222 PHYSIOLOGY 



After this first isolation of a definite chemical substance, further research 

 was undertaken, and gradually a number of these peculiar acids were recog- 

 nized, the lichens examined being chiefly those that were of real or supposed 

 economic value either in medicine or in the arts. In late years a wider 

 chemical study of lichen products has been vigorously carried on, and the 

 results gained have been recently arranged and published in book form by 

 Zopf^- Many of the statements on the subject included here are t9.ken from 

 that work. Zopf gives a description of all the acids that had been discovered 

 up to the date of publication, and the methods employed for extracting each 

 substance. The structural formulae, the various affinities, derivatives and 

 properties of the acids, with their crystalline form, are set forth along with 

 a list of the lichens examined and the acids peculiar to each species. In 

 many instances outline figures of the crystals obtained by extraction are 

 given. For a fuller treatment of the subject, the student is referred to the 

 book itself, as only a general account can be attempted here. 



b. Occurrence and Examination of Lichen-Acids. Acids have 

 been found, with few exceptions, in all the lichens examined. They are 

 sometimes brightly coloured and are then easily visible under the microscope. 

 Generally their presence can only be determined by reagents. Over 140 

 different kinds have been recognized and their formulae determined, though 

 many are still imperfectly known. As a rule related lichen species contain 

 the same acids, though in not a few cases one species may contain several 

 different kinds. In growing lichens, they form i to 8 per cent, of the dry 

 weight, and as they are practically, while unchanged, insoluble in water, they 

 are not liable to be washed out by rain, snow or floods. Their production 

 seems to depend largely on the presence of oxygen, as they are always 

 found in greatest abundance on the more freely aerated parts of the thallus, 

 such as the soredial hyphae, the outer rind or the loose medullary filaments. 

 They are also often deposited on the exposed disc of the apothecium, on the 

 tips of the paraphyses, and on the wall lining the pycnidia. They are absent 

 from the thallus of the Collemaceae, these being extremely gelatinous lichens 

 in which there can be little contact of the hyphae with the atmosphere. 

 No free acids, so far as is known, are contained in Sticta fuliginosa, but 

 a compound substance, trimethylamin, is present in the thallus of that lichen. 

 It has also been affirmed that acids do not occur in any Peltigera nor in 

 two species of Nephromium, but Zopf ^ has extracted a substance peltigerin 

 both from species of Peltigera and from the section Peltidea. 



For purposes of careful examination freshly gathered lichens are most 

 serviceable, as the acids alter in herbarium or stored specimens. It is well, 

 when possible, to use a fairly large bulk of material, as the acids are often 

 present in small quantities. The lichens should be dried at a temperature 



^ Zopf 1907. 



