It was studied in phytotron that the relations between the photosynthesis in wheat and the ecological factors. The results obtained are as follows:1. When the CO2 concentration in the air is 340 to 350ppm and the water content of the soil is moderate, one of the important factories which caused the photosynthetic 'nap' is the obvious decrease of atmospheric relative humidity in the midday.2. When the ecological environment was moderate and more stable, the phenomenon of photosynthetic 'nap' was not observed...