Abstract
Semithin and ultrathin sections of callus tissues of African rice (Oryza glaberrima Steud) after 7 and 14 days of initiation were compared with similar cultures after 3, 5, 7 and 10 days of incubation on a differentiation medium. Histological examinations indicated that at the early stages of development, both types of callus tissue appear structurally similar with small, almost isodiametric and meristematic cells located toward the callus periphery. In addition to this cell type, cultures on a differentiation medium also contained large parenchymatous and highly vacuolate non-differentiating cells. Ultrastructurally, while the former were characterized by a relatively large number of organelles, the non-differentiating cells contained only a thin peripheral layer of cytoplasm. Later in culture, the differentiating cells undergo periclinal divisions resulting in the formation of radial files of tussue which correspond to the meristematic regions where shoot primordia differentiation is generally initiated. There is also evidence to show that the differentiation of shoot and/or root primordia in callus tissues is a high energy demanding process. This energy requirement is fulfilled by the accumulation, mobilization and utilization of starch and other reserve substances prior to the onset of plantlet regeneration.
