CELLS AND CELL PRODUCTS 227 



lecanoric acid (C16H14O7), found in many different lichens, e.g. Roccella tinc- 

 toria, Lecanora tartarea, etc.: whenever there is a differentiated pith and 

 cortex it occurs in the pith alone. Erythrin (C20H22O10), a constituent of the 

 British marine lichen Roccella fuciformis, also belongs to this orsellinic group. 



Group 2. Substances which also form red products with CaCl, but do 

 not break up into orsellinic acid. Among the most noteworthy are olivetoric 

 acid (C21H25O7), a constituent of Evernia furfuracea, perlatic acid (CjsHsoOio) 

 and glabratic acid (C24H26O11), which are obtained from species oi Par me Ha. 



Group 3. Contains three acids of somewhat restricted occurrence. They 

 do not form red products with CaCl, and they yield on splitting everninic 

 acid. They are: evernic acid (CiirHmO,), found in Evernia prunastri var. 

 vulgaris, ramalic acid (Ci^HigO?) in Ramalina pollinaria, and umbilicaric acid 

 (C26H22O11) in species of Gyrophora. 



Group 4. The numerous acids of this group are not easily soluble and 

 have a very bitter taste. They are not coloured by CaCl ; when extracted 

 with concentrated sulphuric acid, the solution obtained is reddish-yellow or 

 deep red. Among the most frequent are fumarprotocetraric acid (CeaHjoOss), 

 the bitter principle of Cetraria islandica, Cladonia rangiferina, etc., psoromic 

 acid (C20H14O9), obtained from Alectoria implexa, Lecanora varia, Cladonia 

 pyxidata and many other lichens, and salazinic acid (CwH^Oio), recorded by 

 Zopf as occurring in Stereocaulon salaziniim and in several Parmeliae, but 

 now found by Lettau' to be very wide-spread. He used micro-chemical 

 methods and detected its presence in 72 species from twelve different families. 

 The distribution of the acid in the thallus varies considerably. 



Group 5. This is called the atranorin group from one of the most im- 

 portant members. They are colourless substances and, like the preceding 

 group, are not affected by CaCl, but when split they form bodies that colour 

 a more or less deep red with that reagent. Atranorin (CigHigOs) is one of 

 the most widely spread of all lichen-acids ; it occurs in Lecanoraceae, Par- 

 meliaceae, Physciaceae and Lecideaceae. Barbatinic acid (C19H20O7), another 

 member, is found in Usnea ceratina, Alectoria ochroleuca and in a variety of 

 Rhizocarpon geographicum. A very large number of acids more or less fully 

 studied belong to this group. 



SuBSERiES II. Anthracene Derivatives 

 The constituents of this subseries are derived from the carbohydrate 

 anthracene, and are characterized by their brilliant colours, yellow.red, brown, 

 red-brown or violet-brown. So far, only ten different kinds have been isolated 

 and studied. Parietin^ (CieHjsOs), one of the best known, has been extracted 

 iromXantkoriaparietina,Placodiu!n murorum and several other bright-yellow 



^ Lettau 1914. 



^ Parietin differs chemically from chrysophanic acid of Rheum, etc.^ 



IS— 2 



