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N. Berg & M. Rice , NNHS © 2016-7 Page 1 of 8

LAB What is in a Leaf? Honors Biology, Newton North High

OBJECTIVES: ! Recognize each of the tissue types and structures found in leaves and explain what they do. ! Recognize the differences between monocot and dicot leaves.

BACKGROUND INFORMATION: Leaves are what plants are all about; they are the sites of photosynthesis. They must perform a delicate compromise between gas exchange and evaporative water loss, taking in enough carbon dioxide for photosynthesis while limiting excessive water loss. Some evaporation is necessary for transpiration, but too much will kill the plant. Leaves also require vascular tissue to transport water and inorganic nutrients into the leaf and to transport the sugars produced through photosynthesis out to the rest of the plant. The leaves of vascular plants contain several different specialized types of tissues, which interact to make the functions organ we call a leaf. You should become familiar with each type of cell and what it does.

While leaves show a variety of shapes, they all do more or less the same job. Some of the differences in shape can be understood in terms of the conflicting requirements that leaves face: absorbing light, exchanging gases, avoiding dehydration, and avoiding predation. Within flowering plants (angiosperms), there are two large groups with different styles of leaves. Dicots (roses, for example) have leaves with a netlike, branching system of veins. Monocots (grasses, for example) have parallel veins. See the picture below for a comparison. Later in this lab you will see more differences between dicots and monocots.

Dicot leaf Monocot leaf

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Cross Section of a (Dicot) Leaf Review 1. Examine the cross section of the dicot leaf below. You should be able to recognize the guard cells

and their stomata (openings) as well as three major regions: the epidermis (dermal tissue) forming a single layer over both top and bottom surfaces, the mesophyll (ground tissue), and the vascular bundle (veins).

Epidermis: A single layer of cells on the top and bottom of the leaf. Each cell has a nucleus, which may be visible as a dot in the cell. However, this is a thin slice of some large cells, and in many cases the slice does not happen to include the nucleus. The epidermis secretes the cuticle, a waxy layer that surrounds the outside of the leaf. The cuticle is not visible in these slides. Stoma (plural: stomata): Openings in the leaf to allow for gas exchange. Stomata are created by guard cells, which can expand or contract to open or close each stoma. Closing the stomata reduces water loss, but can also slow photosynthesis by preventing CO2 uptake. The guard cells are another example of dermal tissue. Mesophyll (ground tissue): Mesophyll means “middle of the leaf,” and these cells fill most of the leaf’s volume. They are the primary sites of photosynthesis, and they are filled with chloroplasts, which are visible as small dots. In dicots, the mesophyll layer is divided into two types: palisade mesophyll and spongy mesophyll. Palisade mesophyll is located directly under the upper epidermis. It has the greatest concentration of chloroplasts. The spongy mesophyll is located directly under the palisade mesophyll, toward the bottom of the leaf. The spongy mesophyll cells are surrounded by air spaces that allow circulation for gas exchange. Vascular bundles (vein): contain xylem and phloem. Xylem forms the system for transporting water and inorganic nutrients from the roots all the way up to the leaves. This transport sometimes involves powerful pressure gradients, so the xylem cells are heavily reinforced. You can recognize them in this slide because they have very thick walls. The phloem cells help do the job of transporting sugars produced in photosynthesis. Phloem transport typically involves weaker pressure gradients than xylem transport. In contrast to xylem cells, the phloem cells are smaller and thinner-walled. In a vascular bundle of the leaf, the xylem is usually on top and the phloem on the bottom.

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1. Label the cross section of the Dicot leaf below using the following terms: upper epidermis, lower epidermis, guard cells, stoma, palisade mesophyll, spongy mesophyll, air spaces phloem and xylem.

Part One: Dicot Leaf 2. Obtain and observe a prepared slide of a Syringa (lilac) leaf cross-section under the high power using

a light microscope. 3. Sketch the Syringa (lilac) leaf cross-section below. (Use colored pencils and draw to scale.)

Label the following leaf structures: upper epidermis, lower epidermis, guard cells, stoma, palisade mesophyll, spongy mesophyll, air space(s) and vein.

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Part Two: Monocot Leaf 4. Obtain and observe a prepared slide of a Zea (corn) leaf cross-section under the high power using a

light microscope. 5. Sketch the Zea leaf cross-section below.

Label the following leaf structures: upper epidermis, lower epidermis, guard cells, stoma, mesophyll, and vein.

