PART II: STRUCTURAL ORGANIZATION OF LEAVES
9.2 Exercise 2 – Internal features of a leaf
All the structures of the leaf can be understood in terms of their role in maximizing the efficiency with which the chloroplasts carry out photosynthesis in different habitats. As you study the structures below, ask yourself, “How does this structure increase the supply of sunlight, carbon dioxide, or water to the chloroplasts?” Examine a prepared slide of a cross section of a leaf under low and high power. Using the numbers in Table 1, locate the structures shown in Figure 1.
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Table 1: Tissue layers of a leaf
Tissue layer (Chloroplast-helping) function
(1) Upper epidermis Protective; secretes waxy cuticle (6) that keeps water in, but (being transparent) lets sunlight through
(2) Palisade mesophyll layer Tightly packed parenchyma cells containing most of the leaf’s chloroplasts (7)
(3) Spongy mesophyll layer Loosely packed parenchyma cells, which also have chloroplasts; 30-50% is air space (8) that allows atmospheric carbon dioxide to diffuse into mesophyll
(4) Vascular bundles (veins) Moves water and minerals into the mesophyll layer and carries away the products of photosynthesis
(5) Lower epidermis Has stomata (9), openings controlled by guard cells (10), through which carbon dioxide and oxygen pass (and water evaporates)
Figure 1: Cross section of a leaf
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9.3 Exercise 3 – Stomata
Stomata are tiny openings in the leaf epidermis which allow gases (oxygen and carbon dioxide) to diffuse in and out. But the leaf loses water for as long as the stomata remain open. To reduce the amount of water lost by transpiration, each stoma is surrounded by two guard cells which regulate the size of the opening. The primary function of stomata and surrounding guard cells is to provide adequate carbon dioxide for photosynthesis while minimizing water loss. During the day when the plant is actively photosynthesizing, the stomata are open to allow the entry of carbon dioxide. At night, the stomata are closed to reduce water loss. Each guard cell is kidney-bean shaped with bands of microtubules running radially around it (See Figure 2). When water enters the guard cells, the microfibrils prevent the cells from expanding their circumference, so they expand their length. The two guard cells are anchored to each other at their upper and lower ends, so lengthening of the bean-shaped guard cells causes their middles to move apart, opening the stoma. The reverse happens when the guard cells lose water; the guard cells lose turgor pressure and the space between them closes. Figure 2. Guard cells and stomata, closed and open
1. Obtain a leaf from the Central Study Area. Use forceps to remove a small piece of the lower epidermis. (It helps to bend the leaf upward so that the lower surface splits open along the fold.) Make a wet mount of the specimen by floating it on a drop of water and covering it with a cover slip. 2. Examine the epidermal tissue under medium power (100x). Then switch to high power (430x) and focus on one of the stomata with its two guard cells. Use the measurement you made of the diameter of your microscope field at 430x in Minicourse 1 to estimate the diameter of the stoma and guard cells. _____ µm 3. In the space below, sketch one of the stomata with its two guard cells. (Don’t just copy the diagram in Figure 2. Draw what you actually see.)