140 FIRST FORMS OF VEGETATION. 
like expansions which they often assume, hard 
and inseparable almost as a portion of the rock 
itself? Then their capacity of extracting their 
nourishment principally from the surrounding 
atmosphere; the curious property which they 
possess of continuing for years without under- 
going any perceptible change; their strong per- 
sistent vitality by which they are able—when 
scorched by the summer sunshine, deprived of all 
their juices, and reduced to shapeless, hueless 
masses, which crumble into powder under the 
slightest touch of the hand or the foot—to revive 
again when exposed to the genial influences of 
the rain, assume their fairest forms and hues, 
and develop their organs of fructification for the 
dispersion of their kind; and lastly, the facility 
with which they can replace portions of their 
substance that have been torn away by storms, 
broken by the tread of man, or eaten by animals; 
all these qualities illustrate the wonderful adapta- 
tion, in their structure and habits, to the unfavour- 
able circumstances in which they are often placed. 
Furnished by such powers as these, wherever 
they fasten their tiny fangs the process of dis- 
integration commences; and though carried on 
slowly and imperceptibly, though ages may elapse 
before any apparent effects have been produced, 
except the increase of individuals and the more 
