LICHENS. 183 
As I have already remarked regarding the medical 
properties of lichens, their tinctorial qualities are- 
equally variable. Orchil manufacturers have found, 
as the result of long experience, that lichens from 
tropical, sub-tropical, and maritime regions, are 
richer as a rule in colouring matter than those 
growing in northern latitudes and inland localities. 
Corticolous species, or those which grow upon 
trees, also yield fainter traces of colouring properties 
than saxicolous species, or those which grow upon 
rocks. Latitude, climate, temperature, moisture, 
exposure, elevation, nature of rocks or trees on 
which they grow, and in general all those conditions 
which affect the botanical character of the lichens, 
also affect their chemical character, and so render 
them more or less serviceable for the purposes of 
the colour manufacturer or dyer. The genus Roc- 
cella itself, which is the most highly esteemed and 
most commonly used of all the dye-lichens, is one 
of the most variable as regards its botanical char- 
acter in the whole lichen-tribe ; there being a great 
many so-called species which might very well be 
referred to one specific type, and regarded simply 
as varieties. And this changeable appearance of 
the plant is accompanied with corresponding modi- 
fications of chemical properties. These properties 
also vary at different stages of growth of the same 
plant; the young thallus presenting, when sub- 
