Scanning Electron Microscope – Field Thistle pollen grain

Looking at pollen grains under the microscope highlights their beauty, complexity and uniqueness. In July 2015 I was lucky enough to spend some time with the Engineering Doctorate team at Baglan Bay Innovation Centre and use one of their Scanning Electron Microscopes. I would like to thank Dr Ian Mabbett for his help as well as that of his team of technicians and postgraduates, in particular Tom Dunlop who operated the SEM, took the photos and measurements. All photos can be viewed at this link

I have previously looked at pollen grains using light or optical microscopes, but the magnification of these of of the ones that I have access to is limted to x1000.

This photo of a field or creeping thistle pollen grain using an optical microscope has been created by stacking several photos together using Heliconsoft stacking software. In this we can make out the three points at which the intine is coming through the exine, but the resolution isn’t that great

2015-09-24_21-31-24 M=C modified

Field Thistle pollen grain x400 light optical microscope

Using a SEM means that I can look at grains up to several 1000 times magnification.

The SEM also means that we can look at things in three dimensions whereas with an optical microscope the focus is on a single layer and to get a 3D image I had to use some stacking software.

Why is the resolution of the image from a SEM greater than an optical light microscope? The only direct explanation that I can found was from Jim Al-Khalili’s book Life on the Edge where he writes about quantum mechanics in biology. In the book he explains the following:

‘The German scientists Max Knoll and Ernst Ruska realized that, since the wavelength (the distance between successive peaks or troughs of any wave) associated with electrons was much shorter than the wavelength of visible light, a microscope based on electron imaging should be able to pick out much finer detail than an optical microscope. This is because any tiny object or detail that has dimensions smaller than the wave falling on it will not influence or affect the wave’

I took along several samples of pollen, but it was the field thistle which had the best results. The grains were brushed onto a glass slide and then had a thin film of gold put on them. The gold is needed to provide focus for the beam as the SEM operates under an electric field of around 20KV.

The results are outstanding, but what we found was that the electric field caused the grains to move around and easily ‘fly’ out of view at times. We also saw some grains expand. Most noticeable though I could easily identify the various elements of the structure of the pollen grain as can be seen below:


Honeybee on Field Thistle

This shows the honeybee on a field thistle flower. In the summer of 2015, the thistles remained in flower for several weeks if not months providing much needed nectar and pollen for many creatures.

Patricia Hodges made detailed drawings of the more common pollen grains and here is her drawing of the field thistle pollen grain

Thistle Patricia Hodges drawing

Patricia Hodges drawing of a thistle pollen grain


Field Thistle pollen x2500

In this first photo of the pollen grain magnified 2500 times we can see various elements of the exine (refer to fig 1). We can see three apertures with the intine coming through located at the top, the left and bottom right of the pollen grain.

The following photos clearly show the ‘spikes’ or ‘ornamentation’.


Field Thistle pollen x2000 diameter 41.4um

RS pollen Exine

Figure 1. Rex Sawyer ‘Honey Identification’ page 16 Pollen Grain Exine

RS pollen layers

Figure 2. Rex Sawyer ‘Honey Identification’ page 16 Pollen grain wall


This photo the accurate measurement of the diameter of the grain and also that of one intine of the pollen grain.


Pollen grain expanding x2000


Field Thistle pollen grain x6000

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