Fluid Transport in Plant Stems
Investigating the Functions of Stems
In order to supply thirsty leaves, a giant redwood tree can absorb and transport 2,500 pounds of water a day from its roots upward through its stem to the unbelievable height of 300 feet (100 meters) or so. In the process, the tree expends the energy equivalent to launching a can of soda into near-earth orbit.
Fluid Transport in Stems
Fluid flow in plants happens in two different directions simultaneously. Water and minerals absorbed into the roots move upward to the leaves where they are needed for photosynthesis. At the same time, food in the form of carbohydrates is moving downward from the leaves to supply the stem and roots with energy.
Fluids in plants move through the vascular system, a series of tubes, channels, and openings that form a continuum throughout the plant. The vascular system is composed of two types of tissue: xylem, which conducts water and minerals upward from the vascular cylinder of the roots, and phloem, which transports carbohydrates (food) downward and outward from the veins in the leaves.
Xylem tissue is composed of two types of cells: tracheids and vessel elements. At maturity, both cell types are dead and consist only of cell walls. Tracheids are long, thin cells with tapering ends and numerous pits in the walls. Tracheids function mainly for support.
Like the tracheids, the side walls of vessel elements are pitted. Vessel elements are attached end-to-end forming a long pipe-like vessel. It is through these vessels that water and minerals flow upward to the leaves.
In phloem tissue the cells involved in the transport of food molecules are the sieve tube members. Unlike the conducting cells in the xylem, the sieve tube members are living cells. The end walls of these cells have several-to-many large pores and are called sieve plates. These pores allow cytoplasmic connections to occur between adjacent sieve tube members providing channels for conduction. The column of conducting sieve tube members is referred to as a sieve tube.
The Process of Fluid Transport in Stems
Fluid transport in plants is a remarkable process for while plants have a vascular system that corresponds to the blood vessels of complex animals, plants have no pump like a heart to move the fluids in these vessels. Clearly, there must be powerful internal forces at work within stems to move that much fluid over those kinds of distances.
Current theory holds that in small plants, such as strawberries and grasses, root pressure alone is sufficient to drive fluids upward to the terminus of the plant. In taller plants, however, a combination of forces is required to overcome the clutches of gravity.
Along with root pressure, movement of fluids upward through the xylem tissues is accomplished by what is called cohesion-tension theory. Water molecules have a strong attraction to each other – cohesion. Simultaneously, they have a strong attraction to the rigid walls of the xylem vessels – adhesion.
As water evaporates from the leaves, the water column in the vessels is subjected to great tension. However, the water column does not break because of the cohesion and it does not pull away from the xylem vessel walls because of adhesion. The only other option is for it to be pulled upward (known as transpirational pull). As water is pulled up the xylem by transpiration off the leaves, more water enters the roots from the soil to replace the water lost off the top of the column.
This combination of root pressure, cohesion-adhesion, and transpirational pull can generate tremendous pressure. And it must, for it takes 150 pounds per square inch to raise fluids 300 feet upward. These combined forces work together so efficiently and powerfully that a mighty oak can move water upward at the rate of an inch every 10 seconds.
Downward flow of carbohydrates (food), some plant hormones, and other organic molecules is currently explained by the pressure-flow hypothesis.
This model proposes that carbohydrates move from where they are made or stored, called a source, to where they are used or stored, called a sink. The process begins when carbohydrates are actively transported into sieve tubes. As carbohydrates enter the sieve tube, water is also transported in by osmosis. Thus, a positive pressure builds up at the source end. This is the pressure part of the hypothesis.
At the sink end of the sieve tube, this process is reversed. Carbohydrates are actively transported out, water leaves the sieve tube by osmosis, and pressure is reduced in the sink.
A single corn plant may transpire (evaporate off its leaves) more than two quarts of water a day, and an acre of corn more than 300,000 gallons of water a day. On a hot day, even a medium-sized tree can lose several hundred gallons of water as vapor off its leaves. Without the fluid transport in the stem to translocate water from roots to leaves, the plant would quickly dry and die.
Microscopic Vessels Allow Fluid Flow in Stems
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