ABSTRACT
The root systems of terrestrial plants perform two primary functions: the acquisition of soil-based resources (principally water and ions), and anchorage. Other root system functions, such as storage, synthesis of growth regulators, propagation, and dispersal, can be seen as secondary. Little is known of the early evolution of the nonaerial parts of plants, although root traces have been found in fossil soils from Silurian deposits (Retallack, 1997), but it is certain that most of the first land plants, such as Cooksonia, Aglaophyton, and Rhynia, dating from the late Silurian and early Devonian periods, had poorly developed root systems (Collinson and Scott, 1987; see also Chapter 1 by Kenrick in this volume). Such fragments as preserved are of large diameter, and sparingly and often dichotomously branched. Early land plants were small and lived in very wet environments; evidence for plants with developed root systems growing in welldrained soils does not appear until the late Devonian (Driese et al., 1997). Neither anchorage nor acquisition of water is therefore likely to have been a serious problem: the tree habit, necessitating deep rooting for anchorage, did not develop until the mid-Devonian, when the evolution of the seed further freed plants from dependence on wet environments (Algeo and Scheckler, 1998; Bateman et al., 1998). It is likely that the most difficult function for early root systems to perform was the acquisition of poorly mobile resources, especially phosphate (Pirozynski and Malloch, 1975; Lewis, 1987). Modern plants with similar underground parts, such as those with “magnolioid” roots, i.e., thick, littlebranched root systems typified by the primitive family Magnoliaceae (Baylis, 1975), or achlorophyllous orchids (e.g., Neottia, Epipogium), are habitually or even obligately mycorrhizal. It is almost certain that Aglaophyton also was mycorrhizal, since arbuscules, the diagnostic structures of the most abundant group of mycorrhizal fungi, have been identified in its fossils (Remy et al., 1994). Mycorrhizal fungal spores have been found in Silurian deposits that certainly predate the evolution of root systems (Redecker et al., 2000).
