A cotyledon is a significant part of the embryo within the seed of a plant. Upon germination, the cotyledon usually becomes the embryonic first leaves of a seedling. The number of cotyledons present is one characteristic used by botanists to classify the flowering plants (angiosperms). Species with one cotyledon are called monocotyledonous and placed in the Class Liliopsida (see monocotyledons). Plants with two embryonic leaves are termed dicotyledonous and placed in the Class Magnoliopsida (see dicotyledons).
The cotyledon of grasses and many other monocots is highly modified, and composed of a scutellum and a coleoptile. The scutellum is a tissue within the seed that is specialized to absorb stored food from the adjacent endosperm. The coleoptile is a protective cap that covers the plumule (precursor to the stem and leaves of the plant).
Gymnosperm seedlings also have cotyledons, and these are typically numerous (multicotyledonous) but variable. For example, a pine seedling usually has about eight or so narrow cotyledons forming a whorl at the top of the hypocotyl (the embryonic stem) surrounding the plumule.
The cotyledons may be ephemeral, lasting only days after emergence, or persistent, enduring a year or more on the plant. The cotyledons contain (or in the case of gymnosperms and monocotyledons, have access to) the stored food reserves of the seed. As these reserves are used up, the cotyledons may turn green and begin photosynthesis, or may wither as the first true leaves take over food production for the seedling.