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1.Crustose-Lichsn stag:- the rock habitat is extremely xeric
water and nutrients? are extremely scarce and the conditions
are aggravated by exposure to sun and wind and extremes of
temp. the first pioneers on such a habitat are the crustose
lichens. They are very slow growers and can stand extreme
dessication. When ever there is rain they can absorb H2O
like a sponge and flourish very rapidly. Migration takes
place by lichen fragments or spores. The lichens secrete
acid a that slowly corrode and decompose rock making it rough.
2. Foliose lichen stage:- the change in the rock is made by
crostose. These occupy the depressions in the rocks and
other weathered portions. Their expanded thalli overshadow
and gradually replace the crustose lichens which eventually
decay and die and humus collect around the foliose. lichens
and acids eat into the rock. Gradually, there collects a
thin layer of soil consisting of rock particles, remains of
lichens and particles of dust. This prepares. The way of
xerophyte masses.
3.Moss stage:- the accumulation of soil and some humus in
the rock crevices makes the substratum of it for invasion of
xerophic mosses. Their rhizoids compete with the lichens and
their erect stems overshadow the latter. They are as drought
enduring as the foliose lichens and sometimes the two occur
together or the moss may come earlier.
Fruticose Lichens like clodonia accompany the mosses and
with them shade the forms which are worsted in the
completion and disappear.
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