Roots

Roots are important to plants for two main reasons: Firstly, they provide anchorage to the substrate; more importantly, they provide a source of water and nutrients from the soil. Roots allowed plants to grow taller and faster. The onset of roots also had effects on a global scale. By disturbing the soil, and promoting its acidification (by taking up nutrients such as nitrate and phosphate[verification needed]), they enabled it to weather more deeply, promoting the draw-down of CO2[71] with huge implications for climate.[72] These effects may have been so profound they led to a mass extinction.[73] But, how and when did roots evolve in the first place? While there are traces of root-like impressions in fossil soils in the late Silurian,[74] body fossils show the earliest plants to be devoid of roots. Many had tendrils that sprawled along or beneath the ground, with upright axes or ‹See Tfd›thalli dotted here and there, and some even had non-photosynthetic subterranean branches which lacked stomata. The distinction between root and specialised branch is developmental; true roots follow a different developmental trajectory to stems. Further, roots differ in their branching pattern, and in possession of a root cap.[4] So while Silu-Devonian plants such as Rhynia and Horneophyton possessed the physiological equivalent of roots, roots – defined as organs differentiated from stems – did not arrive until later.[4] Unfortunately, roots are rarely preserved in the fossil record, and our understanding of their evolutionary origin is sparse.[4] Rhizoids – small structures performing the same role as roots, usually a cell in diameter – probably evolved very early, perhaps even before plants colonised the land; they are recognised in the Characeae, an algal sister group to land plants.[4] That said, rhizoids probably evolved more than once; the rhizines of lichens, for exa

ple, perform a similar role. Even some animals (Lamellibrachia) have root-like structures![4] More advanced structures are common in the Rhynie chert, and many other fossils of comparable early Devonian age bear structures that look like, and acted like, roots.[4] The rhyniophytes bore fine rhizoids, and the trimerophytes and herbaceous lycopods of the chert bore root-like structure penetrating a few centimetres into the soil.[75] However, none of these fossils display all the features borne by modern roots.[4] Roots and root-like structures became increasingly more common and deeper penetrating during the Devonian period, with lycopod trees forming roots around 20 cm long during the Eifelian and Givetian. These were joined by progymnosperms, which rooted up to about a metre deep, during the ensuing Frasnian stage.[75] True gymnosperms and zygopterid ferns also formed shallow rooting systems during the Famennian period.[75] The rhizomorphs of the lycopods provide a slightly different approach to rooting. They were equivalent to stems, with organs equivalent to leaves performing the role of rootlets.[4] A similar construction is observed in the extant lycopod Isoetes, and this appears to be evidence that roots evolved independently at least twice, in the lycophytes and other plants,[4] a proposition supported by studies showing that roots are initiated and their growth promoted by different mechanisms in lycophytes and euphyllophytes.[76] A vascular system is indispensable to rooted plants, as non-photosynthesising roots need a supply of sugars, and a vascular system is required to transport water and nutrients from the roots to the rest of the plant.[6] These plants are little more advanced than their Silurian forbears, without a dedicated root system; however, the flat-lying axes can be clearly seen to have growths similar to the rhizoids of bryophytes today.