Cell fine structure of developing cotyledons and embryonic axes ofPhaseolus lunatusL. seeds was studied relative to stage of seed development and the ability of excised embryonic axes to grow and withstand desiccation. Two stages in the development of the seed were recognized: (a) a ripening stage, characterized by more rapid increase in dry weight than in fresh weight, resulting in a decrease in seed moisture to 60%, and (b) a maturation stage during which seeds lost water without increase in dry weight. Above 70% water, excised embryonic axes failed to grow. Complete potential for growth developed by the time the seed reached 55–50% water. The axes were able to grow before they developed the ability to withstand desiccation. In cells of the youngest axes the ribosomes tend to be grouped into polysome units which surround the endoplasmic reticulum. As the cells mature the polysomes disappear and free ribosomes appear in the cytoplasm. Mitochondria in young axes contain electron‐dense matrices and swollen cristae; with maturation the mitochondrial matrix becomes optically empty and the cristae less swollen. Chloroplasts in young cells contain grana which disappear as the seeds mature. Similar changes occur in the cells of cotyledons. The data suggest that changes in cell fine structure during maturation are not destructive changes caused by excessive loss of water, but they indicate physiological changes which may be necessary to prepare the cells to withstand desiccation.