One, for water and minerals, the xylem, originates in the root and moves water and minerals upward. The second, the phloem, moves dissolved carbohydrates out of the leaves to other plant parts in which they are used for growth or stored.
Vascular plant tissue is designed to meet the nutritional transport needs of land plants. Xylem tissue has two types of transport cells; both are non living when functional.The smaller in diameter is the tracheid. These have a narrow bore and tapering, overlapping ends.
These tapered ends have numerous pits, which are narrow passages to adjacent cells. Water passes through the cell to the next cell through these pits. Because of the narrow openings from one cell to the next, the flow of water tends to be rather small.
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The second cell type in the xylemis the vessel element. These cells have a much larger diameter than the tracheids and have end walls that rarely overlap and are totally open. This second type is very much like a section of water pipe,with no end walls at the end of each section to restrict the flow of water.
The structure of the phloem is similar to that of the xylem in that two cell types are found. Unlike the cells in the xylem, however, only one of the phloem cells, called the sieve element, actually transports fluids; the other is called the companion cell and seems to be involved in unloading the phloem.
Water Movement in the Xylem
One of the early explanations for water and mineral movement in plants was “root pressure.” The water in the xylemin the root contains ten to fifteen times the mineral content of the soil water, an accumulation that cannot be explained by diffusion or osmosis.
Botanists now realize that the root hairs, those finger like outgrowths of the epidermal cells that expand the surface area of the roots to hundreds of square meters, and the cells of the cortex are able to use energy to collect mineral ions, such as magnesium ions, which are then transported into the xylem in the root.
As mineral ions accumulate in the root’s xylem, water diffuses into the root as well. This accumulation of mineral ions and water creates a type of pressure called turgor pressure.
This pressure is identical to the pressure in the water pipes of a house. Any time a faucet is opened, water moves toward the opening. This movement is called bulk flow or convective movement or hydraulic lift.
Root pressure is generally limited by two factors. The first is the large amount of energy needed to accumulate mineral ions from the soil. The second is the problem associated with the narrow passages between tracheids.
As a result, root pressure is generally limited to plants of a few meters in height or less. While root pressure is easily demonstrated in the laboratory, it is clearly not able to move water to the tops of trees, which may reach a height of 130 meters (400 feet) or more.
Cohesion-Adhesion Theory
The means whereby nutrients and water are transported through large plants and trees is explained by the cohesion-adhesion theory. Leaves are the primary sites of photosynthesis in most plants. During photosynthesis atmospheric carbon dioxide is required to make glucose,which is converted to sucrose for transport.
As a result the leaves’ surface is covered by small openings called stomata. As carbon dioxide is entering the leaf, water is evaporating from the leaf in a process called transpiration. Transpiration is driven by the much lower water content of the atmosphere compared to the water content of the leaf.
Transpiration generates a tension,from mega pascals in the xylem of the leaf. This tension is transmitted through the solid water columns of the xylem to the roots and increases the uptake of water in the root. At the more negative part of this range, water should exist as a gas rather than a liquid.
That, however, is not the case, because the cell wall, with its massive number of –OHs, stabilizes the water via adhesion—that is, by forming hydrogen bonds with water molecules, which are also hydrogen bonded to each other via cohesion.
One might expect the water column to break under its own weight (called cavitation), but that does not occur either. Lyman Briggs (1950) demonstrated that a small column of water requires at least 25 mega-pascals before it will cavitate.
For example, as oil in an oil lamp is used in the burning end of the wick (transpiration), more oil is pulled (via adhesion and cohesion) up the wick (similar to a stemor root) from the oil in the base of the lamp (similar to the soil). The moving stream, called the transpiration stream, transports water, mineral ions, and sometimes other materials from the roots to the leaves.
Evidence for the Cohesion-Adhesion Theory
Evidence to support the cohesion-adhesion theory was provided by Per Scholander (1965). He reasoned that if the xylem were under tension when a twig is cut from the plant, the water columns in the xylem would pull back to a position that could be supported by atmospheric pressure.
If this twig were placed in a closed chamber with the cut surface exposed and the pressure were increased gradually in the chamber, it should be possible to force the water columns out to the cut surface of the twig.
The pressure would be a direct measure of the tension in the xylem but of opposite sign, that is if 2 mega pascals were necessary to get the water out to the cut surface the twig, it must have been under -2 mega pascals, since tension is a negative pressure.
Scholander built a small aluminum chamber with an associated pressure gauge and gas supply, called the Scholander Pressure Chamber. He then traveled about and measured the tension in numerous parts of many trees, under a variety of environmental conditions.
He and many others since have reported tension in the –0.1 to –5 mega pascals range for leaves, twigs stems, and roots, with the values becoming more negative as one takes samples higher on the same plant.
The Problem of Transpiration
Transpirational water loss is a major loss to the plant. As much as 95 percent of all the water entering the roots is lost by transpiration. Transpiration is under environmental control. Stomatal opening is controlled by the carbon dioxide level of the interior of the leaf; as carbon dioxide inside decreases, the stomata open.
With the stomata open, a drop in humidity, an increase in temperature, an increase in air currents, or all these conditions will cause an increase in transpiration and the tension in the xylem. Transpiration will continue until darkness or until the water in the plant is so reduced that the plant wilts.
When the xylem is under tension, air bubbles may enter the water columns; these are called embolisms.When a water column is broken by an embolism, it will not transport water. Thus, water transport is reduced. There is some evidence that these embolisms may be repaired.
Movement in the Phloem
Movement of materials in the phloem—again, the conducting tissue that is responsible for moving food manufactured in the leaves to other parts of the plant, including the roots—is driven by pressure rather than by tension.
The leaves are the primary sites for photosynthesis and thus its product sucrose; leaves, therefore, may be called the source. These carbohydrates are loaded into the phloem in the leaf, up to 1.2 moles per kilogram of water.
With more solute in the phloem, water diffuses into the phloem from the xylem. This loading of sucrose and influx of water generate pressure, which is transmitted throughout the phloem.
Any site in the plant, known as a sink, which is actively using carbohydrates for the production of new cells in fruits or the storage of starch in the roots, relieves the pressure, and the assimilate stream, as the fluid in the phloem is called, will move in the direction of the relief, much like a dripping faucet. This assimilate flow will continue as long as carbohydrates are being consumed in an area.
As a result of high levels of carbohydrates in the source and the use of carbohydrates in the sinks, the rate and direction of flow of materials in the phloem are controlled. Sinks may change from hour to hour during the day, sometimes sending materials to the roots, other times sending materials to the flowers or even developing leaves.
Evidence for Pressure in Phloem Transport
Evidence for pressurized phloem transport is easily collected using aphids. Aphids have sharp, hollow snouts, which they are able to insert into sieve cells very accurately. To investigate phloem transport, scientists have allowed aphids to infest a plant.
Once they are settled, it is fairly easy to sever the snouts from the bodies of the aphids. These snouts will continue to “bleed” phloem sap for several days. Bleeding could not occur if the phloem were under tension, which supports the theory of pressurized phloem transport.
Additional evidence for pressurized phloem transport was provided by Ernst Munch in 1927 as a laboratory model.He attached two dialysis bags together byway of a glass tube. Into one of the bags he placed a sucrose solution (the source), and water was placed in the other (the sink).
When both bags were placed in water, the water diffused into the sucrose solution. This generated a pressure that was transmitted via the tube to the other bag and caused water to flow out of the second bag. Thus, the materials in the sucrose bag were transported to the other bag.
