Solution : the plant used for the experiment of the photosynthesis is placed in the dark in starting so as to remove the starch from the leaves . The plant is placed in the dark for 24 – 48 hours so that the leaves are de starched and the starch is stored on the storage organs . The rate of photosynthesis rises  from dawn to midday and decreases as dusk approaches. In the darkness, respiration alone is responsible for changes in the gaseous composition of the plant’s surrounding atmosphere.