pretreatment: a color change from blue to dirty reddish-brown occurs when the iodine reagent slowly diffuses into the water preparation, because the euamyloid reaction appears at lower iodine concentrations than the hemiamyloid reaction. Asci with entirely reactive walls of this type of hemiamyloidity show rainbow-like colours when low-concentrated IKI is applied. Hemiamyloid (red) reaction in IKI prior to KOH, in comparison with euamyloid (blue) and inamyloid (negative). Only the hemiamyloid reaction strongly depends on the applied iodine reagent (IKI, MLZ) and pretreatment with KOH, being negative in MLZ and blue when KOH-pretreated (in IKI or MLZ). Direct application of IKI to a water munt (without KOH, highlighted) is the easiest way to recognize hemiamyloidity. Iodine reaction of hemiamyloid ascus apical rings of Hysteropezizella (Helotiales) in dependence of iodine reagent (IKI, MLZ) and pretreatment with KOH. Occurrence, significance Hemiamyloidity occurs in many groups of ascomycetes. In most members of Lecanorales and Ostropales, whether lichenized or not, the entire outer ascus wall layer reacts hemiamyloid. Roughly 20% of Helotiales have hemiamyloid] ascus apical rings compared to estimated 50% with euamyloid apical rings. In Pezizomycetes and different classes of pyrenomycetes hemiamyloid reactions are rare. Although hemiamyloidity is a very valuable taxonomic marker that permits differentiation between species or genera, this type of reaction, in particular the red reaction in IKI, is often overlooked. This neglect occurred since mycologists switched to Melzer's reagent, which was introduced in 1924 and almost completely displaced the previously used Lugol's solution. Hemiamyloidity was first reported by applying Melzer's reagent which gave a negative result without KOH, but a blue reaction when treated with KOH beforehand. Because of the frequency of hemiamyloidity in lichens, lichenologists generally did not join this change but continued using Lugol's solution. The widespread usage of swelling herborized fungi in KOH before study further contributes to the frequent overlooking of hemiamyloidity. Chemistry The chemical background of hemiamyloidity is not clear. A hypothesis claims that short helical sections of a carbohydrate chain alternate with shorter or longer linear sections. The short helical sections, similar to dextrinoidity of glycogen, would cause the red reaction by inclusion of iodine atoms into the spiral, and the linear sections might curl up under the influence of KOH, resulting in long helical chains which cause a blue stain upon iodine inclusion. The hypothetical spiral structure of these macromolecules seems to be related to the extensibility of the ascus wall, which is