LICHENS. 167 
placed upon the action of the doses administered. 
It is obvious that the chemical qualities of cellular 
plants, whose construction is so extremely simple, 
must vary considerably in different individuals 
and in different situations. The nature of the 
matrix on which lichens grow, and of the medium 
which surrounds them, must, to a great extent, 
determine the presence in them of certain con- 
stituents which are extremely volatile, and depen- 
dent upon such conditions. The lichen that de- 
velops certain qualities when growing on the bark 
of a tree, will not develop them to the same ex- 
tent when growing on a rock; and there will be 
a similar, if not a greater difference between the 
qualities of an individual produced in the shade of 
a dark moist wood, and those of the same plant, 
scorched by the sunshine and swept by the wind 
on a bare exposed rock on the hill-side. It was 
this variable chemical character, and the uncertain 
medical results connected with it, that banished 
the lichens from the druggists’ shops. The dis- 
covery of new and more powerful drugs, obtained 
from tropical plants stimulated by intense sun- 
shine and highly organized soils, hastened their 
exile, and effectually closed the door against their 
return to favour; while at the same time it greatly 
diminished the list of native remedies, the products 
of a cold, moist climate, and of poor and feeble 
