AN ULTRASTRUCTURAL-STUDY ON THE EARLY DEVELOPMENT OF ZEA-MAYS SOMATIC EMBRYOS

Friable embryogenic callus, obtained from immature embryos of Zea mays L., was cultured on N6 medium supplemented with 1 mg/L 2,4-dichlorophenoxyacetic acid, 6 mM proline, and 2% sucrose. Cultured tissue fragments containing several globular embryoids were excised and examined by light and electron microscopy to follow the early development of maize embryoids. The somatic embryos consist of an apical region and a suspensor region. Cells of the apical region are small, cytoplasm rich, and mitotically active. They contain much starch and numerous bundles of microtubules. Suspensor cells are larger and more vacuolated. A high metabolic activity in both cell types is indicated by the presence of many organelles, coated vesicles, and multivesicular bodies. Transition units appear to form intermediate stages between the embryogenic callus cells and the somatic embryo. A transition unit consists of a group of embryogenic cells and shows an apical and a basal region. The unit has many intercellular spaces, and within the cells areas with organelle-free cytosol are frequently observed.