SEED-BEARING PLANTS 427 
some very important fundamental fact or scientific 
principle. 
The formation of pollen in such abundance is one of 
the numerous instances of the "factor of safety" in plant 
organization; and the necessity for it is recognized at 
once when one considers the small chance that any given 
pollen-grain will reach the pollen-chamber of an ovule. 
378. Pollination. Some of the pollen-grains, of course 
reach the carpellate cones, which are usually situated 
higher up on the tree and higher up on the individual 
shoots, than are the staminate cones. This location is 
an advantage, because the light pollen-grains, specially 
buoyant because of their two air-sacs, readily float up- 
ward. Those that reach the carpellate cones, fall between 
the ovuliferous scales, and settle down to the bases of 
the scales. Some of them get caught in the sticky fluid 
that fills the pollen-chamber at this time, and as the fluid 
dries up the grains are drawn close down to the tip of the 
nucellus (Fig. 311). "Here, as always, the deposit of pollen 
on the surface where it is to germinate is called pollina- 
tion. Pollination in Pinus occurs in late May or early 
June, depending on the species, the locality, and the nature 
of the season. 
379. Nodding of the Cone. Soon after pollination the 
stalk of the carpellate cone, in most species, changes its 
relation to gravity, becoming negatively geotropic. One 
side grows more rapidly than the others, thus causing the 
cone to nod and hang pendant (Figs. 307 and 308). This 
position it retains throughout the remainder of its life. 
380. Germination of the Pollen-grain. Very soon after 
pollination, the tube-cell begins to develop a pollen-tube, 
which secretes an enzyme that dissolves the cell-walls and 
