<P> Recent research show that ancient moss could explain why the Ordovician ice ages occurred . When the ancestors of today's moss started to spread on land 470 million years ago, they absorbed CO from the atmosphere and extracted minerals by secreting organic acids that dissolved the rocks they were growing on . These chemically altered rocks in turn reacted with the atmospheric CO and formed new carbonate rocks in the ocean through the weathering of calcium and magnesium ions from silicate rocks . The weathered rocks also released a lot of phosphorus and iron which ended up in the oceans, where it caused massive algal blooms, resulting in organic carbon burial, extracting more carbon dioxide from the atmosphere . Small organisms feeding on the nutrients created large areas without oxygen, which caused a mass extinction of marine species, while the levels of CO dropped all over the world, allowing the formation of ice caps on the poles . </P> <P> Since moss gametophytes are autotrophic they require enough sunlight to perform photosynthesis . Shade tolerance varies by species, just as it does with higher plants . In most areas, mosses grow chiefly in areas of dampness and shade, such as wooded areas and at the edges of streams; but they can grow anywhere in cool damp cloudy climates, and some species are adapted to sunny, seasonally dry areas like alpine rocks or stabilized sand dunes . </P> <P> Choice of substrate varies by species as well . Moss species can be classed as growing on: rocks, exposed mineral soil, disturbed soils, acid soil, calcareous soil, cliff seeps and waterfall spray areas, streamsides, shaded humusy soil, downed logs, burnt stumps, tree trunk bases, upper tree trunks, and tree branches or in bogs . Moss species growing on or under trees are often specific about the species of trees they grow on, such as preferring conifers to broadleaf trees, oaks to alders, or vice versa . While mosses often grow on trees as epiphytes, they are never parasitic on the tree . </P> <P> Mosses are also found in cracks between paving stones in damp city streets, and on roofs . Some species adapted to disturbed, sunny areas are well adapted to urban conditions and are commonly found in cities . Examples would be Rhytidiadelphus squarrosus, a garden weed in Vancouver and Seattle areas; Bryum argenteum, the cosmopolitan sidewalk moss, and Ceratodon purpureus, red roof moss, another cosmopolitan species . A few species are wholly aquatic, such as Fontinalis antipyretica, common water moss; and others such as Sphagnum inhabit bogs, marshes and very slow - moving waterways . Such aquatic or semi-aquatic mosses can greatly exceed the normal range of lengths seen in terrestrial mosses . Individual plants 20--30 cm (8--12 in) or more long are common in Sphagnum species for example . </P>

Moss growing on trees is an example of what