The movement of plants from water to land has necessitated the development of internal mechanisms to supply all the parts of the plant with water. As discussed in Plant Classification, Vasular Tissues , tracheophytes (including virtually all terrestrial plants except for mosses and liverworts), have developed complex vascular systems that move nutrients and water throughout the plant body through "tubes" of conductive cells. The vascular tissues of these plants are called xylem and phloem. The xylem of vascular plants consists of dead cells placed end to end that form tunnels through which water and minerals move upward from the roots (where they are taken in) to the rest of the plant. Phloem, which is made up of living cells, carries the products of photosynthesis (organic nutrients) from the leaves to the other parts. The vascular system is continuous throughout the whole plant, even though the xylem and phloem are often arranged differently in the root than they are in the shoot. The major mechanism by which water (along with dissolved materials) is carried upward through the xylem is called TATC (Transpiration-Adhesion-Tension-Cohesion). It should be noted that TATC, while supported by most scientists, is speculated but not proven to be at work in very tall trees. In this theory, transpiration, the evaporation of water from the leaf, is theorized to create a pressure differential that pulls fluids (held together by cohesion) up from the roots.
Water transport also occurs at the cellular level, as individual cells absorb and release water, and pass it along to neighboring cells. Water enters and leaves cells through osmosis, the passive diffusion of water across a membrane. In plants, water always moves from an area of higher water potential to an area of lower water potential. Water potential results from the differences in osmotic concentration (the concentration of solute in the water) as well as differences in water pressure (caused...
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