6. Answer the following questions based on your observations.

a. Varying position of stomata is one way that plants have adapted to increase water use efficiency. Leaves can be described as hypostomatic, epistomatic, or amphistomatic. Like many scientific words, these words contain Latin prefixes. In Latin, hypo- means “below”; epi- means “above”; and, amphi- means “both”.

i. What type of leaf is Syringa in regards to its stomata? Why?

ii. What type of leaf is Zea in regards to its stomata? Why?

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b. What is different about the mesophyll in a Zea leaf compared to that of the Syringa leaf?

c. There are different types of plants in regards to their photosynthetic pathway. Most plants carry out C-3 photosynthesis, in which both the light dependent reactions and the Calvin Cycle occur within the same chloroplasts in all of the mesophyll cells. Other plants have adapted to extremely hot, bright conditions by carrying out C-4 photosynthesis in an attempt to minimize water loss while still allowing photosynthesis to take place in intense sunlight. During C-4 photosynthesis, CO2 is stored in 4-carbon compounds so that the plant does not need to keep the stomata open at all times. Some C-4 plants also separate the reactions of photosynthesis into different chloroplasts within different types of cells, another energy conserving measure. The light independent reactions occur in the mesophyll cells, while the Calvin cycle occurs in enlarged bundle sheath cells that surround the vascular bundle.

i. Compare the cross-sections of the Zea and Syringa leaves. Which do you think is an example of a C-4 plant?

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Part Three: Pine Needle The leaves of Pinus (pine) trees are called needles. Though their shape is different from the leaves of most angiosperms, they contain more or less the same tissue types. Pines often live in harsh conditions: hot, dry summers and freezing winters. They are good at withstanding environmental stress. Their needles, with a low surface area-to-volume ratio, help reduce damage due to drying out or heavy snows. Pine needles also have some features not seen in Syringa leaves. Transfusion tissue surrounds the vascular bundle, and apparently helps transport materials into and out of the vascular tissue. This tissue is abundant in pine needles, but not in most leaves of flowering plants. Resin ducts are visible in the mesophyll region of the leaf. They are found in roots, stems and leafs of conifers. The resin ducts carry resin, which is a hydrocarbon-containing substance that may help protect the leaves. The cuticle is a waxy layer around the outside of the pine needle. The stomata are sunk into small pits in the epidermis.

1. Label the following in the Pine needle cross-section below: epidermis, mesophyll, stoma

2. Using complete sentences, answer the following questions:

a. What type of macromolecule is produced by resin ducts? How would it aid in protecting the Pinus?

b. Sunken stomata are often referred to as stomata crypts. How do they aid the Pinus in surviving hot, dry summers?

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Part Four: Leaf Adaptations Some leaves have adaptations for surviving in extreme environments:

(1) Mesophytes are plants whose leaves are not adapted to extreme environments. An example is the Syringa leaf.

(2) Xerophytes are plants adapted to arid environments. (3) Hydrophytes are plants adapted to grow in water. (4) Sclerophytes are plants adapted to resist animals, freezing temperatures, and ultraviolet light.

1. Below is a cross-section of Nerium oleander.

The epidermis is several layers thick and is covered by a thick cuticle. On the bottom of the leaf stomatal crypts can be seen. They contain hair-like structures called trichomes. What type of plant would you classify Nerium oleander?

2. Below are cross-sections of the Yucca plant.

The leaves are tough and fibrous with a thick cuticle. Thick mesophyll surrounds the vascular bundles. The stomata occur in long narrow grooves between the fiber bundles. The vascular bundles have fibrous sheets.

What type of plant would you classify the Yucca?

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3. The Nymphaea (water lily) is a hydrophyte. Obtain and observe a prepared slide of a Nymphaea (water lily) leaf cross-section under the high power using a light microscope.

4. Sketch the Nymphaea leaf cross-section below. 5. Label the following in the cross section below: stoma, vascular bundle,

bundle sheath, intercellular space(s), spongy mesophyll, palisade mesophyll, trichome, lower and upper epidermis.

6. Nymphaea leaves also contain astrosclereids in their mesophyll layer. Astrosclereids are branched, pointed, irregular reduced form of sclerenchyma cells. Can you find one in your cross section? Why do Nymphaea leaves have this adaption?

a. Nymphaea leaves are different from lilac leaves. Identify two structural changes that you notice in any of the four regions of its leaf: epidermis, stomata, mesophyll, and vascular bundles.

b. Explain why each structural change is a helpful adaptation for the water lily leaf.

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