A cotyledon, often called a seed leaf, is the first leaf-like structure formed within the embryo of a seed. This structure is a fundamental part of the developing plant and is present before the seed begins to sprout. Cotyledons are a defining feature of flowering plants (angiosperms) and support the young plant through its earliest phase of growth.
Anatomy and Primary Function
A cotyledon is an integral part of the plant embryo inside the seed. It is structurally distinct from the true leaves that develop later from the shoot tip (the epicotyl). The primary function of the cotyledon is to act as a temporary food storage unit for the germinating seedling.
These seed leaves are packed with food reserves, such as fats, starches, and proteins, which fuel the energy demands of germination. This stored nutrition allows the seedling to grow before it is mature enough to create its own food. In many plants, especially those with two cotyledons, the seed leaves are thick and fleshy due to the dense concentration of these reserves. Once the seedling is established, the cotyledon’s role as a nutrient source is complete, and the structure will typically wither away.
Cotyledons and Plant Classification
The number of cotyledons a plant embryo possesses is the basis for classifying the two largest groups of flowering plants. Plants with a single cotyledon are known as monocotyledons (monocots), while those with two cotyledons are called dicotyledons (dicots).
Monocots include plants like grasses, corn, wheat, and lilies, which emerge from the seed with only one slender seed leaf. In many monocots, this single cotyledon acts as an absorbing organ to transfer nutrients from a separate storage tissue called the endosperm. Dicots, such as beans, oak trees, and sunflowers, contain two fleshy cotyledons that often split apart as the seed germinates. These two cotyledons usually hold the majority of the stored food supply needed for the seedling’s early growth.
The cotyledon count is accompanied by other systematic differences. Monocots typically have leaves with parallel veins, while dicots have leaves with a netted or branched vein pattern. This embryonic difference is a foundational characteristic used by botanists to organize the plant kingdom.
Transitioning Roles
The fate of the cotyledons after the seed sprouts depends on the plant’s specific germination method, which has two types. In epigeal germination, the elongating stem section below the cotyledons (the hypocotyl) pushes the cotyledons up above the soil surface. Once exposed to light, these seed leaves often turn green and perform temporary photosynthesis before the true leaves develop.
Conversely, hypogeal germination occurs when the cotyledons remain anchored beneath the soil surface. In this method, the section of the stem above the cotyledons (the epicotyl) lengthens to push the shoot tip upward. The cotyledons transfer their stored nutrients to the emerging seedling while staying protected below ground, eventually shriveling as their food supply is exhausted. Examples of epigeal germination include beans and sunflowers, while peas and corn are examples of hypogeal germination.