Liesegang Rings (or Liesegang Bands) Another kind of Reaction Diffusion System. discovered by accident, actually occurring in chemical reactions but not yet discovered in embryos or animals or plants (And the mechanism of which is not yet known for sure!!) (After a century of intensive research, and ~ 4,000 published research papers!) Recipe: (From the June 1969 issue of Scientific American, "Amateur Scientist" column) a) Dissolve 4% by weight of ordinary gelatin into hot water. Also dissolve into this water, 0.4 grams of potassium dichromate per 100 ml. of water. WARNING: Dichromate is a serious cumulative poison; don't ever let any get on you. b) Pour this hot liquid solution into thin layers in Petri dishes, or onto microscope slides. (In Biology 441L labs, we put these gels between slides, with a one millimeter space between them.) c) Leave your poured plates overnight to gel. d) Either put crystals of silver nitrate directly into contact with the dichromate gel, or dissolve enough silver nitrate into water, and put drops of this solution in contact with the gel. e) Watch precipitation of red silver dichromate crystals in the gel. Instead of forming a continuous mass of silver dichromate crystals, rings of crystals form, separated by clear areas in which crystallization has not occurred. Depending on the geometric patterns of sources of diffusing dichromate and silver ions, various geometric patterns can form, including not just bands but also spots and dotted lines. Instead of forming solid blobs of insoluble silver bichromate, what happens instead is formation of many concentric rings of precipitated silver bichromate, with clear gaps between the rings of precipitate. These gaps are what is unexpected and unexplained. It is also surprising is that rings don't form if you dissolve silver nitrate in a gelatin solution, let it gel in thin layers, and then scatter crystals of potassium bichromate on top of that. That is some kind of asymmetry. However, rings do form if you put a small bichromate crystal on top of an area of bichromate gelatin gel where silver bichromate crystals are already formed. The Wikipedia site on this subject is well done. By using high power microscopes to making time lapse videos of Liesegang rings I have discovered several new and very unexpected phenomena. A couple of these videos were shown in lecture. ****************************************************************** Evidently, all self-organizing phenomena (even the simplest) resist human comprehension. Our brains expect shapes to be imposed by something internal, not as a net result of internal interactions. How would Positional Information explain Liesegang Rings and reaction-Diffusion systems? Could somites be formed by Reaction-Diffusion systems? Could spatial patterns (Colinearity) of hox genes be produced by Reaction-Diffusion systems? ------------------------------------------------------------- Videos shown in class: video of Liesegang bands forming from a thread soaked in silver nitrate solution placed on a layer of gelatin containing potassium dichromate another video of Liesegang bands forming computer simulation of bands forming, shown in class the Pascal program that generated this simulation