In a conventional microscope, light waves are channeled through glass lenses to magnify the image and so resolve greater detail. However, resolution is limited by the quality of lens and, ultimately, by the wavelength of visible light. An electron microscope uses an electron beam instead of light, to give much greater magnification.
An electron beam has waveform characteristics similar to light but its wavelength is 100,000 times shorter. The beam is controlled by adjusting the velocity of electrons within it and it is focused using electro-magnetic lenses. The discovery of this phenomenon led to the development of electron microscopes that can attain magnifications and resolutions far greater than those previously known.
Two types of electron microscope
There are two types of electron microscope. The first to be developed in the 1930s was the transmission electron microscope (TEM). In a TEM, the specimen is an ultra-thin section of material suspended in the column. When electrons hit it, most will pass through it. However, if electrons contact heavy atoms within the specimen they may be knocked out of the beam. The pattern this forms is translated into an image when the beam is refocused by the second magnetic lens (the objective). The image is magnified by a third lens (the projector) up to 1,000,000 times and can be viewed on a fluorescent screen. The diagram below represents the analogy between light and electron illumination.
The scanning electron microscope (SEM) was first commercially available in 1965. Modern SEMs can magnify surface detail up to a million times. The beam of the scanning microscope passes across the surface of a specimen rather than travelling through it. When the beam hits a conductive surface it is split into its constituent electrons at various energy levels. The signal produced by secondary electrons is detected, amplified and displayed on screen in a way similar to that of a normal television image.
The Supra 55VP
There have been scanning electron microscopes (SEMs) at RBGE since 1974. The Supra 55VP is the fourth and was installed in April 2003. Unlike its predecessors, it has a field emission gun that produces a beam 1000x brighter than the conventional SEM. This means that delicate material can be observed at lower kV without loss of detail. It also enhances depth of field and greatly improves resolution. In addition, the Supra has a variable pressure option for viewing unprepared material.
For RBGE's botanists, access to the scanning electron microscope provides an opportunity to study plant surface structure in minute detail. This facility assists the taxonomist in the identification and classification of species and is particularly useful when the characters cannot be seen with the naked eye.
These characters might include the presence or absence of hairs; the pattern of stomata on a leaf or perhaps the appearance of leaf scales. The structures of pollen and seeds are also considered to be taxonomically important.
The images in the SEM image gallery illustrate some of the characters studied and highlight how microscopic structural variations may provide the taxonomist with valuable clues to aid species classification. Images in the Colour SEM image gallery have been pseudo-coloured.