Fig. 10–12. Development of the five-nucleate megagametophyte. Potentilla nivea. Scale bars = 20 µm. 10. The megaspore nuclei (arrows) enter the megagametophyte tubes as they move into the nucellar cap. Two sections (from the same series but a few micrometres apart) have been combined into one picture in order to illustrate the position of the nuclei better. Flower about to open. 11. About halfway to the micropyle the megaspore nucleus divides into two (arrows). The nuclei will stay in this position for some time while the megagametophyte tube continues to elongate. Flower about to open. 12. When approaching the micropylar end of the megagametophyte, the upper of the two nuclei divides into two and immediately again into four (arrows). The lowermost of the four nuclei becomes the second polar nucleus and travels towards the center of the megagametophyte (broken arrow). Flower about to open