plant biology
Cotyledon
Last reviewed 2026-10-04
A cotyledon is a seed leaf that appears during early seedling development. It may contain stored food for the young plant or begin photosynthesis before the first mature leaves form. Plants with one cotyledon and plants with two cotyledons develop in different broad groups, but a gardener mainly sees cotyledons as the seedling's first leaves.
What it means
Cotyledons often look different from the later leaves. They may be smooth, rounded, narrow, or thick depending on the plant. A seedling's first pair of leaves should not be judged by the shape of the mature plant.
How it affects the garden
Protect new seedlings from drying, crusted soil, pests, and temperature extremes. Once true leaves appear, the plant is developing the foliage that will support more sustained growth. Thin seedlings carefully so roots are not competing in a crowded tray.
The seed starting calculator helps plan when to sow, but transplant timing should consider the seedling's development, not just its age.
Example
A tomato seedling emerges with two narrow seed leaves. The next leaves have the familiar tomato shape and are true leaves, not cotyledons.
Common confusion
Cotyledons are not always the same as the first leaves a gardener sees on a cutting or division. They are specific to seed-grown plants and early embryo development.
Sources
University of Minnesota Extension illustrates early seedling development and care.
