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48
Plants
Some of the world’s most important plants are the tiny, single-celled organisms called algae. They are the basis of many food chains, and they also secrete a hard, limy skeleton. Moreover, they occur in vast numbers. For this reason, they have become important rock builders through much of geological time. Many coral reefs are made up mainly of algae. The earliest living things, the stromatolites, are made of mats of algae. The calcified remains of tiny algae, called coccoliths, make up chalk rock.
Algae absorb nourishment through their surfaces. More developed (vascular) plants can extract nourishment from water and soil. Some of the oldest plants, the pteridophytes (horsetails, ferns), reproduced by spores; later, seeds became more common.
Pteriodophytes dominated the land
in the Upper Palaeozoic Era. Seed-bearing plants were already important by the Upper Carboniferous Epoch.
Plants do not provide the easiest way of dating rocks because species are hard to identify from partial remains.
Articulatean
Lycopod
Bennetitalean
Psilophyte
(Above) Forest plants from the Carboniferous Period.