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. BJÖRKMAN, 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.