Of all protoplasmic functions, the one which is, by tradition, most closely linked with our conception of vitality is the function for which the name of respiration has been accepted. It might, therefore, well be expected that every variation of the intensity of the metabolic activity of a cell would be correlated with some change in the respiration of that cell. Before we can decide whether respiration really holds this position as an index of the integrated activities of the cell, we need to accumulate respiration date for different types of plant organs throughout their life-history of development, maturity and senescence. These date must then be examined critically with the hope of establishing the nature of the major and minor determinants of the variations of intensity of respiration. No such collection of data has yet been published. The present paper aims at making a contribution to this collection and other contributions should follow. A good deal of work has already been carried out in the Cambridge Botany School upon respiration of evergreen leaves, which continue to exist in a state of maturity for very long periods of time. In striking contrast with this type of organ is the type of the ripening fruit. The natural biology of the two is so different that it is obviously of importance to establish whether the same fundamental principles are manifest in both. In the ripening fleshy fruit, senescence is the dominant stage of ontogeny. The fruit of the apple, which possesses such striking keeping properties, is most suitable for investigation, since it runs through its ripening senescence at a slow rate.