Cotyledons

A cotyledon is an organ found in embryonic vascular plants before adolescent development. A vascular plant may have one or two cotyledons; of the angiosperms, monocot seeds possess one cotyledon and eudicots possess two. This difference is a distinguishing trait between the clades and is the etymological origin for the clade names.

The cotyledon is found in the sperm and underneath the seed coat after fertilization. Its function and appearance varies depending on whether or not the embryo relies on it for food storage. If the embryo depends on the endosperm for food storage, the cotyledon is not fleshy and is used for absorbing nutrients. If the embryo depends on the cotyledon or cotyledons, the organ is fleshy and full of nutrients, and the endosperm is often absorbed into either the embryo or the cotyledon. In plants where the latter is true, such as garden beans and various nuts, this cotyledon is preserved and eaten as commercial food.

The cotyledon is a round, ovaline structure that borders the embryo axis. Eudicot cotyledons surround the embryo. In both cases, the cotyledon is attached to a single node on the embryo axis, and both the cotyledon and embryo are often bent within the shell. When broken apart in the shell, such as in garden beans and nuts, the cotyledon is broken apart.

In adolescence, the node the cotyledons connect to is represented as the node of the first opposite simple leaves that appear on the stem. These leaves are actually the remaining structure of the cotyledons and are not “true” leaves; they are separated from the roots and the true leaves by the hypocotyl and epicotyl regions of the stem respectively. Once the plant has turned green and is able to photosynthesize, the seed leaves may shrivel up and fall off, leaving behind a notch on the stem axis where they were.

While the embryo develops, the cotyledon is often pulled upwards with the growth of the plant. This represents the formation of the primary root or roots from the radicle, which pulls the cotyledon and the epicotyl of the embryo from the ground.