Lichen

Lichens are the first to appear on a bare rock. The lichens which colonise bare rock are crustose lichens, e.g.,Rhizocarpon, Hinodina etc.

These lichens are resistant to desiccation and temperature extremes. Their propagules, dropped by wind

and temiperature extremes. Their propagules, dropped by wind on a rock and moistened organic acids which corrode the rock surface by rain or dew, develop attaching rhizoids and grow into lichens. The lichens produce This produces depressions and releases minerals which facilite further growth of the lichens. The lichens hold the fine partides of rock, along with the sand particles brought by wind, in the depressions, to initiate soil formation. Dead lichens matter to the forming soil which slowly becomes fertile.

contribute organic Crustose lichens are then replaced by foliose type of lichens, e.g, Dermatocarpon, Parmelia. These lichens have large leaf-like thall They can absorb and retain more water, and are able to accumulate dust particles which help in the further build up of the substratum Thus, some humus becomes accumulated. The weathering of rocks and its mixing with humus results in the development of a fine thin soil layer on rock surface, and thus there is a change in the habitat.