We cannot assert that the mechanisms responsible for water and solute uptake by plant roots are in full measure known. It is agreed that the active solute transport and the osmotic dis-placement of water play an important part in this process but the sufficiency of these mecha-nisms, as well as the spatial organization of their operation, are a subject of discussion. This process has been described using compartmental models in which different radial root layers were treated as compartments separated from one another and the environment by membranes with different characteristics (see [1] and references thereafter). This approach is fairly empirical and does not take into account the main characteristics of the functioning of the root as a distrib-uted mechanical system.
The approach we develop is based on the representation of the water and solute transport from the environment to the xylem as flow of a two-phase fluid through a solid frame-work. Two fluid phases, both containing a generalized solute, are identified with the intra- and extracellular fluids that fill the symplast and apoplast. The partial differential equations of motion are formu-lated for each fluid phase and for the dissolved component in each phase in accordance with principles of multiphase continuum mechanics, with account for hydraulic resistance, diffusion, convection, and (in the intracellular phase) the distributed osmotic force. The interphase mass exchange is regulated by membrane-type relations that take into account the active solute trans-port, the interface permeability to the solute and water, and the osmotic water transfer. Our ap-proach makes it possible to investigate various hypotheses concerning the spatial distribution of tissue characteristics, like the membrane permeability or the ion pump activity. Moreover, it gives a tool for analyzing other possible hypotheses, which can be easily incorporated in this general framework.
On the basis of our model we performed flow calculations for coefficient values varied over ranges consistent with estimates known from the literature. In the calculations we used dif-ferent boundary conditions and different radial distributions of the system characteristics. The computation domain was formed by the space between two coaxial cylinders and the domain boundaries corresponded to the interfaces with the environment and the xylem vessels. On the inner boundary (with the xylem) different boundary conditions were used: the extracellular pres-sure was always assumed to be given, whereas for the solute it was assumed either the given concentration (which corresponds to certain experiments) or a convective entrainment condition, most adequate to other experiments with excised roots and even (with certain limitations) to the root functioning in the whole plant.
It is shown that the known experimental data can satisfactorily be described if we assume the presence of ion pumps in an outer (cortex) part of the root only, a non-uniform radial distri-bution of the membrane permeability to the solute (higher in the outer part), and the presence of a localized inner barrier impermeable for the extracellular phase and the solute contained in it (Casparian bands). It is of interest that in the absence of this barrier the system, although can work as an "osmotic pump" if an osmotic pressure difference is maintained between the input and output, is not able to pump water efficiently with the convective entrainment condition as-signed.
From the continual model developed various compartmental models can be obtained using averaging procedures.

[1] Murphy R. Some compartmental models of the root: Steady-state behavior (2000) // J. Theor. Biol. v. 207. pp. 557-576.