LICHENS. 117 
of circular growth, so that their stability is 
maintained at every point and stage of growth by 
their spherical form, which gives a maximum of 
contents with a minimum of exposure. We see 
in lichens as the form of their whole life, what in 
higher plants we observe only at certain stages of 
growth, and generally late in life, when they have 
exercised their active functions and returned to 
repose in the leaf, the blossom, the seed, the bud. 
And having thus a fuller and larger attainment of 
the spherical in form than other plants, we infer 
that they have a slower growth, a greater amount 
of stability and repose, and consequently a higher 
longevity. Nature, in the foliaceous webs of the 
lichens, works in her warp and woof as Penelope 
wrought at her loom, by fits and starts. She 
unrolls in a season of drought when no growth is 
made for weeks together, and the lichen seems 
withered and dead, what she had accomplished 
during a moist season, when the lichen was 
stimulated to new growth and exhibited the fresh 
green hue and the soft mobile tissue of active life. 
But not only is the form of the lichen thus suited 
to long suspensions of growth; .its substance also 
favours the retention of life in unfavourable cir- 
cumstances, when endurance is the only quality 
which the plant can display. Lichens as a class 
are very largely composed of starch, which 
