Cotyledon Function in Germination Process

By Marcus Williams · October 9, 2026

What Cotyledons Are

Cotyledons are the first leaves to emerge from a germinating seed. Unlike true leaves, they are part of the seed embryo itself. Their primary function is to provide energy to the seedling until true leaves develop and begin photosynthesis. Think of them as the seedling's built-in lunch packed by the parent plant.

Two Types of Cotyledons

Storage Cotyledons (Hypogeal Germination)

In crops like peas, corn, and beans, cotyledons remain underground and serve purely as energy storage. They are thick, fleshy, and packed with proteins and carbohydrates. The shoot grows upward through the soil while cotyledons stay below, gradually transferring their stored nutrients to the growing shoot.

Photosynthetic Cotyledons (Epigeal Germination)

In crops like tomatoes, lettuce, and sunflowers, cotyledons are pushed above ground where they green up and begin photosynthesizing. They are thinner and leaf-shaped. They serve a dual role: first transferring stored energy, then producing new energy through photosynthesis.

The Nutrient Transfer Timeline

Cotyledons contain a finite supply of stored energy. Over the first 7-14 days, this energy is systematically transferred to the developing shoot and roots. As true leaves expand and begin photosynthesis, cotyledons have completed their role.

Yellowing Is Normal

Cotyledons naturally turn yellow and drop as their stored nutrients are depleted. This is not a sign of disease or nutrient deficiency. It is the planned obsolescence of the seed's energy package. Do not remove yellowing cotyledons prematurely — they are still transferring nutrients until they drop naturally.

Monocots vs Dicots

Monocots (grasses, onions, corn) have one cotyledon. Dicots (most vegetables and flowers) have two. The cotyledon count is the defining characteristic that splits all flowering plants into these two groups.

Implications for Seed Starting

Seeds with large storage cotyledons (beans, squash) are more forgiving of poor growing conditions during the first week. Seeds with thin photosynthetic cotyledons (lettuce, carrots) need adequate light immediately upon emergence or they exhaust their small energy reserve before true leaves develop.