tion of the color is explained as follows : Most lichens contain color- 

 less acids free from nitrogen, such as erythrinic acid (formed by the 

 union of erythric acid with erythrite), orsellinic acid, etc., which 

 are very readily transformed into orcin [C 7 H 6 (H 2 0)]; in the 

 presence of an alkaline solution, as ammonia, the colorless crystals 

 of orcin are dissolved, forming a purple solution known as orchil. 

 Litmus is a special preparation of orcin which changes to red in 

 the presence of acids and blue in the presence of alkalies ; for this 

 reason it is extensively used in making chemical tests, litmus paper 

 being ordinary bleached paper dipped in a solution of litmus. Orchil 

 is also used by artists as a pigment. 



Lichens contain a number of chemical compounds which are but 

 little understood and for that reason will not be discussed. Many of 

 them evidently depend upon the chemical nature of the substratum. 



Oil globules are quite common in the spores, less common in the 

 hyphal cells. The rhizoidal " fat cells" in calcareous lichens have 

 already been mentioned. Calcium oxalate crystals are of common 

 occurrence in different parts of the lichen, being most frequently 

 found in the higher foliose species. The more or less imaginary 

 tonic and antifebrile properties of many lichens are due to the bitter 

 extracts (acids, etc.) which were obtained from them. The nutri- 

 tious properties are of course due to the large percentage of lichenin. 



The dark rusty coloration met with in many lichens, particularly 

 in crustaceous forms, is due to an infiltration with some salt of iron 

 (Gumbel). 



The micfochemical examination of lichens reveals the fact that 

 their chemical compositon is variable, depending to a large extent 

 upon the substratum upon which they live. No chemical test is abso- 

 lutely reliable. In general, it is, however, found that a blue colora- 

 tion of the thecium with iodine takes place in most of the higher lichens ; 

 in many of the lower forms, as. Baeomyces, it does not ; and while 

 some respond very quickly to this reaction, others are very slow. 

 All lichenologists have noted the extreme variability in the amount 

 of coloring substance of orchil-producing lichens {Rocella, Pertu- 

 saria, Gyrofhora, etc.). Our knowledge of the chemistry of the 

 lichens is very deficient, notwithstanding the voluminous, though 

 fragmentary, literature on the subject. 1 



1 Most of the publications on the chemical constituents of lichens are to be found 

 in the chemical journals. (Erdmann's Journal fur practische Chemie. 1846 — ). No 

 recent important chemical analyses have been made. 



