Seedling Changes

The initial roots growing from the embryo radical supply the emerging seedling with water and nutrients. Other changes then occur.
 
After the first corn leaves emerge, the hormonal message to the mesocotyl tissue is to stop pushing upwards. Apical meristem, at the tip of the mesocotyl is now below the soil surface where the first leaf is attached. Photosynthesis now drives the metabolism of the young seedling as it switches from dependence upon the seed endosperm for carbohydrates.  Cell division in the meristem produces new leaves, each attached to the young stem under the soil surface and attached in distinct clusters of newly dividing cells called nodes.  These nodes are then stimulated to produce roots at about the time the 4th leaf appears in the young seedling.  Because the roots are being produced from stem tissue, they are called adventitious roots.  As the primary root, that had grown initial seed, loses its energy source, adventitious roots become the main roots for the plant.
 
The first 4-5 nodes of the young stem remain underground, each producing the roots for the plant, even as the first leaves remain attached at the same locations.  What appears to be stem in a 4-5 leaf seedling is a compilation of leaf sheaths tightly wrapped together while the actual stem remains beneath the surface. The underground stem portion, formerly attached to the mesocotyl, with adventitious roots becomes known as the crown.  Eventually the mesocotyl deteriorates as it is deprived on nutrition and loses resistance to the many soil organisms.
 
As the stem growing point eventually emerges above the soil surface a few exposed nodes will often form the brace roots to further support the adult plant. A lot of changes in a relatively short time after seed is planted.