The mature grain contains both the embryo and, within the endosperm, enough stored food reserves to initiate plant growth. The shoot apex of the embryo has three or four leaf primordia and a tiller bud protected by the coleoptile. The root pole has a structure called the coleorhiza, which protects the root until it has broken through the seed coat. After sowing, the germination process starts with the absorption of water (imbibition) and the embryo sends out hormonal signals which induce the synthesis of hydrolytic enzymes in the aleurone. These enzymes degrade the cell walls, starch and storage proteins of the endosperm. The uniformity of this hydration process and the extent of the cell wall degradation are important aspects of malting quality. If the grain starts to germinate in the wheat ear before harvest (pre-harvest sprouting), as it can if the season is cool and damp, flour quality is poor reducing the value and end-use of the crop. Within a few days of imbibition roots start to grow and shortly after that the coleoptile emerges. The coleoptile will grow until it finds the soil surface, after which the leaves will emerge and a seedling will become established.
More information on the chemistry of germination is available here.