Corn seed germination and tropism

The mature embryo in a corn seed includes the cells already programmed to become the first 5 or 6 leaves, the primary root and a few secondary roots as well.  The cotyledon of this monocot is called the scutellum. This is the large white portion of the seed that remains attached to the endosperm and becomes the main conduit of energy for germination.
 
Imbibition, and sufficient heat energy, initiates germination which is mostly a process of cell elongation in the root and shoot areas of the embryo.  Root tip cells are surrounded by a special layer of cells (coleorhiza) that act as a protective covering when the root tissue, also called the radical, pushes through the pericarp of the seed.  At the other end of the embryo, a coleoptile protects the shoot tissue all the way to emergence from the soil.  This becomes the ‘spike’ that we first see in the field.
 
Root tips include special cells with organelles (statoliths) that are heavier than other parts of the cell.  Consequently, they accumulate on down side of the outer layer cells of root tissue.  These cells lead to production of hormone-like chemicals (auxins) that inhibit root cell elongation on the lower side of the emerging root.  With greater cell length on the upper side, the root grows downwards, regardless of the orientation of the seed when planted.
 
Shoot tips cells also produce similar organelles also affected by gravity.  They also produce auxins but these hormones have the opposite affect on shoot cell elongation.  Those cells on the gravity side with more auxin become longer than those on the upper side.  Consequently the shoot grows upwards.
 
Affect of gravity on plant growth direction is called geotropism.  After shoots emerge, phototropism becomes dominant, causing the plant to grow towards light because cells on the shaded side produce more auxin and consequently longer cells.