A cotyledon is the embryonic leaf, or one of the embryonic leaves, inside a seed. Its main function is to help the young plant get established: it may store and supply food, or it may absorb food from the seed’s storage tissue, and in some plants it also becomes an early photosynthetic leaf.
What “cotyledon” means
Cotyledon is the standard botanical term for a seed leaf: a structure that is part of the plant embryo within a seed. It is not slang or an informal expression. The word comes through Latin cotyledon from Greek kotylēdōn, a name used for a cup-shaped plant structure. In present-day botany, the word refers specifically to an embryonic leaf or leaf-like organ in a seed plant.
The number of cotyledons is one of the basic ways botanists describe flowering plants. A plant whose embryo has one is called a monocot; one whose embryo has two is called a eudicot. The terms are useful for classifying plants, but the cotyledons’ appearance and role can vary. They may look unlike the plant’s later leaves, and their work may be mostly complete before those leaves appear.
In a sentence, the word is used as a biological noun: “After germination, the bean’s cotyledons supplied stored food to the seedling.” In everyday gardening or classroom discussion, people may instead say “seed leaves,” especially when pointing out the first structures above the soil.
Where the cotyledon sits in a seed
A seed contains a plant embryo and, in many seeds, tissue that stores food for growth. A protective seed coat surrounds these parts. The embryo includes the young shoot and root, as well as its cotyledon or cotyledons. Their size and position differ among kinds of seeds: in some, the cotyledons occupy much of the seed; in others, they are relatively thin and sit beside a substantial food-storage tissue.
These arrangements matter because a seedling cannot immediately rely on mature roots and leaves. Before it can take up enough water and minerals from its surroundings or make much of its own sugar through photosynthesis, it needs resources to start growing. Stored food, whether held in cotyledons or in other seed tissue, bridges that early stage.
A familiar bean seed makes the arrangement easy to see. When a soaked bean splits, its two large halves are cotyledons; the tiny embryo lies between them. A corn kernel provides a different example: it is a monocot seed, with one cotyledon, and much of its food reserve is in the endosperm rather than in a pair of fleshy seed leaves.
How cotyledons support germination
Germination begins when a viable seed takes up water under suitable conditions. The embryo resumes growth, and its stored resources are mobilized. Cotyledons can contribute by containing reserves such as carbohydrates, oils, or proteins, which the developing embryo and seedling use as sources of energy and building material. The exact reserve and its location depend on the plant.
In seeds where the cotyledons hold much of the reserve, enzymes help make stored compounds available to growing tissues. In other seeds, the endosperm is the main reserve, and the cotyledon helps absorb and transfer nutrients from it to the embryo. Thus, saying that “the cotyledon feeds the seedling” is often a useful summary, but it does not mean every seed stores all its food in the cotyledon.
As the embryonic root grows, it begins anchoring the seedling and taking up water and dissolved minerals. The shoot develops toward the surface, where its leaves can receive light. The cotyledons support growth during this transition; they do not replace the roots or provide an unlimited food supply. Their contribution is temporary, lasting until the plant can sustain growth more independently.
Food storage and transfer: two common patterns
Botanists often describe seeds as having cotyledons that are storage organs or cotyledons that function mainly in absorbing stored food from another tissue. These are broad patterns rather than an absolute division: the details vary by species.
- Food stored in the cotyledons: In many beans and peas, the cotyledons are thick and contain much of the seed’s reserve. They provide materials to the growing seedling and may shrink as those reserves are used.
- Food stored mainly in endosperm: In seeds such as corn, the endosperm remains a prominent food-storage tissue. The single cotyledon, called the scutellum in grasses, absorbs nutrients from the endosperm and passes them to the embryo.
These examples show why seed structure should not be reduced to a single diagram. A seed can have a cotyledon and a separate storage tissue, and the cotyledon’s role may be to connect the reserve with the developing plant rather than to hold most of it itself.
What happens when cotyledons emerge
In some plants, the stem-like region below the cotyledons lengthens and carries them above the soil. This is called epigeal germination. The exposed cotyledons may become green, spread out, and photosynthesize for a time as well as draw on stored reserves. Beans commonly show this pattern.
