**5.6 Germination inhibitors**

Many species' seeds fail to germinate even when the embryos are completely developed when the seed is ripe, even though the environmental conditions are excellent. In such seeds, dormancy is caused by the physiological state of the embryo. The embryos of such seeds will not grow when they initially mature, even if the seed covers are removed. Inhibitors may be present in the embryo (example, in Xanthium), endosperm, (example, in Iris) or in the seed coat (example, in Cucurbita). Abscissic acid (ABA) is one of the most commonly detected inhibitors of germination.
