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The Fine Art of Leaf-Cutting
In order to gain an overview of all of the tissues in a leaf, it's best to make a
cross-section through it. You will need:
• a slide and a cover slip
• the pipette and water
• a razor blade (see page 11)
• the tweezers
• various leaves, preferably thicker ones such as those of a boxwood, ficus, ivy,
or even an evergreen tree
• styrofoam or a piece of carrot
As easy as preparing a cross-section of a leaf may sound, unfortunately it is
usually rather difficult. Use the same procedural principles as those you fol-
lowed on page 38. First cut a slit into the styrofoam and stick the leaf into it.
You can also use a piece of carrot for this purpose. Then cut a layer off with
styrofoam and all. That way, the leaf and the styrofoam are in one plane. Now
place the razor blade on the styrofoam and cut the leaf and styrofoam as
thinly as possible. Go ahead and make 10 or 15 cuts like this while you're at it.
This will greatly increase the chances of their being a sufficiently thin section
among them.
The problem with preparing leaf cross-sections is that many leaves are very
thin and soft and are consequently also very difficult to cut. It's best to use
somewhat thicker or tougher leaves for practice at the beginning — such as
boxwood or ivy — before moving on to more tender objects. But sometimes
it is very difficult to get good, clean cross-sections. Leaves that are too thick
have the unpleasant tendency of folding over when cut, so that what you're
looking at is not the cross-section but a top view of the top or underside of the
leaf. So here's another tip: Cut very thin wedges. You surely won't be able to
see anything on the thick end, but you will generally get a good cross-section
on the thin end. Apart from that, the rule is: Practice makes perfect!
In leaf cross-sections, one finds many round or oval cell groups at the boundary
between the palisade and sponge tissues. These are leaf vessels that you have
cut transversely. You may already be familiar with the types of cells in them
from other objects we have looked at. They contain vascular bundle tissue with
tracheary elements and are usually also surrounded by thick-walled cells of the
reinforcement tissue (see also Chapters 11 and 12).
Needles from firs, pines, or spruces are leaves too. However, since they don't have
clearly defined upper and lower sides, their internal structure is a little different
than that of deciduous plants (in contrast to the evergreen needle trees, decidu-
ous trees lose their leaves each winter). But a nice, thin section of a pine needle is
an aesthetic pleasure that you shouldn't miss. The same also applies to the leaves
of many grasses, which often have very interesting cross-sections.
Did You Know?
The inner surface of the lung of
an adult human is as large as a
4-bedroom home? In fact, the inner
surface is 90 m2 on average! That
makes it clear how effective the fine
branching of our air passages is. A
horse that is able to9 run very fast
can even have an inner lung surface
of 500 m2.
Because of its many branches, the lung has a very
large surface area.
Cross-section through a needle leaf (tamarack)
Cross-section through a lilac leaf
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