SPERMATOPHYTA - 413 

Fig. 465. Longitudinal section of apical portion of megasporangium of 
Cycas rumphii shortly before the time of fertilization 

Above is the nucellus, through which pollen tubes are growing ; below is a por- 
tion of the female prothallus, showing two archegonia 
the gametophytes produced by them have the z, or single, number 
of chromosomes, while the 2 2, or double, number of chromosomes 1s 
restored when a sperm nucleus fuses with the egg nucleus. This 
double number persists throughout the life of the sporophyte and is 
reduced again when a spore mother cell divides to form four spores. 
i seen ied —— 

