An ovule passes through various contiguous phases during its development into a seed (Fig. 1). The first phase, histodifferentiation (or initial morphogenesis) (27), begins with fertilization and rapid cell division, leading to formation of the embryonic axis and of tissues that eventually accumulate reserve materials (3). Following this is the maturation or seed expansion phase, during which there is a marked increase in seed mass; cell expansion occurs as reserve deposition takes place. The 3rd and final phase, maturation drying (or desiccation), is characterized by a general reduction in metabolism as water is lost from the seed, which then passes into a quiescent, and sometimes dormant, state. This last phase is characteristic of temperate zone crops, but does not necessarily occur in tropical plants.