Cytochemical and histological microscopical methods allow for differentiation between the effects of ozone, heavy metals and biotic pathogens. Each stress factor induces a characteristic set of changes at cell and tissue levels. Ozone and heavy metals both induce oxidative stress resulting in HR-like and accelerated cell senescence processes in the leaf blade. Only in the case of heavy metals are these processes also observed in the veins and in the lower epidermis. Insect and fungi can be detected either directly as organisms or by their traces.