Summary Adult leaves, protocorms and seedlings of succulent tropical orchids exhibit diurnal fluctuations in acidity. The titratable acidity varied at different stages of growth. The fluctuation in acidity was barely detectable in protocorms but as they grew older, their capacity for acid accumulation increased. The O2 exchange in orchid protocorms and seedlings was similar to that of adult leaves in their responses to temperature, bicarbonate and light intensity. It is concluded that protocorms and seedlings resemble adult organs in having the characteristics of crassulacean acid metabolism (CAM).