GYMA'OSPERMS : 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 fnjm the fart that severa. archegonia 

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

 archegonia are se.xual orgtms, and since the sexual organs are developed on 

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

 prothallium. In fig. 359 are repjresented 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 \vhich 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 arc dcvelo]>i[Tg the jjollen tube continues its 

 growth down through the nucellar cap, as shown in tig. 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. < )ne 

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

 cell, and the other is called the stalk (.l'11, 

 though it is more propjerly 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. — A'ery soon after the 

 archegonia are mature (early in June in the 

 northern Ignited States) ihc pollen tuVje 

 grows through into the archegonium and 

 empties the two sperm nuclei, the vegctati\'e 

 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 

 sho\\'n in hg. 361. These two nuclei, in the 

 pines, do not fu.se into a resting nucleus, but 



at once organize the nuclear figure lor the "•"■'"? P'"f ''""V'!? °^ theventral 



=' ^ canal cell at the apex cil the 



first division of Ihc embiwo. Two nuclei archegonium. End, endosperm; 



, - , T , 1. . 1 r Arch, archegonium. 



are tlrus formed, and these di^'ide to form 



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



-ArcL. 



Fig. 360. 

 Last division of the egg in the 



