A structure in the embryo of a seed plant
that may form a `leaf' after germination and
is commonly known as a seed leaf. The number
of cotyledons present in an embryo is an
important chararcter in the classification of
flowering plants (angiosperms).
Monocotyledons (such as grasses, palms, and
lilies) have a single cotyledon, whereas
dicotyledons (the majority of species) have
two. In seeds that also contain endosperm
(nutritive tissue) the cotyledons are thin,
but where they are the primary food-storing
tissue, as in peas and beans, they may be
quite large. After germination the cotyledons
either remain below ground (hypogeal) or,
more commonly, spread out above soil level
(epigeal) and become the first green leaves.
In gymnosperms there may be up to a dozen
cotyledons within each seed.