[Relations between CO2-exchange and transpiration in bryophyllum daigremontianum]

1. The transpiration in leaves of Bryophyllum daigremontianum exactly follows the changes in consumption of atmospheric carbon dioxide (caused by the Crassulaceen acid metabolism) during the light and dark periods. After removal of the epidermis no distinct rhythm in the course of transpiration can be observed any more, whereas the characteristic CO2 exchange continues in an unchanged matter. For this reason we assume that the changing rate of CO2 uptake from the atmosphere determines the concentration of carbon dioxide in the intercellular spaces of the leaves and in this way controls the opening of stomata. 2. CO2 uptake from the atmosphere in the light phase decreases faster than CO2 consumption in the dark when the plants are held under water stress conditions. At the endpoint CO2 is fixed only in the dark period. On the basis of the connection between CO2 uptake and movement of stomata we assume a closure of the stomata during the light period (since no extracellular CO2 is fixed). Since evaporation values in the light phase are high under natural conditions, this manner of gas exchange minimizes the loss of water during water stress conditions, and nevertheless guarantees a positive balance of carbon.