Bean leaves were supplied 14CO2 in light or darkness. After a brief flush, the total and specific radioactivity of respired CO2 were measured when leaves were kept in light or darkness either in air or in CO2-free air. Changes in the specific radioactivity of respired CO2 show that photorespiration and dark respiration are different processes having different and separate substrates. Little, if any, CO2 production by the process of photorespiration could be detected in darkness. The process of dark respiration was about 75% suppressed in light. Measurements of ATP levels in light or darkness and in air or CO2-free air suggest that dark respiration is controlled by photosynthetically produced ATP. This is supported by the fact that isolated intact spinach chloroplasts transfer most of their photosynthetically produced ATP to the medium.

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Stem explants, excised from greenhouse‐grown Begonia rex plants, were cultured on basal medium (T. Murashige and F. Skoog, Physiol. Plant. 15: 473–497, 1962) contained in sterile Petri dishes. The medium was supplemented with benzyladenine (0.1 mg 1−1) naphthaleneacetic acid (0.01 mg 1−1) and, according to experimental requirements, with either sucrose (3%) or mannitol (3%). Histochemical and biochemical examination of the starch content of the explant was carried out over several days. There was no starch deposition or organogenesis in tissue cultured on mannitol and carbohydrate‐free growth medium. The most dramatic finding was the heavy accumulation of starch in tissue cultured on sucrose medium. This copious accumulation preceded any organ formation and was mainly in regions which ultimately gave rise to shoot primordia. The heavy build‐up of starch preceding organogenesis was also observed when explants previously cultured on mannitol medium were transferred to medium containing sucrose. During shoot primordia development there was a decrease in the starch content of the cultured tissue indicating the utilization of the polyglucan in the organogenic process.

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