The structure commonly referred to as a “seed leaf” is scientifically known as the cotyledon. This specialized embryonic leaf is formed within the seed’s embryo before germination begins. It is one of the first parts of the plant to appear as the seedling emerges from the soil. The cotyledon is a temporary organ designed to support the initial stages of a plant’s life cycle until it can produce energy through mature foliage.
The Primary Function of the Cotyledon
The purpose of the cotyledon is to provide the emerging seedling with its first source of energy and nourishment. In many species, cotyledons are packed with stored food reserves, such as carbohydrates, proteins, and lipids, accumulated from the parent plant. These reserves are metabolized and delivered to the developing root (radicle) and shoot (epicotyl) as the seed breaks dormancy. This initial energy supply fuels the rapid growth needed for the seedling to establish itself.
In other plants, the cotyledons emerge from the soil and immediately turn green, taking on a different functional role. These photosynthetic cotyledons capture sunlight and perform the earliest stages of food production. This immediate self-sufficiency allows the seedling to bridge the gap between exhausting the seed’s energy reserves and the development of the first true leaves.
The Role in Plant Classification
The number of cotyledons present in a seed is a fundamental characteristic used to categorize the vast array of flowering plants, known as angiosperms. This distinction separates them into two major groups: monocotyledons (monocots) and dicotyledons (dicots). Plants possessing a single cotyledon in their embryo are classified as monocots. This group includes economically important species such as corn, wheat, rice, and all grasses.
Plants possessing two cotyledons are classified as dicots, a group that includes most shrubs, trees, vegetables, and flowers, such as beans, oaks, and roses. This initial difference correlates with a suite of other physical traits that persist throughout the plant’s life. Monocots typically develop leaves with parallel veins and flower parts in multiples of three. Dicots generally exhibit net-like or branched leaf venation and flower parts in groups of four or five.
The Cotyledon’s Appearance and Disappearance
The life of the cotyledon begins with germination, which can follow one of two patterns: epigeal or hypogeal. In epigeal germination, the growing stem beneath the cotyledons, called the hypocotyl, elongates and pulls the cotyledons above the soil surface. Conversely, in hypogeal germination, the cotyledons remain below ground, transferring their stored nutrients to the emerging shoot. The cotyledon eventually undergoes senescence, the process of aging and dying off.
Once the first true leaves, which develop from the epicotyl, fully unfurl and become photosynthetically active, the cotyledons are no longer needed. The true leaves have the specialized internal structure necessary for efficient, long-term energy production. At this point, the cotyledons begin to wither, turn yellow, and eventually drop from the seedling. Their temporary existence ensures the seedling’s survival during its most vulnerable stage until it can sustain itself as an independent, photosynthesizing organism.