The cotyledon is the part of the embryo that lies in the seed of a plant. It becomes the embryonic first leaves of a seedling when germination occurs. Various other parts of a seed exist, including the endosperm -- the food supply that is contained in the seed and sometimes specifically in the cotyledon. In addition, the hypocotyl is the part that develops into the stem, while the plumule is the leaf in the plant's early development.
A valuable function of the cotyledon is to help botanists classify flowering plants, also called angiosperms. For example, flowering plants that have one cotyledon -- or one seed leaf -- are called monocotyledonous, or monocots, and are placed in the Class Liliopsida. Examples of these plants include orchids, bulb plants, greenbriers, sedges, lilies and cattails. Meanwhile, plants that have two cotyledons are dicotyldons, or dicots, and placed in the Class Magnoliopsida. Dicots include blackberries, roses, daisies, sunflowers, maples and oaks. Most trees and flowers are dicots.
The cotyledons in some cases are major photosynthetic producers during germination. As photosynthetic producers, cotyledons essentially can synthesize the organic nutrients it requires for growth through photosynthesis. Photosynthesis is the process in which organisms use the energy from sunlight to produce glucose, a type of sugar, in addition to releasing oxygen. The sugar is the plant's food.
The cotyledon encourages leaf growth in a plant's early stages of life. These leaves are important because they serve as the chief way for plants to get the nutrients they need to continue growing and thriving. When cotyledons become new green leaves, the cotyledons raise over ground level. In another process that involves plants such as peas, however, cotyledons remain underground and solely serve the function of a food source.
Cotyledons transmit the food stored in the embryo to new sprouting plants. Plants such as beans and peas -- which are dicotyldons -- have thick cotyledons that drew nutrients from the seeds prior to sprouting. Meanwhile, grass -- an example of a monocotyledonous -- has a single thin cotyledon that absorbs nutrients from the seeds as they are sprouting.
The cotyledon essentially allows a plant's embryo to begin creating new life following germination. This is a critical function because it shows that without the cotyledon, a plant essentially would not be able to breed. With the help of the cotyledon, plants can spread their seeds and boost their species' population at a rapid rate.