Seeds contain dormant embryos with a food supply and a protective coat. They can usually sense the environment, causing the young plant to resume its growth when conditions favor survival. All seeds need oxygen, warmth, and water to germinate. Some seeds need light too, and others need dark or prolonged chilling or a treatment that weakens the seed coat. All these requirements help to assure the seedling's survival. When the root emerges, the seed has germinated. Seeds often germinate underground, and the delicate shoot tip and young leaves need protection as they force their way through the soil. Here you see a bean seed and a maize grain. Let's review their protective systems. In beans and many other seeds, the cotyledons enclose the delicate shoot tip until they emerge from the soil. In addition, the shoot forms a hook that pulls the cotyledons through the soil rather than pushing. Light tells the shoot it has emerged from the soil; the hook straightens and the cotyledons unfold. The maize cotyledon stays underground; the young leaves are sheltered inside a finger-shaped sheath that pushes through the soil. Light tells the sheath when it has reached the open air. Then it stops growing and the leaves emerge. Other grasses do the same.