Leaf temperatures and energy balance ofWelwitschia mirabilis in its natural habitat.

Welwitschia mirabilis is a perennial desert plant with extremely large leaves (0.5-1.0 m broad, 1-2 m long). Leaf temperatures were measured in the field and the energy budget was calculated. The portions of the leaf which were kept above the ground had leaf temperatures which were only 4-6°C above air temperature. In the leaf portions which were in contact with the ground leaf temperatures were 6-12°C above air temperature (absolute maximum 51°C). The important feature in the energy budget ofWelwitschia mirabilis is its high reflectivity (38% of the global radiation). Only about 56% of the global radiation is absorbed by the thick leathery leaves. The energy loss due to convection is of the same order of magnitude as the reflection and it is abouy the same in the portions of leaf on and above the ground. The difference in leaf temperatures found in these portions is due to the loss of thermal radiation from the section of leaf above the ground to the cooler ground which is shaded by the leaf. The provision of a heat sink due to the large area of shade cast by these large leaves is of significance to the existence ofWelwitschia mirabilis in its arid habitats.