GYMNOSPERMS: WHITE PINE, 



307 



as the endosperm. This endosperm continues to grow until a large part of 

 the nucellus is consumed for food. 



623. Female prothallium and archegonia. The endosperm is the female 

 prothallium. This is very evident from the fact that severa* archegonia 

 are developed in it usually on the side toward the pollen chamber. The 

 archegonia are sexual organs, and since the sexual organs are developed on 

 the gametophyte, therefore, the endosperm is the female gametophyte, or 

 prothallium. In fig. 359 are represented two archegonia in the endosperm 

 and the pollen tubes are growing down through the nucellus. The arche- 

 gonia are quite large, the wall is a sheath or jacket of cells which encloses 

 the very large egg which has a large nucleus in the center. 



624. Pollen tube and sperm cells. While the endosperm (female pro- 

 thallium) and archegonia are developing the pollen tube continues its 

 growth down through the nucellar cap, as shown in fig. 359. At the same 

 time the two cells which were formed in 



the pollen grain (antheridium) from the 

 central cell move down into the tube. One 

 of these is the "generative" cell, or "body" 

 cell, and the other is called the stalk cell, 

 though it is more properly a sterile half of 

 the central cell. The nucleus of the gener- 

 ative cell, about the time the archegonium 

 is mature, divides to form two nuclei, 

 which are the sperm nuclei, and the one 

 in advance is the larger, though it is much 

 smaller than the egg nucleus. 



625. Fertilization. Very soon after the 

 archegonia are mature (early in June in the 

 northern United States) the pollen tube 

 grows through into the archegonium and 

 empties the two sperm nuclei, the vegetative 

 nucleus and the stalk cell, into the proto- 

 plasm of the large egg. The larger of the 

 two sperm nuclei at once comes in contact 

 with the very large egg nucleus and sinks 

 down into a depression of the same, as 



shown in fig. 361. These two nuclei, in the pi 6o 



pines, do not fuse into a resting nucleus, but Last division 'of the egg in the 



at once organize the nuclear figure for the whit f pil ]f Cuttin 8 off the ventral 

 - . . canal cell at the apex of the 



nrst division of the embryo. Two nuclei archegonium. End, endosperm; 



are thus formed, and these divide to form A 



four nuclei which sink to the bottom of the archegonium and there organ- 



--Areh 



