CELLS AND CELL PRODUCTS 225 



the thallus forms these outgrowths, or even becomes leprose more freely in 

 damp weather, the amount of acids produced may depend on the amount of 

 moisture in the atmosphere. 



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. Haenmtomma ventosum, 

 though greenish-yellow in lowland situations is intensely yellow in the high 

 Alps. The same variation of colour is characteristic of Rhisocarpon geo- 

 graphicum 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' and others have noted its more luxuriant growth and brighter colour 

 when it grows in positions where nitrogenous food is plentifiri, 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^ 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 I. 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. 



1 West, W. 1905. ^ LeUau 1914. 



S. L. 15 



