In seeds that dehydrate at maturity, the germination process is initiated by the imbibition of water by the dry seed. If imbibition is prevented by impermeable seed coats, as with hard seeds, germination cannot occur. Seeds with permeable seed coats generally exhibit a three-phase process of water uptake consisting of imbibition, activation or germination sensu stricto, and growth phases. Germination sensu stricto is completed by the initiation of the growth phase of the embryo. Since plant cell growth is driven by water uptake into expanding cells, water relations would be expected to be involved in the transition of seeds into the growth phase. Before considering the water relations of germination, a short discussion of the temperature relations of germination is helpful to establish some fundamental concepts. Most seeds exhibit minimum, optimum, and maximum temperatures for germination. Germination capacity and rate generally increase progressively and coordinately during seed maturation.