SUMMARY: Orchid aerial roots contained chlorophylls and were shown to be capable of photosynthesis. 14CO2 feeding in the light showed that the C3 pathway was operating during the day. Aerial roots also exhibited diurnal acidity fluctuations typical of crassulacean acid metabolism (CAM) and malate was the only labelled compound isolated after night 14CO2 feeding. CO2 exchange patterns in aerial roots were consistent with CAM CO., fixation during the night hours. The rate of photosynthetic CO., fixation in the light was much greater than the rate of CAM CO2 fixation in the night. Measurements of CO2 exchange, however, showed little or no net uptake indicating that these roots were not completely autotrophic.

SUMMARY: The stomata of Arachnis cv. Maggie Oei, Aranda cv. Deborah, Arundina graminifolia, Bromheadia finlaysoniana, Cattleya bowringiana×C. forbesii and Spathoglottis plicata (Orchidaceae) occur only on the lower epidermis of the leaves and are located within hyperstomatic chambers formed by cuticular ledges extending from the guard cells. Arachnis, Aranda and Cattleya have thick leaves which exhibit Crassulacean acid metabolism, and their stomata open when acidity levels are lowest, or shortly thereafter. Aranda and Arachnis require higher light intensities for sufficient deacidification to permit stomatal opening than Cattleya. Stomata of the thin‐leaved Arundina, Bromheadia and Spathoglottis open during the day. The stomatal rhythms, morphology and distribution, as well as the pathways of carbon fixation and light requirements for deacidification, reflect the natural habitat of each species or the parents of the three hybrids.