Heat transfer was studied between intact leaves of vat ious sizes and shapes in vivo under free and forced air conditions. Use of a wind tunnel and a microwave transmitter to heat the leaves facilitated measurements of convective, along with radiative and evaporative, heat losses from plant leaves. Knowledge of input energy, analysis of cooling curves, and established formulae, respectively, formed the basis of the steady-state, unsteady-state, and analytical methods for the determination of heat transfer coefficients.
Typical values of steady-state free convection coefficients for Peperomia obtusifolia varied from 1.5 x 10 -4 to 1.9 x 10 -4 cal cm-2 s-1 C -1 as the temperature difference was increased from 5.9 to 9.6ยฐC, whereas the forced convection coefficient was found to be 4.2 x 10 -4 cal cm-2 s-1 C-1 at 122 cm s -1 wind velocity. For egg-plant, this value was about 9 x 10 -4 cal cm-2 s-l C-l1 at 488 cm s -l wind velocity. Convection coefficients as determined under steady-state conditions are compared with those of the unsteady-state and with analytical values for a single leaf and leaves of three different plants. In general, experimental values were found to be higher than the analytical ones.