Seed germination is an important part of the development of new plants. Germination is the initial phase of growth and emergence from the ground. Monocots and dicots, two different categories of plants, both undergo germination, although the process is different for each type.
Seeds and Germination
When the seed for a new plant falls off its parent, or is otherwise separated, it often waits for specific environmental conditions before it begins to sprout, or germinate. These conditions vary based on the plant in question. All seeds will require the presence of water. Some will have a specific period of dormancy before they germinate. Others might wait until the surface of the seed has been weakened by an outside force, such as an animal's metabolism. Some seeds even wait until the seed has been affected by fire -- this ensures that they grow right after a forest fire occurs, giving them lots of space and rich soil.
Germination of Monocots
Monocots have only one starting leaf, or cotyledon. Grasses are an example of a monocot. A monocot seed consists of the plant embryo, the cotyledon, and the endosperm, which is a nutritious package of food for the embryo. During germination, the embryo consumes the endosperm and also uses the cotyledon for food. The embryo sends a primary root out through the seed coating downward into the soil, and pushes a primary leaf up through the coating and soil to the surface. The primary leaf is covered at first by a coating called a coleptile, which protects it from damage. During the whole sequence, the cotyledon never emerges above ground.
Germination of Dicots
The sequence for dicots is somewhat different. Dicots have two cotyledons, which serve a different role than the single cotyledon in monocots. Beans are a typical dicot. In dicot germination, the primary root emerges above the soil from the top of the seed and dives back down into the soil again. Then the portion of the plant that is above ground, the hypocotyl, grows, bending upward into an arch and eventually lifting the seed above ground. The two cotyledons, which had been protecting and enveloping the embryo and endosperm, open, exposing two primary leaves. The cotyledons go on to produce food for the plant using photosynthesis until they eventually die and drop off.
In monocots, the cotyledon is just a thin leaf that usually does not emerge above ground. Dicot cotyledons contain their seed's endosperm, so they are thicker and fuller. The primary root in monocots grows downward, while in dicots the primary root first grows upward to the surface and then dives again. Both monocots and dicots use their cotyledons as food sources, but only dicots use them to produce more food through their photosynthetic metabolism.