The epistomatal wax crystalloids of potted spruce plants exposed for 20 weeks to motor vehicle emissions along roadside and in a fumigation chamber were studied in the scanning electron microscope. An accelerated structural degradation of the wax crystalloids was observed in comparison with control plants. This structural degradation is considered to cause a structural obstruction of the stomatal antechambers and to contribute to the observed reduction of gas exchange.

The dramatic corrosive effect of motor vehicle emissions on the wax tubules in the epistomatal chambers has been followed in needles of spruce and fir with the scanning electron microscope. Fumes of aromatic hydrocarbons (benzene, xylene) were found to induce the same corrosion within days. The epistomatal waxes are consid^ered, therefore, to be the primary target of such lipophilic compounds of polluted air and the physiological consequences of the ensuing premature stomatal ODstruction to be closely related with the 'Waldsterben'.

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