THE GYMNOSPERMS 



363 



spread apart. They thus catch the flying pollen, which falls 

 down to the base of the scales and is caught in a drop of a 

 viscid substance exuded through the micropyle of the ovule. 

 When this dries it draws the pollen grains up in the micropyle 

 close against the nucellus. The scales now close, and the cone 

 turns downward, in which position the ovules are better pro- 



Fig. 353- 



Section of ovule of white pine, int, integu- 

 ment, pc, pollen chamber; pt, pollen tube; n, 

 nucleus; m, macrospore cavity. 



Fig- 354- 



Last division of the egg in the white 

 pine cutting off the ventral canal cell at 

 the apex ot the archegonium. End, 

 endosperm; Arch, archegonium. 



tected from rain and outside changes in the air. The scales 

 become further sealed with resin. This occurs in May or June. 

 The pollen grain then germinates, the tube cell forming a tube 

 which penetrates the nucellus, and usually branches, feeding on 

 the disintegrated cell contents, and quantities of starch grains are 

 found in the pollen tube. During this process the central cell of 

 the sperm case divides into two cells; one is sterile, while the 



