Download Leaf FAQ

Leaves from the Outside In
This FAQ will answer some questions about the structure of leaves. The main focus is on
the leaf surface: how fungi get through it and how we can study it. This will help you
understand what is proposed in one of the grant proposals that you will evaluate.
What do leaves do?
If a plant’s job in life is to help a seed make more seeds
(think about that!), then the leaf’s job is to gather the energy
needed for the plant to grow, flower, and make more seeds.
What do other parts of a
plant do?
Roots gather water and minerals that the leaves and rest of
the plant need, and they keep the plant from blowing away
or falling over. It is the job of the stems and braches to hold
up the leaves in such a way that they get as much sunlight as
possible. Flowers are a plant’s reproductive structures for
making seeds and thus more plants.
How do leaves gather
energy?
Sunlight is a form of energy. Photosynthesizing cells
(mesophyll) in the middle of the leaf trap that energy.
Chemical reactions in the mesophyll use the energy to make
sugars from water (from the soil by way of the roots) and
carbon dioxide (from the air). All living parts of the plant,
including the leaves themselves, can use the sugars to
release energy for the work of growth and development.
What are leaf veins for?
Leaf veins contain cells specialized for transporting water
and minerals to the other cells of the leaf, as well as other
cells specialized for transporting sugars from the leaf to
other parts of the plant. The veins are the circulatory system
of the leaf.
What makes up the “skin”
of a leaf?
The outermost layer of cells in a leaf is the epidermis.
There is an epidermis on both the upper and the lower
surfaces of the leaf. Most epidermal cells are part of the flat
surface of the leaf, but some are the guard cells mentioned
below, and some form hairs with various functions. The
entire outer surface of both epidermises is coated with a
waxy cuticle. The outermost layer of the cuticle is almost
pure wax, which may be highly sculptured.
What do the cuticle and
epidermis do?
One of the greatest problems faced by land plants is that
they lose water by evaporation to the surrounding air. The
epidermis protects the leaf against losing too much water; in
particular, water and water vapor cannot pass through the
cuticle. But the carbon dioxide needed for photosynthesis
can’t pass through the cuticle, either—now there’s a
problem for you! The plant solves this problem by a
compromise made possible the presence of pores called
stomata (singular: stoma). The stomata can open to allow
carbon dioxide to enter the leaf or close to keep water in.
There is no cuticle over the open stomata. Pairs of cells
called guard cells control the opening and closing of the
stomata.
Are the leaf surfaces of all
plants the same? That is,
if we make something
happen in one plant
species, will it happen in
another?
There are a few differences, some of which might matter for
our purposes. Wheat and other grasses belong to a group of
flowering plants that have long and narrow leaves, with
veins running parallel. Most of their epidermal cells are in
parallel rows along the length of the leaf. These cells are
roughly cylindrical in shape.
The leaves of most other flowering plants, including things
like roses, carrots, and oak trees, don’t look at all like grass
leaves. Their veins are not parallel, and their epidermal
cells are flat and very irregular in shape. These cells often
look like pieces of jigsaw puzzles, unlike the neat rows of
epidermal cells in wheat leaves.
How do fungi attack
leaves?
When a fungal spore starts to grow, the first thing we see is
a tiny hypha called a germ tube elongating from the spore.
For the hypha to invade the leaf and start an infection it
must somehow get through the epidermis. It must either
“drill” through the cuticle and epidermis or find a stoma and
gain entry through it. Some fungi can drill, but Puccinia
graminis (the fungus causing the stem rust of wheat) can
only get in by way of a stoma. In either case, the germ tube
forms a clinging structure called an appressorium, which
sticks tightly to the leaf surface. Another hypha grows out
of the appressorium. In some fungi this “peg” can generate
enough pressure to burst through the cuticle and epidermis;
in others the peg can only reach the interior of the leaf if it is
directly over an open stoma. Once a peg is inside the leaf, it
branches, sending hyphae spreading through the interior.
When these hyphae contact mesophyll cells they can form
haustoria that take up sugars and other nutrients that would
otherwise serve the plant. In this way the fungus derives the
nutrients it needs to grow and reproduce, producing new
spores to spread the disease to other plants.
How can we study the
cuticle and epidermis in
the laboratory?
The plants are usually grown in plant growth chambers, in
which the day length, light intensity, temperature, and
(sometimes) humidity are precisely controlled. Growth in
such chambers yields uniform plants that can be studied as
they undergo experimental treatments.
The most useful tool for observing the fine structure of the
cuticle and epidermis is the scanning electron microscope.
A micromanipulator can be used to place spores at
precise locations on the cuticle or to disrupt the structure of
the cuticle or epidermis. We use a light microscope to see
what we are doing with the micromanipulator.