After meiosis II, the gametangia split apart in the region where they are in contact with each other, creating a small 'copulation aperture' through which fertilization can occur. The male and female gametes bulge out through the aperture and come into contact (Figs 1 and 2). Close inspection reveals that the single functional haploid nucleus of each gamete lies immediately adjacent to the copulation aperture. Next, the gametes fuse and the contents of the male gamete move through the copulation aperture into the female gametangium (Fig. 3: the female gametangium is on the left).
As the male gamete moves out of the male gametangium, the smaller 'supernumerary' cell (Figs 4 and 5, grey) expands to fill the space left.The interface between the male gamete and the supernumerary cell is only slightly curved, suggesting perhaps that the expansion of the supernumerary cell is partly responsible for fertilization, by helping to drive the male gamete out.
The copulation aperture remains small during fertilization and the gametes are protected at all times by the walls of the gametangia. The male gamete's chloroplast becomes distorted and folded as it passes through the aperture (Figs 3–5), but it remains intact and the zygote inherits both chloroplasts contributed by the gametes.