This book reports the proceedings of a meeting held in the 'Limburgs Universitair Centrum' , Diepenbeek, Belgium, August 26 to 30, 1974. In convening this meet ing, my aim was to bring together a small number of specialists working on photosynthesis of course but also always keeping in mind that plants are in fluenced by their environment (temperature, light quality and intensity, air com position, daylength . . . . . ) and can differently react according to their stage of deve lopment. In general, all these specialists work on whole plants cultivated in well known conditions (they are not 'market spinach specialists') but, when necessary, give up the idea of measuring photochemical activities in isolated they don't chloroplasts, enzyme kinetics . . . etc. It is noticeable that about 50% of them are working in laboratories directly involved with applied research in agriculture or forestry. The format of the meeting was intentionally kept small but it allowed generous time for discussion; thanks are due to Drs. O. BJOERKMAN, J. W. BRADBEER, M. M. LUDLOW and C. B. OSMOND for taking the chairs during these discussions. In such a small meeting, the choice of invited scientists was really a personnal one and thus reflected my own fields of interest. When planning the conference, I was continually divided between the wish for inviting other interesting people and the necessity of keeping time free for discussions.