The first leaves to form on a plant are called cotyledons, often referred to simply as seed leaves. These temporary organs are integral components of the plant embryo, representing the earliest stage of leaf development. They perform specialized functions immediately following germination to support the young plant’s survival. Their presence and number are fundamental for botanists to classify flowering plants into two major groups.
The Definition and Embryonic Origin
Cotyledons are the embryonic leaves developed within the seed before germination begins. They are formed during embryogenesis, the process where the basic body plan of the plant is laid out inside the protective seed coat. This makes them fundamentally different from all subsequent leaves, which form much later in the plant’s life cycle.
The number of cotyledons a plant possesses is a primary classification trait for angiosperms, or flowering plants. Plants with a single seed leaf, such as grasses, corn, and lilies, are known as monocotyledons or monocots. Plants that develop with two seed leaves, including common examples like beans, oaks, and sunflowers, are called dicotyledons or dicots.
Function: Initial Energy Supply and Photosynthesis
The primary role of cotyledons is to provide the initial energy and nutrients required for the seedling to emerge and establish itself. In many plants, especially dicots like the garden bean, the cotyledons swell to store food reserves, such as starches and oils, that fuel the first stages of growth. These reserves are mobilized and transferred to the growing root and shoot once the seed absorbs water and germination is triggered.
The specific function of the cotyledon changes depending on how the seedling emerges from the soil. In epigeal germination, the stem below the cotyledons elongates, pulling the seed leaves above the soil surface. Once exposed to light, these cotyledons often turn green, develop chloroplasts, and begin performing photosynthesis to create food for the rapidly growing seedling.
In contrast, plants with hypogeal germination, such as corn and peas, keep their cotyledons underground. In these cases, the seed leaves function exclusively as an absorptive organ, drawing stored nutrients from the nearby endosperm or their own tissues. Because they remain buried and never reach the light, their role is purely nutrient transfer rather than photosynthesis.
Distinguishing Cotyledons from True Leaves
While they are called seed leaves, cotyledons are developmentally and structurally distinct from the true leaves that form later. Cotyledons are pre-formed in the embryo, whereas true leaves develop post-embryonically from the shoot apical meristem. The appearance of the first true leaves signals a significant shift in the plant’s growth strategy.
A key morphological difference is that cotyledons are often simple in shape, such as oval or kidney-shaped, and rarely resemble the mature foliage of the plant. The true leaves, which emerge after the cotyledons, possess the characteristic shape, venation, and complexity of the adult plant’s leaves. For example, a maple seedling’s cotyledons are simple ovals, but its true leaves will have the familiar lobed shape.
Once the true leaves have developed sufficiently to take over sustained photosynthesis, the cotyledons have served their purpose. They typically wither, yellow, and eventually fall off as the seedling transitions to a self-sufficient, independent plant.