1.3 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.
Background Information:
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People Sip, Dogs Lap, But Plants Always Drink with a Straw
The trees and many of the smaller plants of the Bonita Channel are typical of the vegetation
that once lived along stream courses in Southern California. Streams and ponds with plants
requiring water at or near the surface are called riparian habitats. Willows, for example, can
tolerate their root system being completely inundated with water and so can thrive along
flowing streams such Bonita Channel. Higher up the slopes of the channel, where soil remain dry for most
of the year, willows and other water-loving plants such as cattails can not grow.
However, all plants, even the shrubs on the upper dry slopes, require some water. That water must be
collected by the root system and transported to the stems and leaves where it is needed. So, how do you
think that water gets to the tops of those willow trees? The blood in your body, which happens to be mostly
made of water, is pushed through thousands of miles of blood vessels by the beating of your heart. The blood
carries with it all the nutrients to your cells, as well as all the wastes from those same cells. But plants don’t
have hearts, or any other moving parts for that matter. So how do they do it? It all comes down to the very
special properties of one of the weirdest molecules in the universe: Water.
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.
What To Do Continued:
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 are 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:
1
Idea One:
2
Idea Two:
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?
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.