Hydration also triggers metabolic changes in the embryo that cause it to resume growth. Storage materials of the endosperm or cotyledons are digested by enzymes and the nutrients transferred to the growing regions of the embryo. The next step in the change from a seed to a seedling is the shoot tip breaking through the soil surface. In many dicots, a hook forms in the hypocotyl . Growth pushes the hypocotyl above ground.
Light stimulates the hypocotyl to straighten, raising the cotyledons and epicotyl.
The epicotyl then spreads the first leaves which become green and begin photosynthesis. Germination may follow different methods depending on the plant species. In peas, a hook forms in the epicotyl and the shoot tip is lifted by elongation of the epicotyl and straightening of the hook. The cotyledons remain in the ground. In monocots, the coleoptile pushes through the soil and the shoot tip grows up through the tunnel of the tubular coleoptile.