DECEMBER. 379 
even on the bare rock, upon which they are often fixed, will 
enable a few seeds of grass, and other small plants to grow. 
These wither, and in their turn afford nourishment to larger 
kinds, and thus a soil is formed by degrees on these other- 
wise barren spots. They themselves require no other 
sustenance than the moisture of the atmosphere; so that 
the roots fixes them to the rock, but is of no further use. 
They grow almost everywhere, on stone and wood, as well 
as in the soil. Some few are eatable, as the Cladonia rangi- 
Jerina, or Rein-deer Moss, as it is erroneously called, which not 
only forms the food of the useful animal from which it derives 
its name, but is used in this country medicinally. It has 
white stems, looking like wire, and forming a mass, which is 
seen on heaths amongst the green Moss. Others are used by 
the dyer. Various species form the weather-stains on old 
walls, and many beautiful kinds grow on rocks and stones. 
One found on the latter substance, of a green and yellow 
colour, is the Opegrapha saxatilis, or Map Lichen, as it re- 
sembles the marks used in delineating geographical plans. 
These kinds are nearly flat on the surface of the stone, but 
others curl up curiously at the edges, and are of a bright 
orange, sulphur, or ashy-grey colour, constituting the genus 
Parmelia. Others cover the trunks of trees in winter, and 
