Cotyledons

Cotyledons are the embryonic leaves formed during seed development. They are the first leaves to appear when a seed germinates and are commonly referred to as seed leaves. In flowering plants, cotyledons are part of the embryo and may serve as nutrient storage tissues or early photosynthetic organs, depending on the species and seed type.

In dicotyledonous plants (dicots), there are two cotyledons. They typically emerge as the seedling breaks through the soil surface. The cotyledons often either function as the primary photosynthetic organs or, in seeds with large nutrient reserves, act mainly as storage organs that feed the seedling until true leaves develop and photosynthesis can proceed.

In monocotyledonous plants (monocots), there is a single cotyledon. This cotyledon, called the scutellum in many grasses such as wheat and maize, acts to absorb nutrients from the endosperm via the aleurone layer. The monocot seedling commonly develops a protective coleoptile around the shoot and a coleorhiza around the root, while the scutellum supplies nourishment during germination rather than serving as a typical green seed leaf.

The role of cotyledons varies by species. Some cotyledons remain underground and store nutrients, while others become green and photosynthetic soon after germination. The presence and arrangement of cotyledons are key features in classifying flowering plants as monocots or dicots and influence early seedling development and nutrient strategy.