Lab Exercise 7: Leaves (also see Atlas pp. 141-150) In most green plants, leaves are the primary photosynthetic organs. They are well adapted for efficient light absorption, carbon fixation, and conduction of photosynthate to the stem for distribution to the rest of the plant. Leaves vary dramatically in size and shape, and numerous modifications of leaf structure have evolved as adaptations to different habitats, environmental conditions, herbivory, and other factors. Upon completion of this lab, the student should be able to: Identify the main parts of a leaf. Differentiate between simple and compound leaves. Identify the various types and parts of compound leaves. Identify the internal parts and tissues of both monocot and dicot leaves. Explain the patterns of venation found in dicots and contrast them with the typical pattern found in monocots. Define or explain the significance of the boldfaced terms found in the exercise. Explain the differences between mesophytes, hydrophytes, and xerophytes, and be able to recognize each type of plant based on the leaf cross section. I. External Morphology of Leaves Examine the leaves of Pelargonium. The leaf consists of a broad, flattened blade or lamina, a petiole (the stalk attaching the leaf blade to the stem), and a pair of stipules, small leaf- like structures near the base of the petiole, where it attaches to the stem. These basic parts of the leaf blade, petiole, and stipules are highly variable. Some leaves lack a petiole, with the blade attached directly to the stem (sessile leaves). Some leaves lack a blade (e.g. the prehensile tendrils of some plants are modified, bladeless leaves). Stipules are often absent, but can also occur in modified forms as spine- like, tendrillate, or glandular structures (among others). In the space above, make a diagram of a single Pelargonium leaf attached to the stem. Label the blade, petiole, stipules, and stem.
II. Leaf Venation You compared the internal structure of monocot and dicot stems in a previous lab. Monocots and dicots also have different leaf structures. One major and easily observed differentiating characteristic is their leaf venation, the pattern of vascular bundles (veins) in the leaf. Monocots and dicots have distinctive arrangements of veins in their leaves. Monocots are characterized by parallel leaf venation, with vascular bundles running the length of the leaf more or less parallel to each other. There are relatively few cross veins connecting the vascular bundles in the leaf, and the veins rarely branch. Make a diagram of a monocot leaf based on the plants on display, showing parallel venation: Dicots have netted, or reticulate, venation. The veins branch many times, typically in a random manner, to form a network of ever- smaller veins in the leaf. A variety of dicot leaves are on display. Two major variations of reticulate venation may be found: A) Pinnate venation: in pinnately- veined leaves, there is a single main vein (or midrib) extending from the petiole through the blade, with lateral veins branching off at various intervals. B) Palmate venation: Leaves with palmate venation typically lack a single midrib. Instead, several major veins branch off from a common origin somewhere near the base of the blade.
III. Leaf Blade Structure and Types of Leaves Leaves can be categorized or described based on the condition of the blade. When a leaf has a single, intact blade, it is said to be a simple leaf. If the blade is divided into multiple smaller units, the leaf is said to be compound. Each individual unit of a compound leaf blade is a leaflet. The edge of the leaf is referred to as the leaf margin. In many plants, the margin undulates so that the blade is indented or reduced between the major veins. Such leaves are said to be lobed, but are still considered simple leaves as long as the blade is not divided into separate parts. The space or indentation between two lobes is called a sinus. The structure of compound leaves follows the same pattern as venation in simple leaves. I.e., Palmately compound leaves have all of the leaflets arising from a common point at the end of the petiole. Often, the leaflets of a palmately compound leaf will themselves exhibit pinnate venation. Pinnately compound leaves have leaflets arising at intervals from a main axis or rachis, which is an extension of the petiole. There are two types of pinnately compound leaves: those with an odd number of leaflets and those with an even number. Examine the herbarium mounts of leaves from four common trees on display, and determine their leaf type (simple or compound) and venation pattern of the blade (palmate or pinnate). Diagram a single, intact leaf of each. 1) Plant name: 2) Plant name: / / 3) Plant name: 4) Plant name: / /
IV. Internal structure of leaves For this part of the exercise, obtain a prepared slide of dicot and monocot leaf cross sections (both sections are included on the same slide). A. Dicot Leaf The typical dicot used in this slide is Syringa (the common lilac). Note the organization of the three plant tissue systems in the leaf: the dermal tissue (consisting of an upper and lower epidermis), the ground tissue (tissue between the two epidermal layers, known as the mesophyll in the leaf), and the vascular tissue (the veins of the leaf, bundles of xylem and phloem that conduct water and nutrients AND provide a measure of structural support to the leaf blade). 1) Examine the epidermis. In this species, the epidermis is only a single layer of cells. Look for the cuticle on the upper epidermis. The cuticle and the epidermal cells function as a unit. What is their main function? (1) 2) Look for guard cells and associated stomata in the lower epidermis. This is a cross section of a leaf, so these cells will not have the characteristic bean- or crescent- shaped appearance seen previously. 3) The mesophyll is separated into two distinct regions. The palisade mesophyll is found in the upper portion of the leaf, and the spongy mesophyll in the lower portion. 4) Examine the midrib or main vein. Which tissue is on top, the xylem or phloem? (2) The leaf model as well, and understand why the tissues are arranged in this way. 5) In the leaf cross section below, label the palisade mesophyll, spongy mesophyll, guard cells, upper epidermis, lower epidermis, and cuticle.
