LICHENS. 177 
simply composed of short filaments placed close 
together and erect, so that their summits consti- 
tute the surface of the thallus. The summits of 
these filaments and the narrow interstices between 
them are sprinkled with a white powder. When 
saturated with water and rubbed by the hand, this 
powder comes away at once; and when liquefied 
by a solution of hypochlorite of lime, it instan- 
taneously assumes a red colour, which is very 
fugitive. The same colour immediately appears if 
we apply this reactive to the surface of the thallus 
of the lichen. We are thus enabled to say what 
is the quantity of colourable matter which the dif- 
ferent species and varieties of the genus contain. 
It is in fact a sort of immediate analysis. And it 
shows to us that the colourable matter is formed 
and excreted on the outside of the gonidial layer, 
there being but feeble traces of it towards the in- 
terior of the lichen or in the medullary layer ; in 
this respect differing from other lichens, which 
contain the colourable matter underneath the 
gonidial layer and not upon it or in its exterior, 
so that it is necessary to cut their thallus and ex- 
pose the medullary layer, whenever we wish to 
subject them to the test of the hypochlorite of lime. 
In this country there are many species of lichens, 
growing in greater or less abundance, on the moun- 
tain rocks, which might be advantageously sub- 
M 
