This part of the embryo supplies food to the growing young plant.
For something so straightforward, gardening is full of confusing terms, obscure Latin names, and contradictory terms. Although you won't see them used often, they are useful terms to know when you are trying to key out or identify a plant.
Cotyledons
Cotyledons are the first leaves produced by plants. Cotyledons are not considered true leaves and are sometimes referred to as "seed leaves," because they are actually part of the seed or embryo of the plant.
The seed leaves serve to access the stored nutrients in the seed, feeding it until the true leaves develop and begin photosynthesizing.
In the photo provided, the two narrow leaves lowest on the stem are the cotyledons. The small, crinkled leaves on top are the first true leaves of this seedling. The cotyledons will fall off as more true leaves develop. Most cotyledons look similarly nondescript, while the true leaves resemble the leaves of the mature plant.
Monocots and Dicots
Flowering plants are divided into two classes: Monocotyledones (monocots) and Dicotyledones (dicots). As the names imply, the main distinction is the number of cotyledons present in the seed embryo–1 or 2. There are several other differences:
Examples of Both Monocots and Dicots
Monocots include most of the bulbing plants and grains, such as agapanthus, asparagus, bamboo, bananas, corn, daffodils, garlic, ginger, grass, lilies, onions, orchids, rice, sugarcane, tulips, and wheat.
Dicots include many of the most popularly grown garden flowers and vegetables, including legumes, the cabbage family, and the aster family, such as apples, beans, broccoli, carrots, cauliflower, cosmos, daisies, peaches, peppers, potatoes, roses, sweet pea, and tomatoes.
Is Classification Important?
It's one of those things that pops up occasionally in garden books and leaves you scratching your head or feeling a little less knowledgeable, but it shouldn't. While it's nice to know, it doesn't really make a difference in how you grow or care for plants. It's not even all that accurate a way of dividing plants.
Although the idea behind these classifications is to help in identifying plants, there is disagreement over the validity of dividing plants into these two classes. Some of the other traits used to classify can overlap. For example, there are exceptions in the number of flower parts, the arrangement of leaf veins, the vascular tissue in the stem, pollen structure, and root development. That's for the botanists to argue out. For gardeners, it's just good to be aware that you may still find plants classified in this way.
Not All Plants Follow the Rules
Not all plants have cotyledons, which means they are neither monocots or dicots. Plants that form spores, such as ferns, and plants that form cones, as with most evergreens, do not produce cotyledons. However, all plants that flower can be divided into either monocots or dicots.