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Activity 1.3
What Goes In Has to Come Out
Some water is used by plant cells as it move up a plant’s stem and leaves, Much of it, however, leaves the leaf’s surface as part of evapotranspiration. Based on the cohesion of water, this process uses evaporating water’s ability to pull water molecules up the plant stem. As water molecules are pulled off the leaf surface as they evaporate off the leaf surface, they pull the water molecules behind them, much like a metal chain being lifted by pulling only on the end link.

As the plant is covered in a somewhat impassable epidermis (A.K.A. plant skin), the water must pass through special pores on the leaf surface called stomatal pores. These minute pores, usually on the underside of a plant’s leaf can open and close permitting water to leave the cell. The stomatal pores perform double duty in that they also permit the exchange of gases, such as carbon dioxide and oxygen between the plant’s cells and the atmosphere.

What You Need:

  • Several leaves (larger tree leaves are easier to work with)
  • Clear fingernail polish
  • A hand lens (or microscope, if you have one)

What To Do:

  1. Paint some clear fingernail polish on a small area on the underside of the leaf.
  2. Let the polish dry.
  3. Peel off the polish from the leaf surface and examine the polish closely with a hand lens or microscope. Do you see the small spots representing the stomatal pores?

What's Going On:

The stomatal pores are made up of two large cells called guard cells. These cells lie side by side and are curved, so they look a lot like a pair of lips. When the guard cells change their shape, the pore opens and closes.

Take It A Bit Further:

Idea One:
Try and examine a leaf directly with a microscope. You should try and find a leaf that is as thin as possible so that the light can pass through the leaf making the stomatal pores visible. Do you see the guard cells?

Idea Two:
Collect the leaves at different times of the day to see if you can find pores that are open and closed. Is there a relationship between time of day and condition of pores? When it gets too hot, excessive water lose becomes a problem, so many plants will close their pores and literally hold their breath until it cools off.

Go Back to Activity 1.2 Advance to Activity 1.4