When the seed of a plant germinates, that first leaf or set of leaves to unfold is called a cotyledon.
Cotyledons are developed by the embryo of the seed and are sometimes called seed-leaves. They contain stored food reserves from the seed, used to keep the seedling fed until the next set of leaves, considered the "true leaves," appears. Once the second set of true leaves sprouts, the plant will begin photosynthesis. The cotyledons will wither and disappear soon after the true leaves start to grow.
Some plant seeds produce one cotyledon leaf and some seeds produce two cotyledons.
Flowering plants with one cotyledon are called monocotyledonous (or "monocots"). Examples of monocots include daylilies, tulips, grasses, wheat, barley, corn, and onions.
Plants with two embryonic leaves are called dicotyledonous ("dicots"). Examples of dicots include roses, chrysanthemums, carnations, beans, mustard, peppers, squash, and tomatoes.
The cotyledons will fall off as the true leaves emerge. Most cotyledons are nondescript, while the true leaves resemble the leaves of the mature plant.