Lesson 4, Spring 2014 – Plants & Water

Introduction (group):

Q: What are we learning about this quarter?A: Water (may also get bubbles, soap, boats, etc.)

Q: What have we learned about water?A: That it has special properties – like surface tension, which is caused by all the molecules of the water sticking tightly to each other.

Q: Is water important for living things? What would happen to you without water?A: Yes, water is important! Living things, like people, need water to survive.

Q: Do plants need water? What happens if you forget to water a plant?A: Yes, plants need water. This is why we have to water our gardens and lawns in California in the summertime. If you forget to water a plant, it will wilt, dry out, and die.

Q: How does water get into a plant? Can plants drink?A: Water is taken up by the roots of the plant, where it is absorbed.

Q: What happens to the water inside of the plant? Do plants have blood and veins like people?A: Plants don’t have blood, exactly, but they do have veins that they use to carry water and nutrients to the stems and leaves.

Q: Why do plants dry out when it is hot? How do they lose water?A: Water evaporates (leaves the plant as a gas) through tiny holes in the leaves.

Q: Why would the plant want this? Why would it have holes in its leaves?A: Losing water from the leaf surface helps the plant cool down. It is a lot like you sweating on a hot day! However, for a plant it is even more important. Losing water through the leaves pulls new water and nutrients up through the veins, in a process called transpiration. If this didn’t happen, water would stop moving through the plant – sort of as if your heart stopped pumping blood through your body.

Q: Why does losing water from leaves pull water up from the roots and stems?A: For the same reason that we saw surface tension in other lessons – because water molecules like to stick together. When some of them are lost from the leaf, they tug on others further down in the stem, pulling them up in a long “chain.”

We will now watch a short (1-minute) video that illustrates how water molecules stick to each other to move through the plant: https://www.youtube.com/watch?v=At1BJJDcXhk.

Part 1: Celery and Food Dye (small groups)

Introduction: We will use food dye to trace the path of water through a celery stalk. The dye will be sucked up from the base of the celery to the top through the xylem elements (veins of the plant). This allows students to see transpiration in action, and also colors the vein system so it is easily seen.

The basic procedure is to place a celery stalk in water colored with food dye, then let it sit for awhile (at least 15 minutes) to allow the dye to move up through the veins. Most students will have already seen this experiment at some point, or be able to predict what will happen (the celery will turn colors). We will do two variations on this today:

Leafy or Leafless? Students will make a prediction about whether dye is going to move faster up a celery stalk with leaves, or one that is leafless. (We would expect the one with leaves to go faster, because there is more surface area for evaporation to occur over.)

1. Get two celery stalks of the same approximate length, one with leaves and one without. These should be 3-4 inches long (in the interests of the dye reaching the top by the end of the class).

2. Fill a cup with about an inch of water each.3. Add food coloring (~5 drops). Blue works best, red is also okay.4. Place the leafy and leafless celery stalks in the cup. Note the time, and how the stalks look (no

color yet).5. Check the stalks at 5-10 minute intervals to see if color is visible at the top yet. The stalk with

leaves should show faint color in 10 or 15 minutes, and clear color in 30-45 minutes. The stalk without leaves may or may not show detectable color during the class period.

Two-Tone Celery. What would happen if we split the bottom of the stalk on a leafy piece of celery and put each side into a different color dye? Ask students to make predictions (half red, half blue? purple? blue and red patches?), and think about what the prediction would mean for the path of water in the stem. I.e., does it just move straight up? Does it cross over between sides?

1. Get a leafy celery stalk (again 3-4 inches long) and carefully cut the bottom 2” in half with a plastic knife.

2. Get two cups and fill them both close to the brim with water.3. Add food coloring (~10 drops) to each cup, using blue and red.4. Place the celery so that each half of the stalk bottom is in one of the cups. Note the time and

how the stalk looks.5. Check the stalk at 5-10 minute intervals to see if color is visible at the top yet. The leaves should

come out half red and half blue, corresponding to the sides on which the two cups are placed, indicating that water just moves straight up (and does not cross over much within the stem).

Once the leafy celery has clearly turned color, if time allows, students can cut it up and examine the veins. If they cut sections across the stem, they should be able to see the veins as small, bright dots in a semi-circle. If they cut sections the long way, they should be able to see the veins as long, tough threads.

Students can also look leaves and/or sections on the microscope if they are interested. Sections need to be very thin to work well, ideally cut with a razor, and should be mounted on a slide with a coverslip and some water. I will have razors that tutors can use to cut sections (not to be used by students!).

Microscope image of celery leaf with blue food dye in veins

Part 2: Leaf Stomata Peels (small groups)

Introduction: Water evaporates through tiny holes in the leaf surface. These are actually not just plain holes. They are surrounded by a pair of cells, which can open and close depending on conditions (e.g., allowing the plant to lose less water if it is very dry/hot). These structures are called stomata.

All plants have stomata, but they can look very different from plant to plant (many or few of them, large or small, organized in rows or whorls or other patterns). We will use nail polish peels to look at stomata in different plants from the garden and make notes about the in a chart.

Procedure: Instructions are modified from http://www.biologyjunction.com/leaf_stomata_lab.htm.

1. Obtain three leaves from different types of plants in the garden. Remember or write down what kind of plant each leaf came from. Hint: leaves that are not very fuzzy will work best.

2. Paint a patch (at least one square centimeter) of clear nail polish on the underside of the leaf surface being studied.

3. Allow the nail polish to dry completely. Takes 3-4 minutes or so.4. Tape a piece of clear cellophane tape to the dried nail polish patch.5. Gently peel the nail polish patch from the leaf by pulling on a corner of the tape and "peeling"

the fingernail polish off the leaf. This is the leaf impression you will examine.6. Tape your peeled impression to a very clean microscope slide. Use scissors to trim away any

excess tape. Label the slide with plant name.7. Examine the leaf impression under a light microscope at 400X. Make an entry in the chart to

describe what you see.8. Repeat for your other leaves.

The nail polish impressions seem to work pretty well, but there will also be some example slides in case the groups are having trouble getting clear impressions.

If you have extra time: try the same process with other types of plants, or with other parts of the plant (that aren’t the undersides of leaves). Can you find stomata on the other side of the leaf? The stems? Flower petals?

Examples (using the method above, a few stomata circled):

Carnation Tree

Leaf Stomata Chart

Type of plant Picture of the leaf under the microscope

Stomata big or small?

How many stomata?(Count, or

none/some/lots)