xX INTRODUCTION 
cases. Growth is extremely slow, and there is in consequence 
little demand on the metabolic activity of the cells. A plant of 
Parmelia sawatilis kept under observation for a considerable 
period was observed to increase about 1 cm. in diameter in a 
year. The gonidial layer is usually several cell-rows below the 
surface and is often obscured by pigments in the cortical cells ; 
they require therefore abundant light and pure air, and are 
always most luxuriant in well-lighted situations, such as the 
sunny side of a wall, and on the outskirts of a wood rather than 
in its shady glades. ‘They soon die out or lead an impoverished 
existence in the near precincts of a large town, owing to the 
smoke. They are the most cosmopolitan of all plants and the 
pioneers of vegetation, occupying great tracts of mountain and 
arctic regions where no other plants can live. Distribution in 
the case of some genera and species is limited by climate or by 
the nature of the substratum. On the other hand, there are 
species, like the mountain-loving Rhizocarpon geographicum, that 
spread almost from pole to pole. 
ECONOMIC USES OF LICHENS. 
In addition to their use as dye-plants, lichens are to a very 
limited extent valuable as a food supply. The lichenin stored in 
the cell-walls of Cetraria islandica, the Iceland Moss, can be so 
prepared as to be both nutritous and appetising and has been 
often utilised by northern peoples. Species of Gyrophora, “ tripe 
de roche,” have been eaten by travellers when no other food was 
available, but though they contain some nourishment they are 
too bitter for consumption. Reindeer. pasture largely on 
Cladonia rangiferina, and snails, slugs, mites, etc., devour eagerly 
many different kinds of lichens. 
In the economy of Nature a considerable part is played by 
crustaceous saxicolous lichens in breaking up the rock and 
preparing it for plants more dependent on loose soil. Caleareous 
and granite rocks are thus slowly but gradually disintegrated. 
Volcanic rocks with their smoother harder surface are less 
affected. Topographically as well as geographically, lichens are 
indispensable pioneers. Many lichens grow on trees, where they 
are epiphytic, though occasionally their root-bases penetrate the 
living tissue; only one species, a leaf-lichen in the tropics, has 
been recorded as constantly parasitic. Sometimes the growth of 
leafy and shrubby forms is so luxuriant as to cover the entire 
