2 THE LICHENS OF PERTHSHIRE 
have a habit of growing on large flat surfaces of granite or other 
hard rocks, when it is almost impossible to obtain a specimen at 
all. Other species, again, which grow on young living bark cannot 
be obtained without injury to the tree or branch. As in the case 
of other plants, small differences in aspect, amount of moisture, 
S 
bordering the Firth of Tay, it is entirely an inland county. 
Perthshire may be naturally divided into two distinct parts— 
namely, the Highland region, which forms the north-western 
portion, and the Lowland region, which forms the south-eastern 
county. This Lowland tract is, however, traverse 
east to south-west by two ranges of hills of moderate height—the 