In other plants, the cotyledons stay below the soil or near the seed while the shoot emerges. This is called hypogeal germination. Peas are a familiar example: their cotyledons remain underground and supply stored food while the shoot grows upward. These terms describe where the cotyledons are in relation to the soil during germination; they do not indicate that one pattern is better than the other.
Whether cotyledons appear above ground can help identify a seedling, but it is not a foolproof identification method. Cotyledons often differ from the later “true leaves” in shape, size, or arrangement. The first true leaves develop from the shoot and tend to show features more characteristic of the plant.
Cotyledons compared with true leaves
Cotyledons are formed as part of the embryo before the seed germinates. True leaves develop later from the growing shoot. Both can be green and photosynthetic, but their developmental origin and usual role differ. A cotyledon is an embryonic structure; a true leaf is part of the plant’s continuing shoot growth.
This difference explains a common gardening concern: cotyledons may yellow, shrivel, or fall away after a seedling has produced true leaves. That change is often part of normal development, especially when the cotyledons have supplied stored reserves. It does not by itself prove that the seedling is unhealthy. Look at the whole plant, including new growth and the condition of the stem and roots, rather than judging it from one fading seed leaf.
Conversely, damage to cotyledons can matter when a very young seedling still depends on them. The effect depends on the plant’s stage, whether the cotyledons are above or below ground, and how much reserve remains elsewhere. Avoid pulling, cutting, or handling them unnecessarily when identifying or caring for a new seedling.
Monocots and eudicots
Monocots have one cotyledon, while eudicots have two. Corn, grasses, and lilies are examples of monocots; beans, peas, and many broad-leaved garden plants are eudicots. The names refer to the number of embryonic seed leaves, not to the number of leaves a mature plant will have.
This distinction is a useful starting point in plant biology, but it is not a complete guide to identifying every plant. Other traits, such as leaf venation, flower structure, and root patterns, also help describe groups of flowering plants. Nor does the number tell you precisely where the seed’s food is stored: monocots and eudicots show variation in the relationship between cotyledons and endosperm.
Common misconceptions and practical observations
- “Every cotyledon is a food store.” Not exactly. Some cotyledons store much of the reserve; others mainly absorb food from endosperm.
- “Cotyledons are just the first ordinary leaves.” They are embryonic leaves, developmentally distinct from true leaves, even when they turn green and photosynthesize.
- “A seed with two cotyledons has only two leaves.” No. Cotyledon number describes the embryo, not the plant’s eventual leaf count.
- “Fading cotyledons always mean trouble.” They can naturally lose color as their reserves are used, although a seedling’s overall condition still matters.
For observation, compare a freshly opened seed with a seedling at different stages. Note how many cotyledons are present, whether they are above or below the soil, and when true leaves appear. Keep in mind that a seed’s exterior may be tough and that not every seed can be opened without damaging the embryo; a prepared diagram or classroom specimen can be a better option.
Frequently Asked Questions
What is the main function of a cotyledon?
A cotyledon helps the embryo and young seedling get established. Depending on the plant, it stores food, absorbs and transfers food from the endosperm, and sometimes photosynthesizes after emergence.
Do cotyledons become leaves?
They are already embryonic leaves, but they are not the same developmental structures as the plant’s later true leaves. In some species they become green and leaf-like; in others they remain underground or wither as the seedling grows.
Why do cotyledons fall off?
They may be shed after their stored resources have been used and true leaves are supporting the plant. Timing varies, so falling cotyledons should be interpreted alongside the seedling’s new growth and general condition.
How many cotyledons does a seed have?
Flowering plants commonly have either one or two: monocots have one, and eudicots have two. The number is a feature of the embryo, not a count of the plant’s mature leaves.
What is the difference between a cotyledon and endosperm?
A cotyledon is part of the plant embryo. Endosperm is a separate seed tissue that can store nutrients; in some seeds, the cotyledon absorbs and transfers those reserves to the developing embryo.
In brief
The cotyledon is the seed’s embryonic leaf, and its central job is to support early growth. It can store food, take up and deliver food from endosperm, and sometimes contribute photosynthesis after germination. Its form and location vary, but in every case it is part of the seed’s transition from dormant embryo to growing seedling.