Their formation is also strongly influenced by light, as is well shown by the varying intensity of colour in some yellow thalli. _Placodium elegans_, always a brightly coloured lichen, changes from yellow to sealing-wax red in situations exposed to the full blaze of the sun. _Haematomma ventosum,_though greenish-yellow in lowland situations is intensely yellow in the high Alps. The same variation of colour is characteristic of _Rhizocarpon geographicum_ which is a bright citron-yellow at high altitudes, and becomes more greenish in hue as it nears the plains. The familiar foliose lichen _Xanthoria parietina_ is a brilliant orange-yellow in sunny situations, but grey-green in the shade, and then yielding only minute quantities of parietin. West[827] and others have noted its more luxuriant growth and brighter colour when it grows in positions where nitrogenous food is plentiful, such as the roofs of farm-buildings, which are supplied with manure-laden dust, and boulders by the sea-shore frequented by birds. _e._ DISTRIBUTION OF ACIDS. Some acids, so far as is known, are only to be found in one or at most in very few lichens, as for instance cuspidatic acid which is present in _Ramalina cuspidata_, and scopuloric acid, a constituent of _Ramalina scopulorum_, the acids having been held to distinguish by their reactions the one plant from the other. Others of these peculiar products are abundant and widely distributed. Usninic acid, one of the commonest, has been determined in some 70 species belonging to widely diverse genera, and atranorin, a substance first discovered in _Lecanora atra_, has been found again many times; Zopf gives a list of about 73 species or varieties from which it has been extracted. Another widely distributed acid is salazinic acid which has been found by Lettau[828] in a very large number of lichens. E. CHEMICAL GROUPING OF LICHEN-ACIDS Most of these acids have been provisionally arranged by Zopf in groups under the two great organic series: I. The Fat series; and II. The Benzole or Aromatic series. I. LICHEN-ACIDS OF THE FAT SERIES =Group 1.= Colourless substances soluble in alkali, the solution not coloured by iron chloride. Exs. protolichesterinic acid (C₁₉H₃₄O₄) obtained from species of _Cetraria_, and roccellic acid (C₁₇H₃₂O₄) from species of _Roccella_, from _Lecanora tartarea_, etc. =Group 2.= Neutral colourless substances insoluble in alkalies, but soluble in alcohol, the solution not coloured by iron chloride. _Exs._ zeorin (C₅₂H₈₈O₄), a product of widely diverse lichens, such as _Lecanora_ (_Zeora_) _sulphurea_, _Haematomma coccineum_, _Physcia caesia_, _Cladonia deformis_, etc. and barbatin (C₉H₁₄O), a product of _Usnea barbata_.