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 algz, 
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 
