ON THE MOVEMENT OF DESMIDS
William Ells
Coniferae, Walnut Tree Lane, Loose, Maidstone, Kent. ME15 9RG.
Tel. (01622) 744819
Unable to travel as much as I would like, I rely on
friends to provide me with samples. Anne Bruce sends me samples
collected by her husband Alastair. These samples are usually in
tubes about 100mm x 15mm, because they are easily knocked over I
have drilled holes slightly larger than the diameter of the tubes
into a block of wood about 15mm thick to stand them in.
The last batch to arrive consisted of three tubes, I was able to examine the contents of two of the tubes the day they arrived, for reasons I will not bore you with the third tube was not examined until it had stood on my north facing window sill for 48 hours. When I had a look at this sample there was a sediment in the bottom just below the level of the hole in the wood block, on top of this sediment there was what looked like a little green tree 2 - 2.5mm. high. This was carefully taken up with a pipette and placed on a slide, the 'tree' was entirely made up of hundreds of desmids, 12 genera and 27 different species were counted. I have found these little 'trees' before, but they have always consisted of many cells of a single species of the genus Closterium.
Desmids excrete and clothe themselves with mucilage, a transparent slimy substance. Some species in copious amounts that can be seen without staining or differential contrast under an optical microscope. With others it is a thinner substance that only shows up when Indian ink or stain is added to the water (photo's of this have been shown in earlier articles).
Desmids are able to move by extruding mucilage from an apex and moving in the opposite direction - jet propelled ! They are also able to attach themselves to the substrate and to each other. Moving towards the light, sticking together with mucilage, and probably still multiplying by division a 'growth' visible to the unaided eye occurs, made up of desmids that cannot be seen individually without magnification.
Later another 12 species were recorded that did not occur in the 'tree'.
The author gives grateful thanks to Anne and Alastair for this excellent sample from Scotland.
Editor's note: Comments and feedback via email to Bill Ells welcomed.
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