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 
Fig. 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. 
