Leaf and Stem Lab
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Pre-Lab:
LEAVES are specialized organs found on the stems of plants. The most important function of the leaf is
to carry out the process of photosynthesis. All of the necessary raw materials for photosynthesis come together in
the leaf. The XYLEM conducts water to the leaf. Carbon dioxide in the air enters through the STOMATA (singular
STOMA) or pores in the leaf. The CHLOROPLASTS in the leaf capture the light energy needed to convert water
and carbon dioxide into food.
Most leaves have large, thin, flattened sections called BLADES. The blade is attached to the stem by a
thin structure called the PETIOLE. Leaf blades occur in a variety of shapes and sizes. Simple leaves have only
one blade and one petiole, whereas compound leaves have several blades or leaflets that are joined to the stem
by several petioles. The leaflets of some compound leaves spread out like fingers on a hand. The leaflets of
others grow in pairs along a central petiole. The leaflets still of others are arranged on petioles, which in turn, are
arranged around a central pole.
Purpose:
To investigate the vascular tissue of terrestrial plants.
To identify some of the varied and specialized structures in a leaf.
Materials:
5 leaves including monocot, dicot, and conifer, lilac leaf cross-section slide, microscope
Procedure:
Part A: external leaf structure
Note the color of the upper and lower surfaces of the dicot leaf. Identify the blade and the petiole. Use a
hand lens to observe the midrib (central vein). Look for smaller veins that branch out from the midrib to all parts of
the blade. Draw a biological drawing of the dicot leaf. Label the BLADE, PETIOLE, MIDRIB, and SMALLER
VEINS.
Observe the shape of the monocot leaf. Use a hand lens to examine the leaf veins. Not that the lower
edge of the leaf blade surrounds the stem, forming a SHEATH. Draw a biological drawing of the monocot leaf and
label the BLADE, SHEATH, STEM, and VEINS.
Examine the shape of the conifer leaf. Pine leaves are found in clusters that are enclosed by a scaly
sheath. Pine leaves grow in groups of two, three, and five needles. Draw a biological drawing of your confer leaf.
Label the BLADES and SHEATH.
Part B: cross section of a dicot leaf
Examine the prepared slide of the lilac leaf cross section under the low power objective of your
microscope. Notice the arrangements of the various layers of the leaf. Identify the upper epidermis, or top layer
of the leaf. Locate the lower epidermis, or bottom layer of the leaf. Draw a biological drawing of the leaf cross
section. Label the UPPER EPIDERMIS, the LOWER EPIDERMIS, and the CUTICLE.
Switch to the high power objective. Locate the upper and lower epidermis. Note the size of the epidermal
cells. Cells in the epidermis secrete a substance called cutin which covers the whole leaf and prevents the leaf
from drying out. Focus on the lower epidermis. Look for stomata, or pores. Locate the guard cells that border
each stoma. These specialized cells control the opening and closing of the stomata by changing the amount of
water the cells contain. Label the STOMA and GUARD CELLS.
Locate the layer of elongated rectangular cells just below the upper epidermis. These cells are called the
palisade layer and are a type of cell called parenchyma. They contain many chloroplasts, which appear as tiny
green bodies inside the cells. Label a PALISADE PARENCHYMA CELL and a CHLOROPLAST. Observe the
irregularly shaped spongy parenchyma cells just below the palisade parenchyma cells. The spongy cells form a
honeycomb pattern with intercellular (between cells) air spaces, which connect with the stomata just below in the
lower epidermis. Together, the spongy and palisade parenchyma cells make up the mesophyll layer. Label a
SPONGY PARENCHYMA CELL, and AIR SPACE and the MESOPHYLL LAYER.
Locate the vascular bundle in the mesophyll. Label the VASCULAR BUNDLE. Locate the xylem and
phloem. Xylem cells generally have thick cell walls and are bunched near the top of the vein. Phloem cells have
thinner cell walls and are located near the bottom of the leaf. Surrounding and supporting the xylem and phloem
are cells that form a bundle sheath. Label the XYLEM, PHLOEM, and BUNDLE SHEATH.
Observations:
Make labelled BIOLOGICAL Drawings of each of the following:
Dicot Leaf (Live Specimen) – Label the structures described
Monocot Leaf (Live Specimen) – Label the structures described
Conifer Leaf (Live Specimen) – Label the structures described
Lilac Leaf cross-section (Prepared Slide) – Identify and Label the structures described
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Discussion:
1) What are the functions of the following structures: petiole, veins, stoma, guard cells, blade, xylem, phloem,
palisade mesophyll cells, spongy mesophyll cells
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2) Which surface of the leaf, upper or lower, contains the most chlorophyll. Explain how you know.
Explain why this might be.
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3) How are veins important in leaf function?
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4) A cross-section of a pine leaf would show a thickened cuticle and thickened epidermis. How does this design
help the pine leaf survive in the northern parts of Canada?
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5) How do the stomata and air spaces in the spongy mesophyll work together to help the plant survive? /1
6) Some people mist their house plants with water. Do you think the plant can absorb water through its leaves?
Explain why or why not.
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7) In order to keep house plants healthy, why should you periodically remove dust from their leaves?
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8) How is the vascular tissue in plants similar to vascular tissue in the human body?
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