CELLS AND CELL PRODUCTS 221 



an excretion in the cells of hepatics. He grew various species in which oil- 

 cells occurred in the dark and then tested the cell contents. He found that 

 after three months of conditions in which the formation of new carbohydrates 

 was excluded, the oil in the cells, instead of having served as reserve material, 

 was entirely unchanged and must in that instance be regarded as an 

 excretion. 



D. LICHEN-ACIDS 



a. HISTORICAL. The most distinctive and most universal of lichen pro- 

 ducts are the so-called lichen-acids, peculiar substances found so far only in 

 lichens. They occur in the form of crystals or minute granules deposited in 

 greater or less abundance as excretory bodies on the outer surface of the 

 hyphal cells. Though usually so minute as scarcely to be recognized as 

 crystals, yet in a fairly large series their form can be clearly seen with a 

 high magnification. Many of them are colourless; others are a bright yellow, 

 orange or red, and give the clear pure tone of colour characteristic of some 

 of our most familiar lichens. 



The first definite discovery of a lichen-acid was made towards the begin- 

 ning of the nineteenth century and is due to the researches of C. H. Pfaff 1 . 

 He was engaged in an examination of Cetraria islandica, the Iceland Moss, 

 which in his time was held in high repute, not only as a food but as a tonic. 

 He wished to determine the chemical properties of the bitter principle con- 

 tained in it, which was so much prized by the Medical Faculty of the period, 

 though the bitterness had to be removed to render palatable the nutritious 

 substance of the thallus. He succeeded in isolating an acid which he tested 

 and compared with other organic acids and found that it was a new substance, 

 nearest in chemical properties to succinic acid. In a final note, he states 

 that the new " lichen-acid," as he named it, approached still nearer to boletic 

 acid, a constituent of a fungus, though it was distinct from that substance 

 also in several particulars. The name " cetrarin " was proposed, at a later 

 date, by Herberger 2 who described it as a " subalkaloidal substance, slightly 

 soluble in cold water to which it gives a bitter taste; soluble in hot water, 

 but, on continued boiling, throwing down a brown powder which is slightly 

 soluble in alcohol and readily soluble in ether." Knop and Schnederman 3 

 found that Herberger's "cetrarin" was a compound substance and contained 

 besides other substances " cetraric acid " and lichesterinic acid. It has now 

 been determined by Hesse 4 as fumarprotocetraric acid (C^ H^O^), a deri- 

 vative of which is cetraric acid or triaethylprotocetraric acid with the formula 

 CM^O^OCaH^s and not C 20 H 18 O 9 as had been supposed. Cetraric acid 

 has not yet been isolated with certainty from any lichen 5 . 



1 Pfaff 1826. 2 Herberger 1830. 3 Knop and Schnederman 1846. 4 Hesse 1904. 



s Zopf 1907, p. 179. 



