A cotyledon, often referred to as a “seed leaf,” is a component of a plant embryo within a seed. These specialized leaves are among the first structures to emerge when a seed begins to germinate. Distinct from a plant’s true leaves, cotyledons play an early role, providing initial support for the developing seedling.
Essential Roles of Cotyledons
Cotyledons primarily serve as a nutrient source for the emerging seedling. They contain stored food reserves, such as starches, proteins, and oils, which are mobilized to fuel the seedling’s initial development. This stored energy supports the plant until it can establish its root system and produce its own food. For example, the large cotyledons of beans and peanuts are rich in these reserves, supplying energy for germination.
In some plant species, cotyledons also undertake an initial phase of photosynthesis. After emerging above the soil surface, these cotyledons turn green and begin converting sunlight into energy, acting as the seedling’s first temporary leaves. This temporary photosynthetic activity provides additional energy while the plant develops its true leaves. This role is short-lived, as cotyledons are eventually replaced by permanent leaves.
Beyond nutrition and temporary photosynthesis, cotyledons offer protection to the delicate parts of the embryonic shoot. As the seedling pushes through the soil, the cotyledons can shield the plumule, the embryonic shoot tip that will develop into the stem and true leaves. In monocots like corn, a sheath called the coleoptile, part of the modified cotyledon, encases the young shoot as it grows upward. This helps ensure the growing point remains unharmed during emergence.
Cotyledon Development and Variation
The number of cotyledons present in a seed is a characteristic used to classify flowering plants into two major groups. Plants with a single cotyledon are known as monocotyledons, or monocots, examples of which include corn, rice, and grasses. In contrast, plants possessing two cotyledons are categorized as dicotyledons, or dicots, which encompass species like beans, peas, and almonds. This difference in cotyledon count is one of several distinguishing features between these plant groups.
Cotyledons also vary in their emergence pattern during germination, a process categorized as epigeal or hypogeal. In epigeal germination, the cotyledons are lifted above the soil surface as the seedling grows, often expanding and becoming photosynthetic. Common examples include beans and onions. Conversely, hypogeal germination occurs when the cotyledons remain below the ground, serving as nutrient storage organs. Peas and corn are examples of plants that undergo hypogeal germination, where the cotyledons stay protected beneath the soil.
Regardless of their specific function or emergence pattern, cotyledons are temporary structures in a plant’s life cycle. Their lifespan can range from just a few days to several weeks, and in rare cases, up to a year. Once the true leaves develop and become capable of efficient photosynthesis, the cotyledons have fulfilled their purpose. They typically wither, shrivel, or fall off as the seedling becomes self-sufficient.