Dicot-
The primary root is what emerges through the seed coats as the seed is still in the soil. The hypocotyl, which is below the cotlyedons, comes out from the seed coats and pushes its way through the soil. It has a slight arch -hypocotyl arch- to it as it begins to grow up. The two cotyledons protect the plumule, which is the epicotyl-above the cotyledons-and first leaves, from mechanical damage. Once the hypocotyl arch emerges from the soil, it straightens out. This response is triggered by light. In many dicots, the cotyledons not only transfer their food stores to the developing plant, but also turn green and make more food by photosynthesis, until they drop off.
Monocot-
The primary root stabs into the seed's coverings and grows down. Then, the primary leaf of the plant grows up. It is protected as it pushes up through the soil by the coleoptile; which is a hollow, cylindrical structure. Once the seedling has grown above the surface, the coleoptile stops growing and the primary leaf stabs it as well.
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