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Leaves213-page3 - layered hypodermis • The epidermis is...

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Leaves - 3 Leaf Functions Photosynthesis A primary function of leaves is photosynthesis. As you observe the structure of leaves in lab, note the design of the leaf relative to what it needs for photosynthesis: light, water, CO 2 and chlorophyll. Transpiration Transpiration is the evaporation of water through leaf surfaces, which creates a tension (or negative pressure) that draws water upward through the xylem tissue from roots and stems throughout the plant. Gas exchange o CO 2 in for the process of photosynthesis o O 2 in or out, depending on diffusion gradients. Oxygen, a by-product of photosynthesis, is required for cell respiration in all living plant cells. Leaf Structure Epidermis The leaf epidermis comprises the outer layer of leaf cells. Since most leaves are “flattened” there is an upper and a lower epidermis. For vertical leaves the upper epidermis is the inner side of the leaf and the lower epidermis, the outer side The epidermis layer is generally one cell thick; some leaves may have a multi-
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Unformatted text preview: layered hypodermis • The epidermis is covered with a waxy cuticle (thickness varies). • Epidermis Functions Protection Gas exchange • Special cells in leaf epidermis Guard cells that form stomata • Guard cells are concentrated on lower epidermis in most leaves. Guard cells are most often pairs of "bean-shaped" cells with thick, rigid inner walls. They contain chloroplasts. The opening between guard cells forms stomata for gas exchange • Stomata open and close with changes in turgor caused by solute differences between the guard cells and the surrounding epidermal cells. The thinner walls of the guard cell stretch as it swells with water whereas the thicker, inner walls do not. This differential stretching creates the stoma. Chemical signals, photosynthesis and osmotic changes are necessary for most stomatal function. We will discuss this mechanism later....
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