Vascular tissue is not surprising that their form should be similar to pipes. The individual cells of phloem are connected end-to-end, just as the sections of a pipe might be. As the plant grows, new vascular tissue differentiates in the growing tips of the plant. The new tissue is aligned with existing vascular tissue, maintaining its connection throughout the plant. The vascular tissue in plants is arranged in long, discrete strands called vascular bundles. These bundles include both xylem and phloem, as well as supporting and protective cells. In stems and roots, the xylem typically lies closer to the interior of the
Sap century, there has been some controversy regarding the mechanism of xylem sap transport; today, most plant scientists agree that the cohesion-tension theory best explains this process, but multiforce theories that hypothesize several alternative mechanisms have been suggested, including longitudinal cellular and xylem osmotic pressure gradients, axial potential gradients in the vessels, and gel- and gas-bubble-supported interfacial gradients. Xylem sap transport can be disrupted by cavitation—an "abrupt phase change [of water] from liquid to vapor"—resulting in air-filled xylem conduits. In addition to being a fundamental physical limit on tree height, two environmental stresses can disrupt xylem transport by cavitation: "increasingly negative