Roots absorb water and leaves release water, but how does water move up a plant? In this lesson, we will look at how this happens in vascular plants, including the importance of xylem, cohesion and transpiration in the process.
Review of Xylem
You may remember that vascular tissue is the tissue used to transport water and nutrients throughout a plant. There are two types of vascular tissue: xylem and phloem. Xylem transports water and dissolved minerals, while phloem transports food. We will focus on the structure of xylem and how this vascular tissue actually transports water throughout a plant.
Let's first review a few basic aspects of xylem. You may remember that xylem is found in vascular plants but not in non-vascular plants. Since xylem is a type of vascular tissue, this presence or absence of xylem makes sense. Within vascular plants, the arrangement of xylem is different depending on the specific type of flowering plant. In monocots, such as grasses, xylem is found in paired bundles with phloem throughout the stem. In dicots, such as trees, the xylem is found in concentric rings. The xylem is on the inside of the ring and the phloem is on the outside of the ring. We can see the layout of xylem in a monocot and in a dicot below. Remember that in the monocot, the xylem is found throughout the stem while in the dicot, the xylem is found in rings - like those you see in a tree trunk.
Layout of xylem
Xylem is made from vessels, which are continuous tubes from dead, hollow, cylindrical cells lined up end to end throughout the plant. There may also be tracheids, which are dead cells that are tapered at the ends and overlap. Let's look at the vessels and tracheids a little bit more to get a better idea. Think of vessels as round pipes. These are lined up end to end in order to create a longer tube to transport water. Tracheids are different because they are tapered at the ends. While tracheids are useful, they are not as efficient at transporting water as vessels are because there may be gaps between the cells. We can see this in our diagram below. Additionally, vessels may be strengthened by lignin, which is produced by cells before they die to strengthen the cell wall. Lignin makes the vessels more durable.
In tracheids, there may be gaps between the cells.
Review of Cohesion
Cohesion is the forces holding water molecules together. This concept was covered when talking about properties of water and capillary action. You may remember capillary action is like when you have a little bit of water in the bottom of your cup but the water in the straw is a little bit higher than the water in the cup. Because water is polar - meaning it has a somewhat negative end and a somewhat positive end - it is attracted to other water molecules and other substances, such as the straw.
Like water moving up a straw, water moves from the roots and out of the leaves due to cohesion.
Now, you may be asking yourself: what does a straw have to do with xylem? Well, just like the water moving up the straw, water moves up xylem. The movement of water in plants is from the roots up through the shoot and out the leaves. Part of the reason for this movement of water is cohesion. The water molecules within the xylem tend to stick together, which allows them to help pull other water molecules up through the xylem - even against the flow of gravity.
When water is moving through the xylem, it is moving between different vessels or tracheids. It is very important that these elements are tightly held together because if there are air bubbles, the effects of cohesion are diminished. That is, if air bubbles get into the xylem, the water will no longer move up through these tubes. We can relate this idea back to our straw. If you have ever had a straw that has even a tiny hole in it, you know that it is very hard - even with the aid of suction - to get the water through the straw and up to you. This is what happens if air bubbles get into the tubes of the xylem.
Transpiration and Cohesion
Before we can talk about the effects of transpiration on the function of xylem, we need to review what transpiration actually is. Transpiration is the release of water vapor in plants and is regulated depending on the needs of the plant as well as the conditions of the surrounding environment. More than 90% of the water absorbed by the roots is released through transpiration. You may be able to relate transpiration in plants to perspiration in people. Perspiration is when you sweat, so you are releasing water. When plants release water, it is transpiration.
Transpiration occurs in the leaves, and the guard cells will open or close the stomata based on these needs. Remember that stomata are pores in the leaf that allow for gas exchange and are generally found on the underside or bottom of leaves. On either side of the stomata are guard cells that control the opening and closing of the stomata.
When plants release water through transpiration, cohesion is responsible for moving more water throughout the xylem. When the water leaves through the stomata, new water is pulled into the leaf by this pull between water molecules. Think of this as a very crowded restaurant with a long line. There are many people inside and a line of other people waiting to get in. When people are finished eating, they leave the restaurant but are quickly replaced by new people who were waiting in line. The people in the line are like the water found in the roots waiting to move up the stem to get to the leaves or, in our case, into the restaurant.
It is very important to remember that this movement of water up from the roots through the stem and out the leaves does not require energy. It is all based on the need to have water throughout the plant and cohesion between water molecules. The water in your straw may only go up a little bit on its own but in plants, the water can move hundreds of feet based on the effects of transpiration and cohesion.
Xylem is essential to vascular plants because it allows for the transport of water. Without xylem, big trees would not be able to move water from the roots up to the leaves. Remember that xylem is made of vessels, which are dead cells lined up end to end. The vessels are like segments of long pipes used to transport water. Some plants may have tracheids, which are similar to vessels but have tapered ends making them somewhat less efficient at transporting water than vessels.
Because of cohesion and transpiration, water is able to move against the flow of gravity in plants. The attraction of water to other water molecules, or cohesion, allows water to pull other water molecules up through the xylem. Transpiration, or when plants release water, creates the need for new water molecules to replace those that were lost. Remember that we related this action to a crowded restaurant with a long line of people waiting to get seated. Amazingly enough, this vital process of moving water up from the roots to the stem and eventually out through the leaves does not require energy but rather relies on the effects of cohesion and transpiration.
At the end of this lesson, you'll be able to:
- Explain the structure and function of xylem
- Describe cohesion and transpiration