Flower fertilization is the process of sexual reproduction in flowering plants, responsible for producing seeds and the fruits that enclose them. This process is the source of the grains, fruits, and vegetables that sustain many forms of life, making it foundational to the world’s agricultural productivity and biodiversity.
Flower Structures Essential for Reproduction
For reproduction to occur, a flower has male and female parts. The male reproductive organ is the stamen, composed of the anther and the filament. The filament is a thin stalk that supports the anther, which produces and holds pollen grains containing the male reproductive cells.
The female reproductive organ, located in the flower’s center, is the pistil or carpel. It consists of three parts: the stigma, style, and ovary. The stigma is at the top of the pistil and has a sticky surface to capture pollen. The style is a tube-like structure connecting the stigma to the ovary. At the base of the pistil, the ovary contains one or more ovules, which house the female egg cell.
The stamen is structured for pollen dispersal, while the pistil is designed to receive it. Not all flowers contain both male and female parts; some have only stamens (staminate) or only pistils (pistillate). Flowers that contain both are known as perfect flowers.
How Pollen Reaches the Flower
The transfer of pollen from an anther to a stigma is known as pollination, a prerequisite for fertilization. This transfer is facilitated by vectors, with the main mechanisms being wind, water, and animals. These methods ensure male gametes within pollen grains can reach the female reproductive structures.
Wind pollination (anemophily) is common in trees and grasses, which produce large amounts of lightweight pollen. Water pollination (hydrophily) is a rarer method used by some aquatic plants where pollen is transported by water currents. Animal pollination (zoophily) involves insects, birds, and other animals attracted to flowers by colorful petals, scents, or nectar; as these animals forage, pollen adheres to their bodies and is transferred between flowers.
Pollination is also categorized as either self-pollination or cross-pollination. Self-pollination occurs when pollen from a flower’s anther lands on the stigma of the same flower or another on the same plant. Cross-pollination is the transfer of pollen between flowers of different plants of the same species, which promotes genetic diversity.
The Fertilization Process Explained
Once a pollen grain lands on a compatible stigma, the fertilization process begins. The stigma’s surface stimulates the pollen grain to germinate, causing a pollen tube to emerge. This tube grows down through the style, guided by chemical signals from the ovule.
The pollen grain contains a tube cell and a generative cell. The tube cell forms the pollen tube, while the generative cell travels down it and divides to form two sperm cells. The pollen tube eventually penetrates an opening in the ovule called the micropyle, allowing the sperm cells to enter the embryo sac.
Flowering plants (angiosperms) undergo a process called double fertilization. One sperm cell fuses with the egg cell inside the ovule, creating a diploid zygote that will develop into the plant embryo. The second sperm cell fuses with two polar nuclei in the embryo sac. This fusion creates a triploid cell that develops into a nutritive tissue called the endosperm.
Development After Fertilization
After double fertilization, transformative changes occur within the flower. The fertilized ovule develops into a seed. The zygote divides to form the embryo, which contains the rudimentary root (radicle) and shoot (plumule). The endosperm serves as a food source for the embryo, and the ovule’s outer layers harden to form the protective seed coat.
Concurrently, the ovary surrounding the ovules develops into a fruit, which is a mature ovary that encloses the seeds. The ovary wall thickens and can become fleshy, like in apples and peaches, or dry and hard, as in nuts and grains. Fruit development is stimulated by hormones from the developing seeds, and in some species, fruit size is correlated with the number of seeds it contains.
The seed protects the embryo and provides nourishment until conditions are suitable for germination. Seeds can remain dormant for extended periods to survive unfavorable conditions. The fruit aids in the protection and dispersal of the seeds. Dispersal mechanisms include being eaten by animals or having structures like wings or hooks for transport by wind or animal fur.