266 THE VEGETABLE WORLD. 
decorating them with a thousand varied hues of colour. These — 
Thallogens live in the air, never in water. Their existence may 
endure for hundreds of years; their growth and propagation are 
both excessively slow. They are found in all regions of the globe, 
from the tropics up to the poles. They grow on the summit of the 
loftiest mountains, in the plains and valleys, and at all inter- 
mediate heights. Near the limits of eternal snow, where all other 
signs of vegetation have disappeared, on the edge of glaciers, and 
up to the 70th degree of north latitude, that is to say, at the 
nearest point to the Pole which has been approached by ma, 
the Lichens still vegetate. Humboldt and Boussingault found 
them growing near the summit of Chimborazo, and they are the 
last vegetables which are found on the slopes of Mont Blane. 
In form these universally disseminated plants are cellular and 
flowerless, formed of a lobed and foli called a thallus, 
which is formed of a cortical, and medullary layer—the former 
being cellular, the latter both cellular and filamentous. Others 
are crustaceous, when the cortical is coloured and the medullary 
layer is colourless. The reproductive matter is of two kinds:— 
1. Spores, naked, or lying in membranous amylaceous tubes “oF 
thecze immersed in nuclei of the medullary substance, which 
through the cortical layer and colour, and harden by exposure " 
the air in the form of little discs, called shields. 2. The separated 
_cellules of the medullary layer of the thallus. These, call 
gonidia, are of a green colour, and lie singly or in clusters beneath 
the cortical layer of the thallus, and break out in clusters called 
soridia, or in cups called cyphelia. 
Although Lichens, with one or two exceptions, are a a 
immersed in water, they are said in all cases to be developed 4 
humidity, resembling in that state Phycew and Conferve ; 
as soon as the humidity diminishes, the under part dies 
inert leprous crust is formed, which becomes the basis 
plant. Hence the two sorts of tissues—living cellule 
the vegetating part, and dead cellules which have lost thet 
cohesion; the latter have their reproductive powers while every 
part of the living stratum has been ascertained to possess TePr” 
ductive properties. . os 
Some Lichens are employed in medicine, others are subjects 
3 forming 
