since the ovules are protected by carpels, it takes something special for fertilisation to happen. angiosperms have pollen grains made of just three cells. one cell drills down through the integuments, to make a passage for the two sperm cells to flow down. the megagametophyte is a tiny haploid female plant which includes the egg. it has just seven cells. of these, one is the egg cell; it fuses with a sperm cell, forming the zygote. another cell joins with the other sperm, and forms a nutrient-rich endosperm. the other cells take auxiliary roles. this process of "double fertilisation" is unique, and is common to all angiosperms.