Biology         Block____                    Name________________________________________
Dr. Wexler
Lab: Stomata in Lettuce
Date_____

Background:

The guard cells are specialized epithelial cells. These cells are bean shaped. The inner edge has a thicker cell wall than the outer edge. During the day, as photosynthesis occurs, these cells fill up with water and become turgid, causing them to change shape (see diagram below). Because of the difference in the thickness of the cell wall, the cells separate and form the stoma opening. During the night, the cells lose water and become flaccid. The stoma opening grows smaller. Thus, when the plant needs carbon dioxide during the day, the stomates are open.

Many scientists have been interested in how the guard cells regulate the size of stoma. There is evidence that light-dependent active transport is used to bring potassium ions into the guard cells during the day. This causes a change in osmotic balance and water will rush in, causing the cells to become turgid. At night, the active transport no longer occurs and the potassium ions diffuse out. The water will then also diffuse out, causing the cells to become flaccid.

Visual Examination of the Lettuce Leaf:

  1. Pick up and observe the whole lettuce leaf, if available. There is a distinct top and bottom. Note the differences between the two sides. The top is shinier and greener while the bottom is paler. The main rib of the leaf bulges out on the lower side. You will observe the lower epidermis of the leaf and of the rib itself.

  2. Place two drops of iodine solution on your slide.
  3. Hold the lettuce leaf in your hand, bottom side up.
  4. Break a side vein so that part of the lower epidermis is separated from the rest of the leaf. Use your forceps to pull the tissue away from the main rib. Make sure no green mesophyll is connected to the lower epidermis. Pull off an 0.5 cm piece and place it in the iodine solution on your slide .
  5. Cover the tissue on the slide with a coverslip.
  6. Observe the leaf specimen under low power. Note the appearance of the epidermal cells.
  7. Look for the darkly stained, bean-shaped guard cells. Switch to high power and observe.
  8. DRAW and LABEL a low-power and a high-power sketch of the cells and stomates of the lower epidermis of the lettuce leaf. Indicate the magnification used.