The cotyledon, which forms in the embryo of a seed before germination, stores food for the embryo. Along with the endosperm, the cotyledon nourishes the new growth of the plant. It is the part of the seed that emerges from the testa, or hard covering, during germination. It sometimes grows upward, turning into a set of leaves as the seed germinates, using photosynthesis to nourish the newly forming plant further.
The leaves formed by the cotyledon sometimes drop off as the plant's first real leaves, which look different from the leaves formed by the cotyledon, emerge. Other times, they remain a part of the plant for years. A cotyledon may also remain underground rather than shooting up as a set of leaves, where it functions as a storage area for food for the plant.
Seeds can have one cotyledon or a pair of cotyledons. Those with one are called monocots, while those with two are classified as dicots. Seeds that are dicots have cotyledons that function as leaves, while monocots have cotyledons that are appendages for food storage. Seeds that store their food in cotyledons inside the embryo are called exalbuminous. Seeds can remain in a dormant state for years, only germinating when conditions are right.