The life cycle of flowering plants, known as angiosperms, involves a transformation of structures to ensure the continuation of the species. Sexual reproduction begins with the transfer of pollen, leading to the fusion of reproductive cells within the flower’s female organs. This biological process culminates in a developmental shift where one tiny structure changes its form and purpose. The question of what this transformed reproductive unit becomes marks a fundamental step in plant biology.
The Resulting Structure
The fertilized ovule immediately begins a developmental program to become a seed. This structure represents the next generation of the plant, packaged for survival and dispersal away from the parent organism. The seed is a tripartite unit, consisting of the embryo, a food supply, and a protective outer layer.
The embryo is the miniature sporophyte plant, which develops from the zygote formed by the fusion of the egg and one sperm cell. This embryonic plant possesses the beginnings of a root and shoot system, ready to grow once conditions are favorable. The second component is the endosperm, a tissue rich in starches, proteins, and oils, which serves as the primary food source for the developing embryo and the seedling after germination.
The outer layers of the ovule, called the integuments, harden and mature into the seed coat, or testa. This tough covering provides a physical barrier, guarding the delicate internal components from damage, desiccation, and microbial attack until the seed is ready to sprout. The final, mature seed is the direct answer to the question of what a fertilized ovule becomes.
The Ovule’s Pre-Fertilization Role
Before its transformation into a seed, the ovule functions as the megasporangium, the structure that produces and houses the female reproductive cells. Within the flower, the ovule is located inside the ovary, which is part of the pistil, the plant’s female reproductive organ. It is attached to the ovary wall by a stalk-like structure called the funicle, which supplies it with nutrients.
The ovule contains the female gametophyte, also known as the embryo sac, which holds seven cells and eight nuclei. The most important of these cells is the egg cell, the female gamete, which is the ultimate target for fertilization. Also present are two polar nuclei in the central cell, which play a distinct role in a process unique to flowering plants.
Fertilization is initiated when a pollen grain lands on the stigma and germinates, extending a tube down the style toward the ovary. This pollen tube enters the ovule, typically through a small opening called the micropyle, to release two male sperm nuclei into the embryo sac. This leads to double fertilization, where one sperm nucleus fuses with the egg cell to form the diploid zygote.
The second sperm nucleus fuses with the two polar nuclei in the central cell to form a triploid nucleus, which rapidly divides to generate the nourishing endosperm tissue. This simultaneous dual fusion triggers the ovule’s post-fertilization development into a seed.
The Larger Botanical Context
The transformation of the ovule into a seed is coupled with the development of the surrounding flower parts, creating a complete dispersal unit. The ovary wall, which housed the ovules, undergoes a change, maturing into the fruit. The fruit’s primary purpose is to protect the developing seeds and to facilitate their eventual dispersal.
The ovary wall, or pericarp, differentiates into distinct layers in the mature fruit, which can be either fleshy or dry. Fleshy fruits, like berries, drupes, and apples, rely on animals to eat the sweet tissue and carry the seeds away from the parent plant. In contrast, dry fruits, such as nuts and grains, may rely on wind, water, or mechanical splitting for dispersal.
The existence of the fruit is a defining characteristic of angiosperms, providing a layer of protection that gymnosperms, or non-flowering seed plants, lack. This structural innovation has allowed flowering plants to colonize a vast array of environments by evolving diverse dispersal strategies. The development of the seed within the protective confines of the fruit ensures the successful propagation of the new generation.