Plant fertilization is the biological process where male and female reproductive cells, or gametes, fuse to form a single cell called a zygote. This fusion restores the diploid state of the plant, meaning the new cell contains two sets of chromosomes, one from each parent. Fertilization is the final, defining act of sexual reproduction in plants, which is necessary for the development of the seed.
Setting the Stage: Pollination Versus Fertilization
The process that must precede fertilization in flowering plants is pollination, which is often confused with fertilization itself. Pollination is a physical, external event involving the transfer of pollen grains from the anther to the stigma, requiring agents like wind, water, or animals. Fertilization, by contrast, is an internal, genetic, and biochemical process that occurs only after successful pollination. The actual fusion of the male and female nuclei to create the zygote is the distinct event known as fertilization.
The Mechanics of Plant Fertilization
Once a pollen grain lands on a compatible stigma, it must first rehydrate and then germinate. This germination involves the growth of a specialized tube, known as the pollen tube, which extends down through the style. The pollen tube acts as a microscopic conduit, carrying the two non-motile sperm cells toward the ovule deep within the ovary.
The tube’s growth is carefully guided by chemical signals released by the female gametophyte, directing it toward the ovule’s entry point, the micropyle. The pollen tube eventually penetrates the ovule and enters a small structure called the embryo sac. It enters one of the two synergid cells, which then degenerates, causing the pollen tube to rupture and release its two sperm cells.
Why Double Fertilization Matters
Flowering plants, or angiosperms, utilize a unique process called double fertilization, involving two separate fusion events within the embryo sac. This mechanism utilizes both sperm cells delivered by the pollen tube and is a defining characteristic of this plant group.
In the first event, one sperm cell fuses with the egg cell to form the diploid zygote, which is the first cell of the new plant embryo. Simultaneously, the second sperm cell travels to the center of the embryo sac and fuses with the two polar nuclei. This second fusion event, called triple fusion, forms a triploid cell that develops into the endosperm, a nutrient-rich tissue that serves as the primary food source for the developing embryo inside the seed.
The Transformation into Seeds and Fruits
The success of double fertilization triggers a cascade of developmental changes within the flower’s female organs. The fertilized ovule, which now contains the newly formed embryo and endosperm, begins its transformation into a seed. The protective layers surrounding the ovule, known as the integuments, harden and develop into the seed coat, which shields the delicate internal structures. Concurrently, the ovary wall, which houses the ovule, begins to swell and mature into the fruit. The fruit’s primary function is to enclose and protect the developing seeds, providing a mechanism for their eventual dispersal.