5i8 BRITISH FUNGI 



scarce is eaten by the inhabitants. This lichen is supposed to have 

 been the " manna " of the Children of Israel. 



Dyes of various colours are yielded by lichens, the best known 

 and most important being variously known as " orchil," " cudbear," 

 " orseille," " litmus," etc., which has been used from the earliest 

 historical times. As " litmus," it is still used in chemical research 

 as an agent for determining the presence of acids or alkalies re- 

 spectively. It is obtained from a lichen called Rocella tinctoria, 

 found in many parts of the world, growing on rocks by the sea- 

 shore. ' ' 



The most remarkable feature in connection \\\i\\ the study of 





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Stii-ta piiltiioiiaria, a liclien growing on trees 



lichens is that of their dual nature, a subject which can only be 

 briefly alluded to in this book. 



During earlier times it was naturally assumed that lichens, like 

 mosses, ferns, fungi, algae, and other plants, were entities. Some 

 time ago, however, it was clearly demonstrated tliat all lichens are 

 in reality composed of a fungus and one or more algie, living to- 

 gether and collectively constituting a lichen. The fruiting portion 

 of a lichen is always produced by its fungal constituent. This 

 combination of two plants belonging to totally different families 

 to form a third plant having marked characteristics of its own may 

 at first sight appear to be strange, to say the least, but it has its 

 parallel, in a less marked manner, in other groups of the vegetable 

 kingdom. 



In dealing with the fungi it has been stated that parasitism is 

 an acquired habit, and that ah degrees of parasitism, in the case of 



I 



