Plants require a continuous supply of water. Water serves multiple purposes, from participating in photosynthesis to maintaining structural rigidity. This fluid must travel from the roots, where it is absorbed, to the furthest leaves and growing points. Plants achieve this through specialized tissues in their stems.
The Plant’s Water Highway
The xylem is the primary tissue responsible for water transport in plants. This specialized vascular tissue forms a complex network throughout the plant, extending from the roots, through the stem, and into the leaves. Xylem functions as the plant’s internal plumbing system, moving water and dissolved minerals absorbed from the soil upwards.
The xylem is composed of several cell types, including tracheids and vessel elements, which are elongated, tube-like structures. These cells are dead at maturity, forming hollow conduits for unimpeded water flow. Their walls are reinforced with lignin, which prevents collapse under pressure and provides mechanical support to the plant.
Driving Water Upward
Water movement through the xylem is driven by transpiration. This involves the evaporation of water vapor from tiny pores, stomata, on the surface of leaves. As water evaporates, it creates negative pressure, or tension, within the xylem, drawing water upwards from the roots.
Two properties of water, cohesion and adhesion, are important for this upward movement. Cohesion is the attraction between water molecules, allowing them to stick together and form an unbroken column within the xylem tubes. Adhesion is the attraction between water molecules and the inner walls of the xylem vessels, helping to counteract gravity. These forces, combined with transpirational pull, enable water to ascend against gravity, even to the tops of the tallest trees. This process is passive, requiring no direct cellular energy.
More Than Just Water
While xylem transports water and minerals, plants also have another distinct transport system. This system, the phloem, moves organic compounds. The phloem distributes sugars, produced during photosynthesis in the leaves, to all other plant parts, including roots, fruits, and growing regions.
Unlike the unidirectional flow of water in the xylem, sugar transport in the phloem can occur in multiple directions, depending on where sugars are produced and needed. This ensures energy-rich compounds are efficiently supplied throughout the plant for growth, storage, and metabolic activities. Xylem and phloem work in tandem, with distinct roles in transporting water versus food.