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How to make a surface section
Stomata
A bath sponge has an enormous surface
You can also make impressions of leaf surfaces using nail polish (see page 40). Very
beautiful growth patterns on the epidermal cells then often become visible.
Death By Thirst or Hunger?
At the same time as the chloroplast-containing cells on the inside of a leaf have to
absorb carbon dioxide from the air, the leaf cannot allow water to evaporate from
inside it in an uncontrolled manner. Otherwise, many plants would dry out before
they even begin fabricating starches. In order to reduce the evaporation, the
surface of a leaf is covered with a relatively thick layer of wax called the cuticle.
The cuticle ensures that the leaf is well-insulated from the outside — so well, in
fact, that practically no water is able to escape. However, no carbon dioxide is able
to get through either. Nature's inventive spirit was able to resolve this dilemma.
In the epidermis on the underside of a leaf, you will often find differently shaped
cells. They are scattered as little bean shapes across the leaf's surface. These open-
ings are referred to as leaf pores or stomata (singular: stoma). They are, in a cer-
tain sense, the leaf's doorkeepers. When carbon dioxide is needed, or even when
the humidity in the air is high, they open up and allow the gases to pass through.
If too much water is in danger of escaping, especially when the temperature is
high and humidity is low, they close again. The cells require energy to open and
close the stomata. They generate this energy with their own chloroplasts.
But the evaporation of water from the stomata has a positive side too. You already
learned how water gets from the ground into the other plant organs (see also
Chapter 11). It so happens that the driving motor for the transport of water in a
plant's vessels is the evaporation of water from the leaves. This creates a sort of
suction that draws fresh water upward from the root.
On Sponges and Palisades
What do a bath sponge, your lung, and the leaf of a plant have in common? Not
much at first glance. But the answer lies in the detail.
Air reaches one of your two lungs through the trachea. But the lungs are not sim-
ply empty sacks. In the lungs, the bronchia branch out into extremely tiny vessels.
This is advantageous in that your lung increases its surface area many times over.
It's through this surface that the lung absorbs oxygen from the air and passes it on
to the blood. A sponge also has a very large inner surface area. That's the reason
why sponges possess such excellent absorbency. Most sponges that are used in the
household are made of plastic. But there are natural sponges too. Natural sponges
are actually animals (but they are not alive when bought in the store). Sponges
feed themselves by filtering the smallest of nutrient particles or small animals and
plants from the water. The larger the inner surface of a sponge, the more water it
is able to absorb at the same time.
But where is the analogous large surface area in leaves? A cross-section gives it
away. It is generally possible to distinguish two layers of cells on the inside of a
leaf. An (upper) layer made of oblong (or palisade-shaped) cells and a (lower) one
made of cells of a wide variety of shapes. The cells of the upper layer are packed
tightly together and contain many chloroplasts. They are chiefly responsible for
the absorption of sunlight. The lower cell layer has a much looser construction.
There are large gaps between the individual cells. Here, it is precisely this unique
layout of cells that serves to enlarge the surface area. In nature, it is usually true
that any place where substances are absorbed or exchanged, there are structures
that enlarge the surface area through complex branching or folding. This lower
cell layer in leaves is also referred to as sponge tissue due to its appearance and is
primarily responsible for the absorption of carbon dioxide from the air.
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