The Evolution of Plant Roots

Roots are essential to the development of large plants because they provide a means of anchoring and maintaining an upright position.Most land plants are literally rooted to the spot.Roots also play a key role in water and nutrient acquisition.More significantly still, roots have a tremendous impact on the environment. They can break up rock, bind loose particles together, and prov idea conduit for the movement of water and dissolved minerals,all of which are essential to the development of soils.

In piecing together a fossil plant to form a conceptual whole, it is usually the rooting system that remains the final piece in the puzzle. It is often the case that roots are poorly studied or completely unknown.Although the fossil record of roots is therefore less complete than that of other plant organ systems, it is possible to discern some general trends.The earliest land plants, like modern mosses and liverworts,did not have well-developed root systems.These plants simply bore absorbing hairlike cells on stems and leaves that grew flat along the ground.From their fossils,some very early plants are known to have borne branches that appear to be specially modified for rooting. In other cases, roots were able to form from dormant buds on aerial stems.Fungi are also known to have played a key role in these early rooting systems, as they do in modern plants.Fungal symbionts—fungi that live in mutually beneficial relationships with another organism-have been recorded in the petrified plants of the 400million-year-old Rhynie Chert fossil site in Scotland, demonstrating a link with mycorrhizal fungi that goes back to the dawn of the land flora.These tiny, shallow rooting systems were adequate for small plants (30-50 centimeters tall), but larger organisms required something more substantial.

By the Late Devonian and Early Carboniferous eras (385 to 300 million years ago), an enormous variety of rooting structures had evolved.The evolution of large erect plants, and in particular trees,placed increasing demands upon the anchoring and supply functions of roots.These problems were solved mainly through the development of more extensive underground systems.The evolution of the cambium, the layer of living cells between wood and bark,enabled continuous perennial growth and long-term survival of roots in soils.

One important consequence of all this was that there was a progressive and massive increase in root biomass during the Devonian, which had an enormous impact on the development of soils. Prior to the Devonian,soils, if developed at all, are thought to have been predominantly thin and of microbial origin. By the Middle Devonian,soil penetration depths of roots were still shallow(less than 20 centimeters), but this increased to 1 meter or more as forests spread.The diversity of soils also increased.This change was brought about by root-induced weathering and mixing. By the end of the Devonian, there was an increase in soil clay content,structure,and differentiation into distinct layers—a development that correlated with increases in depth of root penetration. Soils with modern profiles(series of layers) are recognizable at this time.

The impact of roots on the environment extends beyond their immediate effects on the development of soils.The presence of roots in soils increases the natural weathering of calcium and magnesium silicate minerals.This apparently mundane fact turns out to have extremely important consequences for climate and temperature globally. Under natural circumstances, calcium and magnesium silicates react chemically with a dissolved form of the gas carbon dioxide (a process referred to as weathering), which comes from the atmosphere.This produces calcium and magnesium carbonates,which are transferred through the groundwater system to rivers and ultimately to the oceans, where they accumulate in the form of limestone and dolomite rock.Across the surface of the Earth, these chemical reactions occur on a vast scale, removing carbon dioxide gas from the atmosphere and locking it up as carbonate in rock formations. This reduces the so-called greenhouse effect, which leads to lower global temperatures. In other words, the widespread development of roots in land plants affected the chemistry of the atmosphere and the oceans, which, summed over millions of years,added up to changes in climate on a global scale.

1.Roots are essential to the development of large plants because they provide a means of anchoring and maintaining an upright position.Most land plants are literally rooted to the spot.Roots also play a key role in water and nutrient acquisition.More significantly still, roots have a tremendous impact on the environment. They can break up rock, bind loose particles together, and prov idea conduit for the movement of water and dissolved minerals,all of which are essential to the development of soils.