Seeds represent the fundamental unit of reproduction for flowering plants (angiosperms) and conifers (gymnosperms). Each seed encapsulates a miniature, undeveloped plant, known as an embryo, along with a stored food supply, all encased within a protective outer layer.
Defining a Seed and Its Purpose
Seeds are biological structures that serve multiple functions for the plants that produce them. They are crucial for the dispersal of plant offspring, enabling species to spread to new locations and colonize diverse terrestrial environments. Seeds also allow plants to survive through unfavorable conditions, such as drought or winter, by entering a state of arrested development called dormancy. This protective mechanism ensures that the delicate embryo remains viable until conditions become suitable for growth.
The stored food within a seed provides the young plant with initial nourishment, offering a significant advantage over single-celled spores by giving the seedling a robust start. Seeds, therefore, are not merely reproductive units but sophisticated survival and dispersal vehicles.
Anatomy of a Seed
A typical seed consists of three primary components: the seed coat, the embryo, and a food supply. The seed coat, an outer protective covering, shields the internal structures from physical damage, pathogens, and environmental stresses like extreme temperatures. A small pore, called the micropyle, is often visible on the seed coat, facilitating the entry of water during germination.
Inside this protective shell lies the embryo, the miniature plant itself. Key parts of the embryo include the radicle, which is the embryonic root that will anchor the plant and absorb water. The plumule is the embryonic shoot, containing the first true leaves, destined to grow upwards and form the plant’s stem and leaves. Additionally, the embryo possesses one or more cotyledons, often referred to as “seed leaves.” These cotyledons can serve as nutrient reserves or, in some species, perform initial photosynthesis upon emergence.
The third main component is the food supply, which provides energy for the developing embryo. This stored food is commonly found in the endosperm, a nutritive tissue rich in starch, oil, or protein. In some seeds, particularly many dicots like beans, the endosperm is largely absorbed by the cotyledons during seed development, and the cotyledons themselves become the primary storage organs. In contrast, monocots like corn typically retain a substantial endosperm that nourishes the growing embryo.
From Flower to Seed: How Seeds Form
The formation of a seed begins within the flower, specifically after the process of pollination and fertilization. Pollination involves the transfer of pollen, which carries the male reproductive cells, from the anther to the stigma of a flower. If compatible, a pollen grain germinates on the stigma, and a pollen tube grows down through the style to reach the ovule located within the ovary.
Fertilization occurs when a male gamete from the pollen fuses with the egg cell inside the ovule. This fusion results in the formation of a zygote, which then develops into the embryo. In flowering plants, a unique process called double fertilization also occurs, where a second male gamete fuses with central cells to form the endosperm, the primary food storage tissue for the developing seed.
Following successful fertilization, the ovule undergoes significant changes, maturing into a seed. Simultaneously, the surrounding ovary of the flower begins to ripen and develop into a fruit, which encases and protects the developing seeds. This transformation protects the seeds and aids in their dispersal.
Bringing Life: The Process of Germination
Germination is the process by which a seed emerges from its dormant state and begins to grow into a new plant. This process requires specific environmental conditions to be met, primarily water, appropriate temperature, and oxygen. Some seeds also require specific light conditions—either light or darkness—to trigger germination.
The initial step in germination is imbibition, where the dry seed rapidly absorbs water through its seed coat. This water uptake causes the seed to swell and softens the seed coat. Water also activates enzymes within the seed, which begin to break down the stored food reserves into usable energy for the embryo.
Following imbibition, the radicle, the embryonic root, is typically the first part to emerge from the seed. It grows downwards, anchoring the seedling and beginning to absorb water and nutrients from the soil. Soon after, the plumule, the embryonic shoot, emerges and grows upwards, developing into the stem and leaves. The emergence of the radicle marks the completion of germination, as the young plant transitions from relying on stored reserves.