Taking desmids to the next dimension …

Participants at the British Phycological Society / Quekett Microscopical Club field weekend at the Freshwater Biological Association, September 2017.  Scale bar: one metre (= 1,000,000 micrometres).

A theme that has run through this blog over the years has been that what you see down a microscope is often a highly distorted view of reality and at the end of our weekend of desmid hunting, Chris Carter gave a talk that also made this point, using desmids as a case study.  In essence, we had spent much of Saturday and Sunday morning peering down microscopes at three-dimensional objects that appeared, as a result of the very shallow depth of field that is characteristic of high magnification images, two dimensional.   We were then matching these to two-dimensional representations in the Floras and identification guides that we had to hand.  Dave explained a few tricks that experts use, such as applying gentle pressure to a coverslip with a fine needle, to turn desmids in order to see them from other angles but, mostly, we were restricted to very flattened views of desmids.

Chris has tackled this problem from several directions over the years, including experiments with anaglyphs (see “Phworrrrhhh …. algal sex in 3D!“) as well as the very careful manipulation of a long, cylindrical Pleurotaenium that won the Hilda Canter-Lund prize earlier this year.   He has also produced a number of plates with desmids laid out almost as if on an engineer’s drawing board, with front, top and side views.   Several of these are on Algaebase, but one example is reproduced below.  Microscopists learn to use the fine-focus control to appreciate the depth of the objects that they are examining and Chris also shows how it can reveal the nature of surface ornamentation on different parts of the cell.  The temptation, given a series of photos such as these (excluding the side view) would be to use “stacking” software to produce a single crisp image.  This is appropriate in some situations but you are, in truth, just producing a crisp two-dimensional image rather than offering any insights into the true shape of the cell.

Staurastrum furcatum from Botswana, photographed by Chris Carter.

Another technique that can be used to generate three-dimensional images is, of course, scanning electron microscopy.  However, this is beyond the budget for anyone outside a major institution.  This has helped greatly get a better understanding of the morphology of diatoms, in particular, but the third dimension comes at a price.   Scanning electron micrographs take us to an opaque, monochrome world, purged of the vivid colours that the microscopic world usually offers us.

Chris’ pièce de résistance, however, was a three-dimensional model of a Staurastrum, produced by the 3D printing company Shapeways and loosely-based on various pictures of S. furcatum and presented to him as a 70th birthday present by his son.  The main point is to demonstrate the symmetry and gross features of a typical Staurastrum rather than to be a taxonomic blueprint. The designers were very helpful but it does hint at what is possible with modern technology.

Chris Carter’s three-dimensional model of Staurastrum.  It is about six centimetres across.   You can buy your own copy from Shapeways by following this link …

The missing ingredient in this recipe is imagination.  Or, to be more precise, the viewer’s imagination as Chris has clearly demonstrated that he is not lacking in that department.  Once you have a sense of the three-dimensional form of a Staurastrum, you be able to use that knowledge every time you look at a two-dimensional image of a desmid through a microscope.   Seeing, as Ernest Gombrich reminds us in his great book Art and Illusion, is as much about using prior experiences to interpret the raw data collected by our optic nerves as it is about the patterns of light that stimulate our retinas.   Just as a child can look at a two-dimensional image of a cat in a book and match this to the real creatures that he or she encounters, so knowing about Staurastrum’s third dimension helps us to interpret the flat shapes that we see.

At a more basic level, all identification is a matter of matching the objects we see either to schemata stored in our memory or to patterns in books.  This, in turn, helps us to understand why the microscopic world seems so strange and mysterious to those who do not study it.  It all comes down to having (or not having) the prior experiences that generate recognition.   At one level, there are gasps of astonishment as people with none of these schemata in their memories encounter the beauty of desmids for the first time.  And then there is Frans Kouwets, another speaker at the meeting , who is busy cataloguing 750 different species of one genus, Cosmarium.   And in between there are the rest of us …

Frans Kouwets explains his fascination with Cosmarium to the British Phycological Society / Quekett Microscopical Club field meeting at the Freshwater Biological Association in September 2017.


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