NAME:
Station #
DATE:
PER.
HOW DOES LEAF STRUCTURE RELATE TO ITS FUNCTION?
Internal Leaf structure and Photosynthesis
Procedure and Observations
As you progress through the examination of a prepared Dicot Leaf
label the following structures on the diagram:
cuticle, upper epidermis, palisade cells,
chloroplasts, spongy cells, vein, xylem cells,
phloem cells, lower epidermis, air space,
stoma, guard cells.
Examine a prepared slide of a leaf cross section under low power (40x) of your microscope. Be sure to observe the top
to the lower surface and from one margin to the other. Observe that the leaf is composed of three tissues: dermal,
mesophyll, and vascular tissue.
Dermal
Study the upper epidermis. (a) How many cells thick is it? (b) Do you see any chloroplasts?
The cells of the epidermis are covered by a waxy lipid layer called a cuticle. (c) Suggest a function for this hydrophobic
layer.
Mesophyll
The mesophyll is the largest area of the leaf and is composed of two regions. The first of these is made up of palisade
cells which lie just below the upper epidermis.
Study this area under high power (400x). (d) Describe the cells and their orientation to the upper epidermis. Predict the
reason for this orientation. [hint: think about the surface area of the leaf]
Locate some chloroplasts in the palisade cells. (e) Suggest a function of the palisade layer. (f) Why is the long, columnar
shape of the palisade cells important?
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Study the second region of the mesophyll. Locate the spongy layer of cells below the palisade cells. (g) Of the two,
which layer is more compact? (h) Do chloroplasts appear to be as numerous in the spongy cells as they are in the
palisade cells? (i) Predict why this may be the case.
Note the numerous spaces among the spongy cells. These are the air spaces. (j) On the basis of their relationship to
other tissues in the leaf, what do you think their function is?
Vascular
The spongy layer is penetrated by numerous veins. Move the slide until you are able to locate a large, central midvein.
(k) How do the veins appear compared to the other tissues of a leaf?
Examine the vein closely. Locate empty cells with thick walls in the upper parts of the section. These are the xylem cells.
(l) Suggest two functions of the xylem cells based on your observations and the description of these cells.
The thin-walled cells that form a cluster below the xylem cells are the phloem cells. (m) What is the function of the
phloem cells?
Dermal
Examine the lower epidermis. (o) How many cell layers compose it? Closely examine the lower epidermis. Try to find
tiny pores with small, rounded cells on either side. The pores are the stomata and the rounded cells are the guard cells.
(p) Determine the relationship between the stomata and the air spaces of the spongy tissue. (q) Suggest a function of
the stomata.
(r) Compare the number of stoma on the upper and lower epidermis. Predict why there may be a difference in the
number of stoma on these two layers.
Draw your leaf
cross section here.
Guard Cells and Stomata
Obtain a specimen of a Tradescantia leaf from your teacher. Place a flat
section of the leaf bottom-side up on a slide as a dry mount. Locate the
guard cells — two bright green bean-shaped cells on either side
of the stomate. Draw a neat, clear diagram of a single stomate with its
guard cells on the leaf in the space below.
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