Unit 4, Step 2A

For lab points, do the following lab activity entitled “Leaves and Stomata.” The instructions for this lab are below, but this lab must be done in the Biology Learning Center. There are worksheets for this activity in your Lab worksheet packet. When you have completed the activity, take the results to the front desk in the Biology Learning Center for lab points.

Stomata are pores in the surface of the leaf. The pore is surrounded by two guard cells. When these two guard cells take up water, they bend and open up the pore between them. When these guard cells begin to lose water, they shrink back, and the pore closes up.

There are two major functions of stomata.

  • The first function is exchange of gases with the atmosphere. Plants need to take in carbon dioxide and release oxygen as part of the process of photosynthesis. Plants also take in oxygen and release carbon dioxide during cellular respiration. Since leaves usually have both an impermeable epidermis and a waxy cuticle to prevent uncontrolled water loss, the plant needs a way to get these gases to and from the cells. The stomata serve as channels into the leaf, allowing the gases to diffuse directly into and out of the cells from within the leaf tissue.
  • Stomata also allow controlled release of water molecules into the atmosphere. Although the plant cannot afford to lose too much water to the environment, the plant must have a way to carry water and minerals from the roots, up the stem, and out to the leaves. By allowing some water molecules to escape the leaves through the stomata, there is a negative pressure gradient created that helps draw additional water up through the plant. This is called transpiration. In addition to bringing water and minerals to the leaves, transpiration also helps keep the plant cool on hot days - a method of evaporative cooling.

The illustration above shows the presence of stomata on the underside of the leaf, but not the top of the leaf. Is this typical? To find out we'll have to look under the microscope, since stomata are very small. However, leaves are too thick and too dense to view directly with our microscopes, especially since the light has to come up through the specimen. So instead of looking at an actual leaf under the microscope, let's look at an impression of the leaf's surface instead!

For this lab, you will need:

  • a microscope from the microscope cabinet (if there isn't one at your desk)
  • Kit # 168

Take a look in the kit, and see if you can figure out what we will use to make an impression of the leaf. How about nail polish?! Our goal will be to put nail polish on the top and bottom surfaces of a leaf, then peel off the nail polish, put the peelings on microscope slides, then observe the slides under the microscope. If stomata are present on the surface of the leaf, we will be able to see impressions of the stomata on the peeled nail polish.

For our leaves, we will use Pothos ivy, which is growing on a shelf near the north wall of the lab. Find the gray drawer with Kits #85-89, turn around, and then you will see the shelf and plant. Important: DO NOT remove the leaf from the plant. You can make impressions of the leaf surfaces while the leaf is still on the plant. The advantages to working with a leaf that is still on the plant is that you won't damage the leaf or the plant for this experiment.

Here's how to make the impression. (The photos are of a leaf that has been removed from the plant, in order to make it easier to take the pictures! Remember: DO NOT remove the leaf from the plant, even though it is pictured that way below! )

First, take a small piece of tape (about a half inch long), fold one end under just a little (to make it easy to peel off again), and place it on the top, shinier side of the leaf. (If you don't have tape in your kit, just come up to the front desk for some tape.) Then take another small piece of tape, and place it on the bottom side of the leaf. The tape will provide a “handle” to help you peel the nail polish off the leaf.

Next, take the nail polish, and paint on a thin coat about the diameter of a penny. Half of the nail polish should be on the leaf, and half on the tape. Paint on a coat of polish on both sides of the leaf, then wait a few minutes for the polish to dry.

While you wait for the nail polish to dry, get two microscope slides and the wax pencil out of the kit.

When the nail polish is completely dry and no longer sticky, carefully peel off the tape from the top of the leaf. Keep peeling, and the dried nail polish should stick to the tape and peel off the leaf. (If it doesn't come off neatly, then use the forceps in the kit.) Then carefully place dried layer of nail polish onto a microscope slide. Use the wax pencil to label the slide (ie, “T” for top surface of the leaf). Then repeat for the bottom surface of the leaf, carefully peeling off the tape and polish, placing on a slide, and labeling (ie, “B” for bottom).

Now take a look at both slides under the microscope, starting with the impression from the BOTTOM SURFACE of the leaf. Why start with the bottom? If there really are stomata on the bottom and not the top, then start with the bottom so that you can see what stomata look like!

When you have the slide in focus under the microscope, count the number of stomata that you can see in one “field.” What is a field? A field is the area of the slide that you can see under the microscope without moving the slide. On your worksheet, record the number of stomata for both the top and bottom surfaces of the leaf, as well as the magnification you were using.

After recording your observations, LEAVE THE MICROSCOPE SET UP, WITH THE SLIDE SHOWING THE MOST STOMATA STILL IN FOCUS UNDER THE MICROSCOPE. In a minute, you will go and get someone from the front desk, and bring them back to your workstation to get checked off for lab points. But first, answer the following question on your worksheet:

  • State the two major functions of stomata. If you are not sure, just read these lab instructions again from the beginning.

When you have recorded your observations and answered the question, take your worksheet up to the front desk for lab points. Be sure to leave your microscope set up so that someone can come back to your desk to look at your slides.

Once you have received lab points, please clean the microscope slides, and return the kit and the microscope to their proper place.

That's it!

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