With the arrival of spring come the emergence of flowers and the sudden bounty of allergy-inducing pollen, phenomena that persist throughout the growing season. While most people identify these as signs of the season, for plants they represent important reproductive processes that will ensure the survival of the next generation. One of the traits of flowering plants and functions of pollen is double fertilization, an adaptation that increases the likelihood that a seed will germinate.
Double fertilization accomplishes two goals. The most obvious, of course, is that the sperm fertilizes the egg, producing a zygote that will develop into an embryo inside the seed. At the same time, a second sperm cell fuses with maternal tissue, which provides a food source for the embryo after it emerges from the seed.
Male flower parts called anthers produce pollen, each grain of which contains two sperm cells. When pollen sticks to the female flower structure, a special cell called a germ cell begins to divide, growing into a tube that extends into the flower and enters the ovule. The two sperm cells journey down the pollen tube, where one fuses with the egg located inside the ovule. The second joins with two maternal cells called polar bodies. This cell begins to divide, forming endosperm, a nutrient-rich substance that fills the inside of the seed.
If you cut open the seed of a flowering plant, such as a bean seed, you will see the products of double fertilization. Inside the seed, you will see the plant embryo pressed to one side, the result of the first fertilization. The smooth substance that fills the rest of the seed is the endosperm, the result of the second.
The evolution of endosperm occurred fairly recently, being a characteristic of the flowering plant or angiosperm plant division, the most recently evolved of plant groupings. Very early plants reproduced with spores and didn't produce seeds at all. The development of the seed put the plant kingdom on the fast track to evolutionary success, but the earliest seed-bearing plants did not utilize double fertilization and did not produce endosperm. Research published in the American Journal of Botany by biologists Jeffrey S. Carmichael and William E. Friedman reveals rudimentary double fertilization occurring in plants believed to be the ancestors of the flowering plant division. Today, double fertilization and endosperm are considered traits of flowering plants.
Double fertilization added another reproductive advantage for the flowering seed plants. Now, in addition to the extra protection afforded by the seed, seedlings had a food supply right in the seed with them. Some plants use this food to begin developing inside of the seed, while others have a supply should growing conditions prove less-than-ideal when they emerge.