LICHENS. 269 
freely upon them. We are even informed that, with 
the inhabitants of Palma, one of the Canary islands, 
they are sometimes made to supply the place of bread. 
It is deserving of remark that, when the root of the 
fern is cut obliquely across, it presents a kind of figure 
of the Imperial or Russian eagle ; from which circum- 
stance Linnaeus was induced to name it Pteris aquilina, 
or " Eagle brake." 
281. The LICHENS constitute a very numerous family of 
plants, which grow on the bark of trees, on rocks, stones, and 
other substances ; and have an indistinct fructification, in scat" 
tered wart-like tubercles, or excrescences, and smooth saucers or 
shields, in which the seeds are imbedded. 
Some of them have a pozcdery appearance, and others are 
crustaceous, leaf-like, shrub-like, herbaceous, or gelatinous. 
282. The CALCAREOUS LICHEN (Lichen calcareus) 
consists of a white crust zoith black tubercles. 
This plant, which is found on lime-stone rocks in 
Wales, and the north of England, is used in dyeing 
woollen and other cloths a scarlet colour. 
283. CRAB'S-EYE LICHEN (Lichen parellus) is a 
crustaceous, whitish, and granulated vegetable substance, with, 
cups of the same colour, which have a thick and blunt border. 
From this lichen, which is found on rocks and stones 
in mountainous countries, and sometimes on stones 
near the sea-shore, is prepared the bluish pigment 
called litmus. It is chiefly collected from rocks in the 
north of England, packed in casks, and sent to London 
for sale. 
284. TARTAREOUS LICHEN (Lichen tartareus) is a 
whitish, crustaceous, vegetable production, zvilh yellow cups or 
sjiields, which have a whitish border. 
The inhabitants of the Highlands of Scotland gather 
this species of lichen from the rocks, and, after cleaning, 
and some further preparation, which is kept a secret 
by the manufacturers, they form it into cakes. These, 
