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Cotyledon is one of the multiple parts of a seed and it is formed inside the seed, during the process of embryo-genesis. The number of cotyledons within a seed separates the plants in two groups: monocotyledons (with one cotyledon) and dicotyledons (with two cotyledons). The cotyledon in dicotyledons functions as leaves in plants, nourishing the plant at the beginning of growing process.
The cotyledons can be also classified with regard to its position: epigeal cotyledons whose location is above the ground and hypogeal cotyledons whose location is under the ground. The position of cotyledons decides if they become photosynthetic, or not, hence, the epigeal cotyledons (usually found in dicotyledons) are photosynthetic, while hypogeal cotyledons are not photosynthetic, they only helping the nutrition process.
The cotyledons can also be classified with regard to the time they last, since duration of some cotyledons can be measured in days, while the duration of other cotyledons can be measured in years.
A cotyledon can also be referred to as a seed leaf. With that being said, for a seed plant, it is the leaf of the embryo of the seed. How many leaves are present determine how the plant is classified. If one cotyledon is present, the plant is considered monocotyledonous and put in the Class Liliopsida, while two of them means the plant is dicotyledonous, and put in the Class Magnoliopsida. What distinguishes a cotyledon from an actual leaf, is that cotyledons are formed during germination, while actual leaves are formed afterwards.
On the link below, I've attached a picture showing the difference between a monocot and a dicot.
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