Observations on the origin of Chloroplasts and Mitochondria in the leaf cells of higher plants

The chloroplasts and mitochondria in embryonic and mature mesophyll cells of maize, spinach, and barley were studied by electron microscopy. The observations are discussed in relation to theories of organelle status and origins. Some of the e!ectron micrographs can be interpreted as showing that mitochondria arise from chloroplasts in mature leaf cells, and lend support to the view, based on cinephotomicrographic studies, that interconversions between chloroplasts and mitochondria like bodies occur in living mature leaf cells. In contrast, other electron micrographs can be interpreted as showing that mitochondria do not arise from chloroplasts, and that the cinephotographic observations refer to pre-existing mitochondria that have been enclosed previously by the deformable chloroplast. Electron micrographs of developing cells, in which the number of organelles increases, do not support the possibility of interconversions of organelles in young cells. To the contrary, the data for the young cells suggest that both organelles undergo divisions, supporting the view that the organelles are autonomous bodies. Some of the difficulties of correlating and interpreting electron microscope and light microscope observations of the same events are stressed.