B. Monocot Leaf Now examine the corn leaf cross section, located on the same slide. Remember, corn is a monocot. Notice the mesophyll consists of relatively homogenous parenchyma cells. Can you distinguish palisage and spongy mesophyll layers? Notice too that guard cells and stomata are found in both epidermal layers (this is not unique to monocots, btw). The epidermis contains enlarged bulliform cells (bulliform = swollen or round). It is thought that these cells lose water more rapidly than surrounding epidermal and mesophyll cells. As they lose water, they collapse, causing the leaf to fold during dry conditions, possibly preventing excessive water loss. Are the bulliform cells more abundant in the upper or lower epidermis? How do you know which epidermis is the upper epidermis, anyway? (3) Examine the smaller vascular bundles. They are surrounded by several large cells known as bundle sheath cells. The distinctive appearance of the bundle is referred to as Kranz anatomy; it is an important feature of plants with the C4 photosynthetic process. The larger vascular bundles resemble those of the stem (having a face- like appearance). These larger bundles have conspicuous bundle sheath extensions of sclerenchyma fibers extending to each epidermis. Label the upper and lower epidermis, bulliform cells, bundle sheath, phloem, xylem, mesophyll, and bundle sheath extension in the photo below.
IV. Leaf Modifications Plants that grow in moderate environments that are neither extremely moist nor extremely dry are called mesophytes. An example of a mesophyte is Syringa (lilac), whose leaves were examined above as an example of a typical dicot leaf. The leaf anatomy of mesophytes is generally unspecialized. Plants adapted to grow in more arid conditions are known as xerophytes, while plants that actually grow in water called hydrophytes. The leaves of xerophytes and hydrophytes often exhibit characteristic adaptations related to survival in their extreme habitats. Several demonstration microscopes have been set up in which you can observe xerophytic and hydrophytic leaves. A. Xerophyte: Nerium oleander Examine the slide of Nerium oleander, the commonly- planted oleander plant. Nerium is an evergreen shrub thought to be native to the arid Mediterranean region, Northern Africa, or the Middle East. In this leaf cross section, you can observe several ecologically- significant features: 1. Thickened cuticle. The top of the leaf has a very thick cuticle, a layer of waxy, hydrophobic substances and forms a barrier against water loss. All plant epidermis has a cuticle, but it is generally significantly thickened in xerophytes. 2. Multiple epidermis layers. These additional cell layers lining both the upper and lower epidermis may function as water- storage cells. 3. Stomatal Crypts. The guard cells and stomata are located only in the lower epidermis, within small cavities in the leaf surface. Called stomatal crypts, these cavities provide shelter to the stomata, and are a common adaptation in xerophytic plants. The stomatal crypts of Nerium are also packed with tiny hairs or trichomes. B. Hydrophyte: Nymphaea sp. Examine the prepared slide of a cross section of water lily (Nymphaea) leaf. Notice that there are many stomata, but they are located on only the upper epidermis. The cuticle is very thin (not noticeable), and the epidermal cells are small and thin- walled. Throughout the mesophyll are scattered numerous star- shaped sclereids (astrosclereids). Their function is not known, but they may help to support the leaf structure or deter predation. What other adaptation can you see that indicates this leaf floats on the surface of the water?
In the Nerium leaf cross section above, label the cuticle, upper epidermis, lower epidermis, palisade mesophyll, and stomatal crypt. In the Nymphaea cross section below, label the upper and lower epidermis, palisade and spongy mesophyll, air chamber, and astrosclereid. (This lab exercise was adapted in part from Exercises for the Botany Laboratory, Kazmiersky 1999).
Name: BIOL 241 Lab 5 Questions: Leaves 1) Name three features characteristic of xerophytic (aka xeromorphic) leaves: 2) How does the mesophyll of a monocot (corn) differ from that of a typical dicot (like Syringa, lilac)? (2 pts) a. monocot (corn): b. dicot (lilac): 3) How do stomatal crypts aid survival in xerophytic plants? (2 pts) 4) A leaf cross section is pictured below. Do you think it is from the leaf of a xerophyte, mesophyte, or hydrophyte? What evidence is your conclusion based on? (3 pts)