INTRODUCTION

Plants are able to shed surplus heatloads in a number of ways, two of which are especially important but only one of which leads to the coolness we associate with places where plants are abundant, such as forests and meadows.

CONVECTIVE VERSUS EVAPORATIVE LOSS OF HEAT

If the temperature of a leaf is higher than its surroundings, air circulation will remove heat from its surface mainly by convection. As this warm air rises, it cools, becomes more dense, and sinks, creating a convection current which removes heat from plant surfaces.

Evaporation of water from leaf surfaces withdraws heat from a plant because energy is absorbed by water as it changes from liquid to vapor (the latent heat of vaporization of water is 44 kJ mol−1). Evaporative cooling can occur even if the temperature of the leaf is below that of the surrounding air.

Which of these two ways of heat loss is the most important to a plant depends on its environment. If there is an ample supply of water then loss of it from leaves can be high without causing damage to the plant; evaporation can be a major means of cooling. Plants adapted to growing in hot, dry conditions, on the other hand, have evolved ways to conserve rather than shed water; convective air currents become the main route for shedding heat.

A typical leaf at moderate temperature dissipates about half its heatload by evaporation of water, half by convection.