The Victorians introduced the plane tree to London, creating the city's lungs. The London plane tree is actually a hybrid between an American plane and a European plane, and can be found all over London. They became popular because they were so hardy. Around a plane tree you will find lots of pieces of bark, and the bark is covered in lots of tiny pores which get clogged with dirt. By shedding the bark on a regular basis, it manages to always have some clean bark to breathe through. When the leaf buds grow in April they develop tiny hairs that protect the leaf from the sun. But these hairs also trap lots of particles of soot. They then drop these leaves at the end of the summer, cleansing the air in the process. London plane trees in city parks can therefore remove 85% of soot from the surrounding air. But people were not so lucky. During the days of London smog, 15% of the city's children had rickets due to a lack of vitamin D.