LICHENS 517 



of the plants belonging to this group. They can resist without 

 injury greater extremes of heat and cold than any other known 

 plants, and also possess great power of adaptability. They are of 

 very slow growth, and appear in many instances to live for a very 

 long period of time, more especially those species forming crusts 

 on rocks. 



Lecidca subuiitdoiiata, a lichen growing on rocks 

 in Alpine districts 



In the cold regions of the north lichens frequently serve as an 

 important source of food to animals, as in the case of the reindeer, 

 who during a certain season of the year depend almost entirely on 

 a certain lichen, Cetraria rangiferina, curiously enough called 

 reindeer moss, for their food. As food, lichens have more than 

 once saved the lives of explorers in the Arctic regions. The nutri- 

 tious property of lichens turns on the presence of a substance 

 called lichenin, or lichen-starch, whose chemical composition is the 

 same as that of starch proper. In the higher forms of lichens this 

 substance is present to the extent of 40-65 per cent of the entire 

 bulk, A lichen called Lecanora escidenta grows loosely attached to 

 rocks in the north of Africa, from which it is removed and carried 

 for long distances by wind, and in desert districts where food is 



