There is a tendency to compare the mesocotyl of the corn plant and hypocotyl of the soybean plant and there are some similarities.  For example, with corn the lengthening mesocotyl pushes the coleoptile to the soil surface.  With soybeans, the lengthening hypocotyl drags the cotyledons and growing point to the soil surface.  However, this is about the limit of the similarities.  Here is a summary of soybean germination, emergence, and the importance of the hypocotyl…

The soybean seed contains embryonic structures that will develop into the typical soybean plant.  Three recognizable embryonic nodes are present in the seed and include the cotyledon node, the unifoliate node, and the first trifoliate node.  Initial cell division begins about 36 to 48 hours after the soybean seed imbibes water.  The first recognizable structure to emerge from the soybean seed is the radicle.  The radicle grows downward and initially anchors the seedling for the next phase of growth.  Once the radicle has emerged and anchored the seedling, the hypocotyl begins elongation and drags the cotyledons (seed leaves) and growing point to the soil surface.

The top of the hypocotyl forms a bend or “arch” as it drags the cotyledons to the soil surface.  If an insect or pathogen attacks the hypocotyl arch, or if the soil is compacted, the arch may break.  When this happens, the soybean plant is finished.  In the same way, if the hypocotyl, itself, is damaged by pest feeding, disease infection, or mechanical injury the whole emergence process can come to a halt and the soybean seedling is finished.

Swelling of the hypocotyl is symptomatic of soil compaction or sometimes herbicide injury.  A number of pests may attack the growing hypocotyl.  These may include white grubs, cutworms, seedcorn maggots, slugs, millipedes and others.  Pathogens that are known to attack the soybean hypocotyl include Pythium, Phytophthora, and Rhizoctonia.  Some of these pests also attack the hypocotyl arch.

Under favorable growing conditions, the hypocotyl (with its arch at the top) drags the cotyledons and growing point upward until the hypocotyl arch is exposed to sunlight.  When light hits the hypocotyl arch, the arch straightens and lifts the cotyledons and growing point free of the soil surface.  Continuing cell division results in familiar stem growth and node development, with the cotyledon node followed by the unifoliate node, followed by the first trifoliate node.  The cotyledon and unifoliate nodes are at opposite sides of the stem.  The first, and subsequent, trifoliate nodes are all in an alternate arrangement on the stem.

Under favorable soil conditions, the soybean hypocotyl was historically capable of lengthening far enough to foster soybean emergence from a seed planting depth of 4 inches.  However, newer soybean varieties may only be capable of emerging from depths a little over 2 inches.  This apparent reduction in hypocotyl elongation does not appear to have been intentional but is a by-product of selecting for higher yielding genetics.  In any case, soybeans should not be planted deeper than 2 inches, and 1 to 1½ inches may be more reasonable.  When in doubt, test a variety’s capacity for hypocotyl elongation by planting representative seeds in field soil in a bucket or pot prior to planting the seed in the field.