GYMNOSPERMS: WHITE PINE. 



211 



division of the cells takes place here, just as the female prothallium develops 

 in the cavity of the nucellus, from the macrospore. The nucleus of the vege- 

 tative cell passes down the cavity of this 

 tubular sac. The antherid cell, which is the 

 smaller cell of the pollen grain, in the pine, 

 divides by a cross wall into a so-called stalk 

 cell, and a mother sperm cell, the latter 

 corresponding to the central cell of the an- 



Fig. 271. 



Section through upper part of nucellus and 

 endosperm of white pine, showing upper por- 

 tion of archegonium, the entering sperm cells, 

 and track of pollen tube ; nc, nucellus : pt, 

 pollen tube ; spc, sperm cells. 



ArcK 



Fig. 272. 



Last division of the egg in the white 

 pine cutting off the ventral canal cell 

 at the apex of the archegonium. End, 

 endosperm; Arch, archegonium. 



theridium, there being no wall formed. The sperm mother cell also passes 

 down the tubular sac, and divides again into two sperm cells, as shown in 

 fig. 270. About this time, or rather a little earlier, with the pollen tube part 

 way through the nucellar cap, winter overtakes it, and all growth ceases 

 until the following spring. 



426. Fertilization. In the spring the advance of the pollen tube con- 

 tinues, and it finally passes through the nucellar cap about the time that the 

 archegonia are formed and the egg cell is mature, as shown in fig. 271. The 

 pollen tube now opens and the sperm cells escape into the archegonium, and 

 later one of them fuses with the egg nucleus. The fertilized egg is now 

 ready to develop into the embryo pine. 



427. Homology of the parts of the female cone. Opinions are divided as 

 to the homology of the parts of the female cone of the pine. Some consider 

 the entire cone to be homologous with a flower of the angiosperms. The en.- 



