Water Vapour and Heat Transfer in Leaves

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Abstract
Factors connected with the formation of water droplets in leaves by distillation from the mesophyll to the epidermis were investigated in a number of species. It was concluded that in illuminated leaves water droplets form principally on the inner walls of guard and subsidiary cells, and sometimes below the anticlinal walls of epidermal cells, because these sites are cooler than the rest of the leaf. Under more isothermal conditions any water droplets that had formed disappeared. With increasing water stress water droplets did not form so readily, though distillation was occurring. Few water droplets were observed in leaves out of doors that had open stomata. Significant temperature gradients were measured across leaves with thermocouples, but these were larger than were gradients calculated from measured thermal conductivities of leaves. The evaporation resistances of the inner walls of the epidermis and of the mesophyll were found to be similar. This led to the conclusion that the hydrophobicity of the surfaces of these tissues is similar. Water transfer in leaves in the vapour phase was found to be more responsive to temperature than to water stress gradients.

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