THE LICHENS 235 



LICHENS 



227. General characteristics. The lichens are not simply 

 fungi. A lichen is not even a single plant, but is a com- 

 bination of fungi and algae living together in such a close 

 relationship that it looks like a single plant. There may be 

 many individual fungi and many individual algse in this rela- 

 tion, but the combination is spoken of as the lichen plant. 

 The fungal part of the lichen is usually, though not always, a 

 member of the ascus-bearing class of fungi, and consequently 

 lichens are often classified with ascomycetes. This is obvi- 

 ously a somewhat questionable classification, but for lack of 

 a better one we shall dis- 

 cuss the class in this con- 

 nection. The algse that 

 enter into the formation 

 of lichens are usually uni- 

 cellular forms resembling 

 Pleurococcus, but may be 

 filamentous green algse or 

 even some of the blue- 

 green algse. FlG * 19 * A foliaceous lichen (Parmelia) 

 upon a piece of bark 



Lichens live upon bark Natural size 



of trees, stones, and upon 



soil (Fig. 190). They thrive under conditions of exposure and 

 in moisture and temperature variations which do not permit 

 most plants to grow. They are found at as great altitudes 

 and with as great range north and south as any plants. 



In stony places lichens often form heavy mats made up 

 of lichen bodies, mosses, and decomposed rock. These masses 

 when upon upright faces of rock may by their own weight 

 fall and become the soil for growth of other plants. New 

 growths soon start where the old ones were, and by a con- 

 tinuation of this process these plants may slowly wear 

 away large masses of stone. It is probable that consider- 

 able chemical action is exerted upon the rock by the hyphse, 



