Figure 1. Fertilization in flowering plants.

(A) Microscopy image of a pollen tube at the opening cleft of a fertilized ovule in the plant species Arabidopsis thaliana (scale bar = 20 µm). (B) During plant fertilization two sperm cells (red) are carried by a pollen tube (yellow) to the ovule, which contains the egg cell (blue) and the central cell (grey). (C) When the pollen tube reaches the opening of the ovule (green) it fuses with one of two accessory cells (yellow), which then starts to degenerate. (D) The pollen tube bursts open, releasing the two sperm cells, one of which fertilizes the egg cell to form the zygote, with the other sperm cell fusing with the central cell to form an endosperm cell.