SEED-BEARING PLANTS 427 



5ome 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. PoUination 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 



