About five to ten days after planting, the new seedling arches through the soil surface (this is called emergence). The hypocotyl ‘hook’ (the emerging portion just below the cotyledons) begins to lengthen pulling the remainder of the seed upward, and the oval seed leaves (called cotyledons) open up. The cotyledons provide the seedling with a temporary source of food (plant useable nutrients originally stored as the seed was formed). The cotyledons quickly turn green and begin making additional food by photosynthesis. Shortly after the first set of true photosynthetic leaves is formed, the cotyledons drop off.