This experiment was carried out to study the causes for "midnap" of wheat, i.e. photosynthetic rate decrease of wheat at midday. The main results are as follows: before or after noon, close number of the stomata in the flag leaf of wheat with the photosynthetic rate decrease is intimate interrelated; photosynthetic endogenous rhythm has nothing to do with photosynthetic rate decrese for the "midnap"; a short shading at midday improves photosynthesis and increases the biological yield.It indicates that the p...