Mesocotyl growth

The mesocotyl is technically part of the stem tissue because it developed from the initial apical meristem in the embryo.  As the seed imbibes water and begins germination, the cells of the mesocotyl begin to elongate.  Individual cell growth was once described in a plant physiology class that I attended just a few years ago (Professor Loomis at Iowa State University about 56 years ago).  He said it was like a balloon swelling with water.  The cumulative affect of the cell enlargement pushes the rest of the stem tissue towards the soil surface. The leading tissue is the shield-like coleoptile. Cell elongation continues until the coleoptile is exposed to the red light component of sunlight.  This stimulates production of a plant hormone that is transmitted to the mesocotyl cells causing them to stop elongating.
 
A few things can go wrong with this process. Temperatures influence the rate of growth of the mesocotyl cells, prolonging the exposure of the tissue to pathogens such as Pythium, a fungus that tolerates the lower temperatures.  If the soil is not adequately compacted and thus allows light to penetrate, the stop growth signals come before emergence of coleoptile.  If too compact, cell elongation is insufficient to push through.  Growth regulating herbicides, combined with cold soils may cause the mesocotyl growth to stop. Cell elongation being dependent upon water for swelling makes the pushing of the coleoptile to the soil surface as well.  Fortunately, all of these potential problems are rare, and the remaining development of the corn plant becomes less dependent upon the mesocotyl.