Although this arrangement is typical, plant species show a wide variation in floral structure. The modifications produced in the evolution of flowering plants are used by botanists to find relationships among plant species.
Flowers are an important evolutionary advance made by flowering plants. Some flowers are dependent upon the wind to move pollen between flowers of the same species. Their pollen grains are light-weight. Many others rely on insects or birds to move pollen.
Theirs are heavier. The role of flowers is to produce seeds, which are contained in fruit. Fruits and seeds are a means of dispersal. Plants do not move, but wind, animals and birds spread the plants across the landscape.
Since the ovules are protected by carpels, it takes something special for fertilisation to happen. Angiosperms have pollen grains comprising just three cells. One cell is responsible for drilling down through the integuments, and creating a passage for the two sperm cells to flow down. The megagametophyte 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 dedicates itself to forming a nutrient-rich endosperm. The other cells take auxiliary roles. This process of "double fertilisation" is unique and common to all angiosperms.