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Investigation 9 – Leaf Structure And Function
The leaf is the major organ of photosynthesis in most autotrophs. It has many specific structural features that
are designed to maximize the rate of conversion of carbon dioxide and water into the required carbohydrates
for the plant, while at the same time reducing water loss. Leaves are specifically adapted to their particular
habitats as their environments may differ in many of the factors required for photosynthesis, e.g. light intensity,
water availability and carbon dioxide concentration.
The aim of this practical is to investigate the structure of leaves and understand how the structure enables the
plant to carry out photosynthesis efficiently. You will do this by making observations of epidermal tissue and a
leaf in cross-section and oblique section.
Materials
Compound light Microscope
Lens cleaning tissue
Tissues
Teat pipette
Forceps
Slides and Coverslips
Hand lens
New single edged blade Carrot or elder pith
100 cm3 Beaker
Fresh leaf tissue Micrometer eye piece
Toluidine blue or iodine green stain
Super glue
Part A: Preparation Of Epidermal Tissue
METHOD
1.
Carefully clean a glass slide and coverslip.
2.
Obtain a leaf and hold it between the thumb and forefinger of each hand with the underside facing upwards.
Gently tear the leaf by pulling one hand towards you at an angle. This should reveal a thin relatively clear
lower epidermal layer. With a sharp razor blade cut a small section and place it centrally on the slide on a
drop of water. Add another drop of water with the pipette and place your coverslip in position taking care
to avoid air bubbles. Clean up any excess water with paper towelling.
3.
Focus first under low power then high. You should be able to see a thin layer of cells of 2 types (a) epidermal
cells and (b) guard cells.
The epidermal cells are regular in their shape and surround the guard cells. The guard cells occur in pairs
and are clearly visible with the dark shaded oval space between them. Green chloroplasts should be clearly
visible in the guard cells.
4.
At high power (around ~×400) magnification observe the cells and draw them carefully showing as much
detail as you can of 2-3 epidermal cells and a pair of guard cells in the space provided. Clearly label
your drawings showing the cell wall, cell membrane (location only), cytoplasm, vacuole (if seen) and
chloroplasts.
5. On the drawing indicate estimates of the sizes of both epidermal cells and guard cells. Estimate to the
nearest 10 μm with the aid of the micrometre eye-piece.
Note: if a micrometre eye-piece is not available then a micro-grid slide may be used to determine the size of
the field of view to allow measurement of the size of the cells.
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Investigation 9 – Leaf Structure And Function
Part B: Preparation Of A Leaf Cross-section
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Method
1.
Using a carrot block and sharp razor blade (illustrated below) cut extremely thin sections of one of the
leaves and float the sections into water in the beaker.
Alternatively you may use two razor blades held together and drawn across a leaf which is resting on a
wooden block.
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The leaf section may be much thinner at one end and this is the part that will be most useful to you.
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(a) use a new razor blade
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The secrets to cutting good thin sections are:
(b)keep the blade and the carrot surface moist.
(c) use a sawing motion while working your way across the leaf section.
(d) use a fine brush and not your fingers to transfer the sections to the Beaker and then to the slide.
Warning: Sharp instrument.
razor blade
carrot block
protruding leaf
2.
Pick the thinnest one or two sections and make a wet mount slide. Apply a cover-slip and clean up any
excess moisture.
3.
Observe first under low power then high power.
4.
Make a clear, labelled drawing of a cross section of the leaf. You do not need to draw all the cells that you
see but you must show detail of no more than 5 of each type. In your drawing you should;
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Indicate the numbers of layers of palisade cells, mesophyll cells and epidermal cells and indicate the
internal structure of the cells.
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Show the approximate thickness of the cuticle
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Label clearly the cuticle, upper and lower epidermis, palisade mesophyll, spongy mesophyll, vascular
bundle (if seen) and the stoma or guard cells if seen.
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Include measurements to the nearest 10�m to show the thickness of the leaf and the size of a typical
palisade cell.
© S.T.A.R. 2007 This page may only be legally copied under the conditions of sale to the purchasing school.
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Investigation 9 – Leaf Structure And Function
Part C: Preparation Of A Leaf Oblique-section
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Method
1.
Cut strips of leaf tissue to 1cm x 3cm. Glue the strip lower side down on a slide using super glue. When
ther glue has set, use a razor blade cut a very oblique section towards one end of the leaf (see below).
Warning: Sharp instrument.
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Razor blade
Leaf tissue
Stain the cut surfaces in iodine green or toluidine blue stain for 1 to 2 minutes. Soak up the excess stain
using filter paper and add a few drops of distilled water. Cover with a cover slip and observe under medium
power.
3.
Cut another section only this time place the leaf upper side down on the slide. Stain it up the same way and
discuss your observations.
4.
Use this method to investigate the internal structure of leaves from different plants.
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Results
Record your observations accompanied by suitable drawings. Discuss how the leaf is adapted to carry out the
process of photosynthesis through the functions of trapping light, gas exchange, control of water loss and the
transport of materials.
Ensure that the drawings are fully labelled, titled, magnifications specified and estimates of the sizes are shown.
Your calculations of the cell sizes should also be shown in the boxes
LARGE, CLEAR, WELL LABELLED DRAWINGS IN PENCIL SHOWING DETAILS ARE ESSENTIAL.
Design
You are now able to conduct your own investigation into the microscopic structure of plants.
© S.T.A.R. 2007 This page may only be legally copied under the conditions of sale to the purchasing school.
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