the time of pollination, it will be found to consist of a single 

 cell containing a single nucleus surrounded by cytoplasm. Very 

 shortly afterward, however, the nucleus divides repeatedly to form a 

 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 (prothallus) 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. 



