Lichens on corroded glass - from Ethel Mellor's doctoral thesis
This photo shows a small part of a glass surface. You can see a number of pits as well as areas of smooth, uncorroded glass. The letter "a" indicates the rosette-like growth of a lichen thallus that has established itself within one of the corrosion pits. Mellor identified this lichen as Placodium murorum var. cinnabarinum by Mellor. There are different levels of lichen growth within some of the other pits as well. The letter "b" indicates a corroded area with an irregular outline.
Source: Photo 3 (on plate 1) of Ethel Mellor's work: Les lichens vitricoles et la détérioration des vitraux d'église, Paris, 1921.