THE USES OF LICHENS. 19 



All lichens contain a larger or smaller percentage 

 of a starch-like substance known as lichenin, or lichen- 

 starch. It has the same chemical composition as 

 starch (Cg H^o O5) but differs in that it usually does 

 not give the blue reaction with iodine. It is to this 

 substance that lichens owe their nutritive properties. 

 From time immemorial the poor of various countries 

 have made use of lichens as an article of diet, just 

 when it was first so used is impossible to determine ac- 

 curately. Excavations of prehistoric cave-dwellings 

 ''^ermany) have revealed the presence of lichens 

 (Cladonia rangiferind) among the bones of various 

 animals, which would indicate that man of that early 

 period had already made some economic use of lichens. 

 The " miraculously " supplied manna of the Israelites 

 (Exodus xvi, 14, 15) in the wilderness is supposed to 

 have been a species of Lecanora (Z. esculentd). This 

 lichen occurs very plentifully in the mountainous dis- 

 tricts of Tartary, Algeria and other parts of northern 

 Africa. The plant occurs in small nodular masses, 

 grows and spreads quite rapidly under favorable con- 

 ditions. These lichens are only loosely attached to 

 the substratum, so that they are readily torn loose and 

 carried by the wind into the valleys below, where the 

 ignorant and oftentimes hungry peasants suppose them 

 to be bread rained from heaven. Travellers in the above 

 countries have reported several noteworthy and exten- 

 sive "rains of manna." The Kirghiz Tartars eat this 

 lichen under the name of '-earth-bread." 



Iceland moss {Cetraria islandicn) h;is been most 

 extensively used as an article of diet because it con- 



