Water ascent in tall trees: does evolution of land plants rely on a highly metastable state?

The root system of a plant is as complicated as the shoot in its diversity, in its reactions with the matrix of substances, and with the myriad organisms that surround it. Laboratory studies blind us to the complexity found by careful study of roots in soil. This complexity is illustrated in the much-studied corn root system, covering the changes along the framework roots: the surface tissues and their interactions with the soil, the water-conducting xylem, whose gradual elaboration dictates the water status of the root. A conspicuous manifestation of the changes is the rhizosheath, whose microflora differs from that on the mature bare zones. The multitude of fine roots is the most active part of the system in acquiring water and nutrients, with its own multitude of root tips, sites of intense chemical activity, that strongly modify the soil they contact, mobilize reluctant ions, immobilize toxic ions, coat the soil particles with mucilage, and select the microflora.