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Water movement in higher plants is treated as a symplastic fluid flow incorporated into a unified hydrodynamic system comprising the apoplast and vessels (or tracheids). Since water flow is of vital necessity for algae (phylogenetic ancestors of higher plants), it can be stated that higher plants colonized land, still keeping connections with their former water habitat. It is argued that colonization of terrestrial areas by plants became possible due to the appearance and maintenance of a gradient of water chemical potential between the rhizosphere and atmosphere, which drives water flows. Autonomization of flows in the symplast is considered as a vector of evolution, whereas the gradient of water activity is a factor of evolution. The osmotic models of water uptake by roots are analyzed; the role of potassium circulation in water-transporting system is determined; and a mechanism of automatic coupling between CO2 uptake through stomata and water evaporation from leaves is presented. An inherent property of the systems to explicitly or implicitly duplicate its structural or functional elements substantiates possible interactions between the mechanisms underlying opposite water flows in plants.