-12.57%

convection coefficient, whether the galls are spherical or needle-shaped (Table 9.1). Spherical galls will have a greater effect in this regard than will needle galls, although the difference is slight. Galled leaves should therefore be warmer than ungalled leaves. If real galls have a similar effect on real leaves, then the presence of galls should increase the leaves' temperature increment.

Leaf Galls and the Energy Balance of Infested Leaves What does this prove, exactly? Simply demonstrating that leaf galls can alter a leaf's convection coefficient is not enough to say that the change will have any adaptive value. To get there, we must ask two other ques tions. First, by how much will the actual temperatures of galled leaves change? Second, to what degree will this change of temperature affect the net photosynthesis of a galled leaf? Unfortunately, the answers to these questions are completely unknown, but let us not let a little thing like ignorance stand in our way—let us follow the thread and see where it leads.

To take the first question: what change of temperature should we expect for the 10-15 percent reduction in a galled leaf's convection coefficient? Equation 9.6 for Tleaf shows that the effect of any change in the leaf's convection coefficient will vary with the magnitude of the net heat exchange. I have made a few calculations of what temperature increments should be expected for values of qnet that more or less span the range that real leaves experience (Table 9.2). These calculations indicate that in some circumstances galls will have a small effect, but in others the effects will be pretty large. At low values of qnet—which might indicate conditions of low radiation, or high radiation with a considerable rate of evaporation—galled leaves should be less than one degree warmer than ungalled leaves. At higher values of qnet, though, which might occur under sunny, more humid conditions, galls on a leaf should warm leaf temperature by a few degrees. Under all circumstances, though, galled and ungalled leaves

Table 9.2 Temperature increments (AT, in °C) estimated for model leaves under various regimes for qnet. The first three rows tabulate temperature increments for model ungalled leaves, model leaves with simulated needle galls, and model leaves with simulated spherical galls. The bottom two rows indicate the difference in temperature increments (¿7) between model leaves with galls and model leaves with no galls. These numbers indicate how much galls raise the temperature of the leaf above that of a leaf without galls.

Table 9.2 Temperature increments (AT, in °C) estimated for model leaves under various regimes for qnet. The first three rows tabulate temperature increments for model ungalled leaves, model leaves with simulated needle galls, and model leaves with simulated spherical galls. The bottom two rows indicate the difference in temperature increments (¿7) between model leaves with galls and model leaves with no galls. These numbers indicate how much galls raise the temperature of the leaf above that of a leaf without galls.

Experimental condition |
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