Abstract: Maturation drying is a terminal event in the development of orthodox seeds, The desiccation tolerance of embryos becomes more tolerant as seeds mature and less tolerant as seeds germinate. The protective substances, such as sugars, proteins and antioxidant &fence systems, are observed to accumulate in orthodox seeds when tolerance is acquired and to degrade when tolerance is lost. Maturation drying could be a "switch" in the transition from seed development to germination, and decline the syntheses of developmental proteins and developmental mRNAs. And the developmental events are terminated and germinant events are stimulated by maturation drying. Recalcitrant seeds, which do not undergo the developmental stage of maturation drying, are highly sensitive to desiccation throughout seed development.