Plants: Essential Processes
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 by the presence of rigid cell walls) between two regions. The relationship between the amount of dissolves solute and water potential is inverse: where there is a lot of dissolved solute the water potential is low.
Most of the water that a plant takes in enters through the root hairs. The water diffuses easily (and osmotically) into the root hairs because the concentration of dissolved materials in the plant's cellular cytoplasm is high. As discussed in Plant Classification, Root Hairs, there are two pathways through which water travels from the outside of the root to the core, where it is picked up by the xylem. The first of these pathways is the symplast, in which water moves across the root hair membrane and through the cells themselves, via channels that connect their contents. An alternate route for water is the apoplast, in which water travels along cell walls and through intercellular spaces to reach the core of the root. Once in the xylem, the water can be carried by TATC to all the other parts of the plant.
Overall, water is transported in the plant through the combined efforts of individual cells and the conductive tissues of the vascular system. Water from the soil enters the root hairs by moving along a water potential gradient and into the xylem through either the apoplast or symplast pathway. It is carried upward through the xylem by transpiration, and then passed into the leaves along another water potential gradient. In the leaf, some water is lost through evaporation from the stomata and the remaining fluid moves along a water potential gradient from the xylem into the phloem, where it is distributed along with the organic nutrients produced by photosynthesis throughout the plant.