The xylem of vascular plants distributes water and dissolved minerals that are taken up by the roots to the rest of the plant. The cells that transport xylem sap are dead upon maturity, and the movement of xylem sap is a passive process.
Tracheids and vessel elements transport xylem sap
Tracheary elements are the transport cells of the xylem. They lack cytoplasm and organelles when they are mature and are considered part of the apoplast of the plant because they connect directly with the extracellular space. There are two types of tracheary elements: tracheids and vessel elements.
Tracheids are elongated cells with lignified walls that contain small gaps called pits, which conduct xylem sap from one cell to the next in places where their walls overlap. Seedless vascular plants and most gymnosperms, or cone-bearing plants, have only tracheids, which are thought to have evolved before vessel elements.
Vessel elements are wider lignified cells that stack vertically to form vessels. They are connected by perforation plates, specialized cell end structures that have spaces through which xylem sap can flow. The larger diameter and the more efficient structure of perforation plates means that vessels made up of vessel elements can move a much larger volume of sap. Most angiosperms, or flowering plants, have both tracheids and vessel elements.
Active transport of minerals creates a water pressure gradient from roots to leaves
While water enters a plant passively through permeable root cells, active transport is required to move minerals into the xylem. The resulting high concentration of solutes in the roots creates a gradient in the pressure potential of water within the xylem, with higher pressure in the roots and lower pressure elsewhere in the plant, where solutes are less concentrated. Water will then move toward the lower pressure areas; however, this gradient is only a minor contributor to the overall transport of sap through the xylem.
Physical forces on water molecules hold fluid within the xylem
Transport of xylem sap through a plant is made possible in part by some of the physical properties of water itself. The cohesion-tension hypothesis for transport of sap through the xylem was first proposed in the 1890s. Cohesion between water molecules is relatively strong because all three atoms of a water molecule can participate in hydrogen bonding with other water molecules. This means that transpirational pull in the leaves can affect water molecules throughout the xylem, like links in a chain, all the way to the roots.
Another force, adhesion, allows water molecules to stick to surfaces within the plant, such as the cell walls of mesophyll cells in the leaf, where water surface tension is essential for drawing sap out of leaf vessels when water vapor is transpiring from the leaves. Adhesion of water molecules to the walls of xylem vessels prevents sap from seeping downward and out of the plant through the roots when the stomata close and the tension produced by transpiration ceases.
