39 inches for most. That was stored in the seed. The cob will be almost full size, and the silks of the ear will begin to darken and dry out. Later planted corn will need a much longer fall.
At this period, the coleoptile is fully visible, yet no leaves are fully developed. Hybrid seed are the product of cross-pollination between 2 different parent plants, resulting in a new plant/seed that is different from the parents.