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Tuesday, 29 November 2011

Looking at pollen grains

What you need:
The first thing you need is a collection of flowers. I use a set of neat little plastic containers that screw together. They are sitting on my desk as I write this, and the top two contain millipedes that I collected this morning, and which may be the basis of a future entry, depending on what I can see.

You really need a microscope with a magnification of x400 or better, and the patience to piece together different views. Pollen grains are too small to see with the eye, or even with a hand lens, but they are too large for you to focus on the whole grain at one time under high power.

That means you need to manipulate the images with a neat bit of open-source software from the National Institutes of Health in the USA, called ImageJ.  This is really neat, but not entirely intuitive to non-geeks, so I really urge you to read the documentation.

Almost every species has a distinctive pollen, and there is even a science of pollen study called palynology. This science is useful in archaeology, criminal investigations—and even in determining the origins of honey.

Here are two shots of the same pollen, one under low power (left) and one under high power (below).  You need to adjust the focus up and down if you want to see the distinctive patterning (called sculpting) on the surface.

The pollen is from a pea, Gompholobium, which was quite infamous in the early days of Australian settlement, because it poisoned sheep which ate it. The poison, sodium fluoroacetate, is used today under the trade name "1080" (ten-eighty) to get rid of pests, because Australia's marsupials are able to resist the poison which is good against rabbits and foxes.

I didn't get very far with this before deciding to drop it from the book, so the lighting in my examples is quite variable, simply because I had lots of stuff to write, and couldn't chase too far down the blind alleys.  I will get back to this, one day, some time. Take these two shots (below) of cobbler's pegs (Bidens sp.) pollen:

In each case, I took three shots at different levels, trying to capture the sculpting. Here is a last example, Kunzea (bachelor's buttons, Myrtaceae).  I'm far from an expert in this, but there's the idea for you.

There are two problems: pollen grains are sometimes hard to wet, so air bubbles cling to them, and once they are wet, some of them will burst, sending out a pollen tube—this is explained in the next section. I use a tiny amount of detergent in the mounting water and that seems to help get rid of the bubbles. To beat the bursting problem, all you can do is work fast.

Professionally, the clearest views come from scanning electron microscopes, but those are a bit more than a private individual can afford. Under x200, you will be able to just make out the 'sculpting' on the surfaces of pollen grains, and you may even be able to see that they are different shapes. Under x400, it will be much easier to see.

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