Seed development can be a prolonged and complicated process. When the ovule is large and has a significant ontogenetic potential its volume may increase manyfold. On the other hand, when the ovule is small, seed development is often simple and of short duration. Seeds may contain specialized structures, such as a hypostase and an epistase. Both terms indicate a specialized tissue of often thick-walled, sclerotized cells with sometimes tanniniferous contents at the level of the insertion of the two integuments and in the micropylar part of the seed, respectively. Both structures have a blocking function by closing the chalazal opening and the micropyle in the mature seed. The seed coat develops from the ovular tissues and has as such the genetical characters of the mother plant. After fertilization the ovule is characterized by a great deal of mitotic activity without much cell expansion. Seeds may exhibit structural adaptations to regulate or facilitate imbibition or germination.