lab4_leaves_2007

lab4_leaves_2007 - Laboratory 4 Leaves: Specialized Plant...

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Leaves: Specialized Plant Organs Laboratory 4
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72 Laboratory 4: Leaves OBJECTIVES After completing this lab you will be able to: 1. Describe and name leaf patterns 2. Identify the structural differences between dicot and monocot leaves 3. Prepare a short synopsis of the movie “Close Encounters of the Floral Kind” LAB PREPARATION In preparation for this laboratory you should do the following: 1. Read and study this laboratory. 2. Review pages 720-728 in Campbell (7 th Edition). 3. Bring your copy of Photo Atlas for Biology to lab. INTRODUCTION Leaves, perhaps more than any other plant organ, vary greatly in external form and internal structure. As the main photosynthetic organs of the plant, leaf morphology is largely influenced by the amount of sunlight they receive. Additional environmental factors, such as water availability, wind, temperature, and herbivores, also affect the morphology and arrangement of leaves on a plant. A. Transpiration and Leaf Anatomy The carbon a plant uses to make carbohydrates comes from atmospheric carbon dioxide (CO 2 ). Plants sequester CO 2 through special openings in the leaf surface, called stomata . When the stomata are open, some water vapor is lost from the interior of the leaf. This process of water vapor loss is called transpiration . As water vapor is lost by transpiration in the leaves, more water is pulled into the roots and up the plant body, via the xylem (see Chap. 36 in Campbell for a discussion of transpirational pull). While transpiration is an integral component of the mechanism responsible for water movement throughout the plant, excessive water loss can threaten the plants continued existence. For this reason, various leaf adaptations have evolved that minimize or control transpirational water loss in most plants. In this laboratory, we will point out some of the more common leaf adaptations. B. Morphology of Leaves Leaves generally consist of two parts, an expanded portion called the blade (or lamina), and a stalk-like portion called the petiole . In some leaves, smaller leaf-like structures may exist at the point where the leaf attaches to the stem. These are called stipules. Also, in most
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Laboratory 4: Leaves 73 monocots (and some dicots) the base of the leaf is expanded into a sheath which encircles the stem. 1. Leaf Arrangements (Figure 4-1) a. Opposite : two leaves at a node (point of leaf attachment) on opposite sides of the stem b. Spiral (alternate): one leaf per node with the second leaf being above the first but attached on the opposite side of the stem (alternate), or where the successive leaves attach above the first in a spiral. c. Whorled : three or more leaves are attached at one node. Can you see why an alternate leaf arrangement is just a special
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lab4_leaves_2007 - Laboratory 4 Leaves: Specialized Plant...

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