118 FIRST FORMS OF VEGETATION. 
appears in cereal grains, in the tubers of potatoes, 
in fruits, in the wood of forest trees, and in all the 
parts of plants in which active life is suspended, 
owing to completion of growth or function, or 
unfavourable circumstances, such as those of win- 
ter or a dry season, and the organism returns 
to a state of rest. It is nature’s admirable pro- 
vision for keeping the fire. of life in existence, 
until such times as it can start forth afresh in 
more favourable circumstances, by covering it over 
with its own ashes, as the thrifty housewife does 
with the embers on her hearth. When rain comes, 
and the lichen awakes from its dormant state, 
the starch which favoured its hybernation is 
utilized and transformed into materials of growth ; 
just as in our own muscles and liver, where starch, 
which was long supposed to be a peculiarly 
vegetable product, has been recently found in the 
form of glycogen as a normal constituent, is 
consumed in muscular and digestive action, and 
forms part of the fuel with which our muscular 
and hepatic engines are fed. In our own bodies, as 
in the humble economy of the lichen, starch contri- 
butes alternately to the repose and activity of life. 
In their geographical distribution, lichens to a 
certain extent obey the same laws to which the 
higher orders of vegetation are subject, being 
‘influenced by temperature, altitude, and the 
