Abstract: The invention relates to a novel method for simple and direct determination of the actual photosynthesis (the gross photosynthesis), the light respiration and the uptake and emission of CO2, O2 and H2O in connection with photosynthesis and respiration in plants. The invention also relates to a device for carrying out the method and use of the method for evaluation of the growth of algae and plants. The method of the invention is based on the new recognition that photosynthesis is not a chemical process but rather a one step physical reaction. The method is very different from the methods applied so far and much more accurate than these known methods. It has a great potential utility in agriculture and forestry. The method can be worked from great heights using aeroplanes or satellites. It can be used in all types of apparatuses, computers and computer programs designed for estimation of plant productivity and photosynthesis.