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Gunk spewing from the exhaust pipes of cars and lorries clogs up the
lungs of plants just as much as those of pedes-trians. A plant’s lungs are
the microscopic pores in its leaves, which let in carbon dioxide needed
for photosynthesis and let out oxygen. These two pictures, taken in a scanning
electron microscope, show the difference between life in a country churchyard
and life amid the traffic jams.

The underside of a holly leaf taken from a churchyard in an unpolluted
part of Norfolk (left) is perfectly clean. But the other leaf, from a holly
tree growing in the city of Norwich, is cluttered with debris from car exhausts.
The debris is made up of soot particles from unburnt fuel and lead from
leaded petrol.

Pollution not only clogs the pores, reducing the movement of gases in
and out of the leaf, but also coats the surface of the leaf, reducing the
amount of light reaching it. Both hinder the process of photosynthesis,
‘starving’ the plant. Added to this, the particles of soot trap other pollutants,
such as sulphur dioxide and nitrogen oxides, which cause chemical damage
and lower the plant’s resistance to disease.