A seed is an embryonic plant enclosed in a protective outer covering, known as the seed coat, and typically includes a food supply for its early development after germination. This package represents an evolutionary adaptation that allows plants to survive seasons of stress and disperse their offspring. The formation of seeds is the defining feature of the seed plants, which dominate most terrestrial environments today.
The Primary Reproductive Structure
For the majority of plants, specifically the flowering plants (angiosperms), seed production begins within the flower. The flower functions as the plant’s sexual reproductive unit, facilitating the transfer of genetic material necessary for creating a new generation. Its structure contains the specialized parts that transform into the seed and the fruit that encloses it.
The female reproductive organ, often called the carpel or pistil, houses the structures that develop into the seed. This organ consists of the stigma, which receives the pollen, the style, and the ovary located at the base. These specialized structures protect the female gametes while they await fertilization.
Anatomy of Seed and Fruit Formation
The specific structure within the flower that transforms into the seed is the ovule. Ovules are small bodies located inside the flower’s ovary, and they contain the female egg cell. After fertilization, the ovule’s internal contents develop into the embryo and nutritive tissue, and its outer layers toughen to form the protective seed coat.
The ovule’s transformation is linked to the development of the fruit, which forms from the surrounding ovary. The ovary wall thickens and ripens into the fruit (pericarp), which protects the developing seeds and aids in their dispersal. In flowering plants, the ovule becomes the seed, and the ovary becomes the fruit.
The Essential Step of Fertilization
For the ovule to mature into a viable seed, fertilization must occur. This process begins with pollination, the transfer of pollen (containing the male genetic material) to the female part of the flower. The pollen then germinates and grows a tube down into the ovary to reach the ovule.
The fusion of the male nucleus from the pollen with the female nucleus within the ovule forms a zygote, which develops into the embryo inside the seed. In flowering plants, this involves double fertilization, where a second male nucleus fuses with other female nuclei to form the endosperm, a specialized food reserve tissue.
Seed Production in Cone-Bearing Plants
Seed production in cone-bearing plants (gymnosperms) operates differently because they do not produce flowers or fruits. Instead of being enclosed in an ovary, their seeds are considered “naked,” developing on the surface of specialized reproductive structures known as cones. These plants, which include pines, firs, and spruces, produce separate male and female cones.
The female cone produces and protects the ovules and, eventually, the seeds. These cones consist of spirally arranged scales, and the ovules are located on the upper surface of each scale. After fertilization by pollen transferred from the male cones, the ovule develops directly into the seed, which then matures exposed on the cone scale until it is dispersed.