Adjustment of leaf temp. to incident energy was measured by means of thermocouples. Using a sunshine C arc source at 2.5 ft., with energy content of 0.229 cal./cm.2/min., temp, equilibrium in leaves of Pelargonium was attained in 89 sec; in Begonia leaves in 120 sec; in Caladium leaves in 115 sec. These time periods are several times longer than theoretical calculations based on previous energy relations studies would demand. It is suggested that all of the energy relations should be remeasured with specific leaves in the hope that empirical observations and theoretical calculations might be brought into satisfactory agreement.