What is the Need for a Continuous Water Column in Xylem

Several processes work together to transport water from where a plant absorbs it (the roots) upward through the rest of its body. To understand how these processes work, you first need to know one key feature of water: Water molecules tend to stick together, literally.

Water molecules are attracted to one another and to surfaces by weak electrical attractions. When water molecules stick together by hydrogen bonds, scientists call it c o hesion. When water molecules stick to other materials, scientists call it adhesion.

A familiar example of the stickiness of water occurs when you drink water through a straw — a process that's very similar to the method plants use to pull water through their bodies. You apply suction at the top of the straw, and the water molecules move toward your mouth.

Because the molecules cling to each other on the sides of the straw, they stay together in a continuous column and flow into your mouth.

Scientists call the explanation for how water moves through plants the cohesion-tension the o ry. It involves three main factors:

• Transpiration: Transpiration is the technical term for the evaporation of water from plants. As water evaporates through the stomata in the leaves (or any part of the plant exposed to air), it creates a negative pressure (also called tension or suction) in the leaves and tissues of the xylem.

  The negative pressure exerts a pulling force on the water in the plant's xylem and draws the water upward (just like you draw water upward when you suck on a straw).

• Cohesion: When water molecules stick to one another through cohesion, they fill the column in the xylem and act as a huge single molecule of water (like water in a straw).

• Capillary action: Capillary action is the movement of a liquid across the surface of a solid caused by adhesion between the two. When you a place a tube in water, water automatically moves up the sides of the tube because of adhesion, even before you apply any sucking force.

  The narrower the tube, the higher the water climbs on its own. In plants, adhesion forces water up the columns of cells in the xylem and through fine tubes in the cell wall.

Environmental conditions like heat, wind, and dry air can increase the rate of transpiration from a plant's leaves, causing water to move more quickly through the xylem. Sometimes, the pull from the leaves is stronger than the weak electrical attractions among the water molecules, and the column of water can break, causing air bubbles to form in the xylem.

The sudden appearance of gas bubbles in a liquid is called cavitation.

To repair the lines of water, plants create root pressure to push water up into the xylem. At night, root cells release ions into the xylem, increasing its solute concentration. Water flows into the xylem by osmosis, pushing a broken water column up through the gap until it reaches the rest of the column.

If environmental conditions cause rapid water loss, plants can protect themselves by closing their stomata. However, after the stomata are closed, plants don't have access to carbon dioxide (CO₂) from the atmosphere, which shuts down photosynthesis. Some plants, like those that live in deserts, must routinely juggle between the competing demands of getting CO₂ and not losing too much water.

For questions 1–5, use the terms that follow to demonstrate the movement of water through plants by labeling the figure.

Credit: Illustration by Kathryn Born, M.A.

a. Cohesion

b. Osmosis

c. Adhesion

d. Transpiration

e. Stomata

The following is how the figure should be labeled:

1. d. Transpiration

2. e. Stomata

3. c. Adhesion

4. a. Cohesion

5. b. Osmosis.

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