Rock Lichens 



We have given so much because this lowest of all 

 associations brings out a curious mixture of co-opera- 

 tion and competition. Each individual, plant or animal, 

 is attempting to overgrow or to destroy its neighbour. 

 But yet one might say that every form is directly or 

 indirectly of service not only to its neighbours but to 

 the trentepohlia association as a whole (see Chap. I.). 



At the edge of such trentepohlia crusts, however, 

 the green feltwork of pioneer moss protonemas may be 

 found crawling over the heath. Moss tufts will even- 

 tually occupy the stone surface which has been first 

 worked over by trentepohlia. 



One might venture a little farther and say that 

 Nature is by no means careless about the individual 

 bacterium or trentepohlia. Very ruthless methods are 

 employed to take care that it is a thoroughly adequate 

 bacterium. If not, the minute research by which its 

 body is investigated insures that everything which it 

 has formed during life is available for the common 

 good. But on most walls, on exposed mountain rocks 

 or in the cold rock-deserts of the far north, one finds 

 not trentepohlia but lichens. 



These lichens are double plants consisting of a fungus 

 " body " which contains a green layer of resident algae. 

 They ought theoretically to have a double name (that 

 of the alga and that of the fungus), but this is quite 

 unnecessary in practice. 



These lichens are exceedingly hardy and can live, 

 e.g., on black rocks jutting out of an Alpine snow-field. 

 In a wild part of Dalmatia known as the Karst, the 

 rocks are exposed for hours together to a blazing sun, 

 and yet, though the temperature on the surface may be 

 58 to 60 C., lichens occur on them. 



These, the hardiest of the group, are usually mere 

 stains or crusts upon the rock surface, generally grey 



65 E 



