Cambridge International A Level Biology Revision Guide

Chapter 7: Transport in plants
Figure 7.17 The Fatsia plant on the left has plenty of water
and remains turgid. The plant on the right is wilted because it
has lost more water by transpiration than has been taken up
by its roots, and so does not have sufficient water to maintain
the turgidity of its cells.
BOX 7.2: Epidermal peels and impressions
Epidermal peels
It is often easy to strip the epidermis from the underlying
tissues of leaves by grasping the epidermis with a pair of fine
forceps and peeling a strip of epidermis away from the leaf.
The epidermis is a single layer of cells, so can be clearly seen
if placed on a slide and viewed with a microscope.
A suitable leaf is provided by an onion bulb. This is
mainly made up of layers of fleshy storage leaves. It is very
easy to remove the epidermis from one of these leaves by
separating a leaf, making slits in its concave surface in the
shape of a small square using a scalpel, and gently peeling
off the square of epidermis using fine forceps. This can be
mounted in water on a slide for viewing.
The onion is a monocot, so its epidermal cells are long
and narrow and lined up in neat rows. The epidermal cells in
dicot leaves tend to be irregularly distributed and rounder in
shape, but with wavy edges which interlock with each other
rather like jigsaw puzzle pieces, as shown in Figure 7.18.
Many dicot plants may be used to make peels; lettuce and
geraniums are suitable. Use the lower epidermis. Apart from
Figure 7.18 Epidermal peel of a dicot leaf (× 150).
the stomata, note the pair of guard cells either side of each
stoma. These contain chloroplasts and control the opening
and closing of the stomata.
This practical lends itself to further investigative work.
For example, you can investigate whether stomata are open
or closed under different conditions, or estimate stomatal
density (number of stomata per unit area). The latter can
be done by counting the number of stomata in one field of
view and then finding the area of the field of view (and hence
the surface area of epidermis viewed) using the formula πr 2 .
The radius is found by using a calibrated eyepiece graticule
(see Worked example 1 on page 7) to measure the diameter
and dividing by two. Factors affecting density could be
investigated, for example:
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compare upper and lower epidermis
compare monocots and dicots (upper and lower
epidermises)
different parts of the same leaf (are stomata more
frequent near veins?)
age of leaf, particularly growing compared with fully
grown
habitat – wet or dry
variation within a species or within one plant
variation between species
succulent plants and cacti.
Epidermal impressions
Impressions (or replicas) of the epidermal surface can be
made with clear nail varnish. Very hairy epidermises should
be avoided. The surface of the leaf is coated with a thin layer
of nail varnish which is allowed to dry. It can then be carefully
peeled off using fine forceps and mounted dry or in water
on a slide. Alternatively, if this is too difficult, the nail varnish
can be peeled off using transparent sticky tape which is then
stuck on the slide.
You will be able to see whether stomata are open or closed
at the point in time the peel was made. Stomatal densities
can be estimated as with epidermal peels.
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