176 FIRST FORMS OF VEGETATION. 
being valued at from 460,000 to £80,000 ; but 
so great has been its consumption of late 
years, that the best quality, which generally 
sells for £200 a ton, and has in times of scarcity 
been actually sold for the enormous sum of 
41000, or about gs. a pound, has become ex- 
ceedingly rare, and what is now commonly im- 
ported from other countries is worth little more 
than £30 the ton. By far the most valuable dye- 
lichens known are various species of Roccella 
growing on trees by the sea-side, at Zanzibar and 
along the coast of Eastern Africa. They come 
to this country partly by Bombay, and partly 
through Portugal and France, and have sup- 
planted in the British market all other species of 
lichens. And yet in their native country it is 
not known that they are capable of yielding 
dyes or of being otherwise utilized in the domestic 
arts. In this country the sap of the Roccella tinc- 
toria is of a deep yellow, staining the fingers when 
gathered. The colouring matter of all the species 
separates itself and is easily obtained, if we rub the 
lichen between the fingers when immersed in water, 
which in consequence becomes milky. When set- 
tled a whitish powder is deposited, which contains 
erythric acid. The colouring matter is thus easily 
expressed, because the surface of the thallus is not 
covered by any cuticle, the cortical layer being 
