The Water Circuit of the Plants - Do Plants have Hearts ?

There is a correspondence between the circulation of blood in all higher animals and the circulation of sap in all higher plants-up to heights h of < ∼ 140 m-through the xylem and phloem vessels. Plants suck in water from the soil, osmotically through the roothair zone, and subsequently lift it osmotically again, and by capillary suction (via their buds, leaves, and fruits) into their crowns. In between happens a reverse osmosis-the endodermis jump-realized by two layers of subcellular mechanical pumps in the endodermis walls which are powered by ATP, or in addition by two analogous layers of such pumps in the exodermis. The thus established root pressure helps forcing the absorbed ground water upward, through the whole plant, and often out again, in the form of guttation, or exudation. We do not know a plant that could grow without water. Extreme cases of modest needs of water are fungi as well as all desert plants, but even such creatures decay when the air moisture drops permanently below some threshold, of order 60%. Water is primarily required for the transport of materials, both from the ambient soil towards the solar factories in the leaves-or even branches: Pfanz et al (2002)-and from the leaves towards all the domains of growth, above and below ground. Water from the soil provides the indispensable anorganic nutrients; it is conducted upward through the xylem vessels to the leaves, in which photosynthesis converts carbon dioxide (from the ambient air) into energy-rich hydrocarbons, mainly into starch, with nitrogen-and phosphor-containing intermediate products. After the upward transport, water is again required for taking the photosynthesis products from the leaves through the phloem to all sites of growth, in the buds, branches, and stems, roots, blooms, and fruits. Water must therefore permanently circulate through every plant, as sap with dissolved materials of varying concentrations. Moreover, water is the basic substance of the fluid inside cells, the cytoplast, as well as the fluid between cells, the apoplast, and is also an indispensable building material of the cell membranes and walls as well as all other hardware of plants, like carbohydrates, lipids, proteins, and nucleic acids. The problem of how plants manage to lift their water from the soil to their leaves has been with biologists at least since 1726, when S. Hales discussed it in the first edition of the Proceedings of the Royal Society of London. So much has been …