PLANT TRANSPORT
Plant Transport relies for the most part on two types of tubes,phloem and xylem. Xylem are tubes that carry water upwards in a plant. (see key terms). Remember that water moves from area of high water potential to low water potential and that water potential depends on gravity, pressure and solute concentration. Water moves from the roots to the leaves by following a water potential gradient. In order to allow carbon dioxide to filter into the plant, the plant must open its stomata. Consequently, water is lost by evaporation through the stomata in a process called transpiration. Water that gets pulled from the leaf by transpiration is replaced by water from surrounding cells (moving from an area of high water potential to low water potential). Other researchers claim that water adheres to cellulose in the walls of plant cells and water literally creeps along cell boundaries. This pulling force is called transpiration pull. The water in the xylem is like a long column. As the top of the column is pulled (transpiration), the rest of the column is pulled with it, called tension. Water can be pulled so high in plants because of the small size of the individual xylem and the cohesive properties of water. The same hydrogen bonds that account for water's high specific heat give water a cohesive property that allows the xylem to act like a contiguous column. As long as the root has a lower water potential than the surrounding soil, water will flow into a plant's roots. Water moves through porous cell walls in the epidermis of the root to cells of the endodermis. Cells in the endodermis are able to regulate their mineral concentrations to affect water potential and osmotic pressure. However, other scientists believe that bulk flow is the main component of water uptake in the roots. Water is then directed by Casparian Strips into the Stele. The stele is the grouping of xylem and phloem. Once the water reaches the xylem it is directed up through the plant.