From Seed to Seed:
Plant Science for K-8 Educators


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What is transpiration?

You've probably noticed that plant tissues are generally moist inside. Leaf tissues contain a lot of water-in the form of both liquid and vapor. Remember the layer of spongy cells in the mesophyll, the area between the epidermal layers? The spaces between those cells is where gas exchange takes place, as oxygen and carbon dioxide gases are formed and consumed. Those spaces also contain water vapor that has evaporated from the moist surfaces of the cells.

Once again, during photosynthesis, the stomata must open to take in carbon dioxide. When the stomata open, water vapor from the area around the spongy cells escapes into the surrounding air. It is this evaporation that drives some very important functions.

Did you ever wonder how plants actually take up water? Since they do not have "muscles," how do plant roots pull water into their tissues and transport it all the way up to the treetops? You guessed it: The engine driving the uptake of water is transpiration. As water evaporates from the surface of the leaves, it creates a negative pressure-something like a vacuum-within the xylem cells. The xylem cells respond by drawing in more water.

In order for water molecules to be drawn up through the plant in this manner, they have to stick to one another. It turns out that water is quite cohesive (a property that allows it to withstand a lot of tension during pulling) and adhesive (a property that enables the water molecules to adhere to the walls of the xylem cells and pull themselves up through the plant). As a result, our understanding of how plants take up water is often referred to as the cohesion-tension theory (although it could also be called the cohesion-adhesion-tension theory).
From soil, through plant, into atmosphere

The following analogy isn't perfect, but, when used as a demonstration, it may help you to introduce your students to the process. If you suck on a straw in a glass full of water, you pull water up the straw. By sucking on the straw, you are creating a negative pressure at that end and the water moves up the straw. Now imagine the xylem cells in a stem as a system of conduits, and you can understand how the negative pressure at one end would draw water through the conduits. What creates the negative pressure? The surface tension of water evaporating from the surface of the spongy mesophyll cells creates the pulling force.






















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