How does water travel from the soil into the plant and make the journey upward into the leaves? Most of the water a plant uses is absorbed by the roots; without a pumping heart, how does the water move against gravitational forces, sometimes for several hundred feet?
The force at work is called transpirational pull, which can be visualized by imagining sipping a drink through a straw. Evaporation of water through stomata (tiny openings in leaves) creates the suction, pulling water continuously upward through the stem. The hotter and sunnier the day, the faster the evaporation and the faster the transpirational pull. Problems begin when the rate of transpiration exceeds the ability of the roots to absorb water from the soil (usually because the soil is too dry) and the plant wilts due to loss of turgor, or water pressure, within their cells.
How It Works
When water enters the root, it crosses the outer epidermis and moves toward the xylem (water-conducting tissue) by moving in the spaces between the cells or actually through them. Once water reaches the endodermis however, it runs into the impermeable band called the casparian strip that seals off the pathways between cells, thus forcing the water to keep moving only by going through the cells.
Tiny channels called aquaporins allow absorption and transport toward the xylem by osmosis. All of these layers serve as a filtration system. Once the water reaches the xylem, which is a series of open tubes, it moves quickly upward, continuing through xylem branches in leaf petioles, then into the midrib and tiny veins in the leaves, and ultimately into the cells themselves through osmosis.
Bonds That Can Be Broken
In order to keep the whole thing working, it is important that water keeps moving in an unbroken flow. This is possible because water molecules bind together, or are cohesive. The bonds are strong enough to keep the molecules stuck together even as they travel up to great heights. However, if the transpirational pull becomes too great (as on a hot day), the cohesion is broken and an air bubble, or embolism forms. Unless the bonds can be reestablished (sometimes water will enter the xylem from surrounding cells and fill the gap and force the air to dissolve), the flow of water is permanently interrupted. Water can be diverted around the embolism by moving laterally into other xylem tubes, but if too many embolisms are present the part of the plant above them will die.
Other causes of embolisms are drought (transpirational pull eventually exceeds the rate of water coming in from the roots, so cohesion is broken) and freezing, where air is forced out of water as it freezes within the plant.
Capon, Brian. 2005. Botany for Gardeners. Timber Press: Portland, Oregon
McElrone, A.J., Choat, B., Gambetta, G.A. and Broderson, C.R. (2013) Water Uptake and Transport in Vascular Plants. Nature Education Knowledge
ÖrdÖg, Vince, Molnár, Zoltán (2011) Plant Physiology. University of Debrecen, University of West Hungary, Pannon University