In mycology a tissue or feature is said to be amyloid if it has a positive amyloid reaction when subjected to a crude chemical test using iodine as an ingredient of either Melzer's reagent or Lugol's solution, producing a blue to blue-black staining. The term "amyloid" is derived from the Latin amyloideus ("starch-like"). It refers to the fact that starch gives a similar reaction, also called an amyloid reaction. The test can be on microscopic features, such as spore walls or hyphal walls, or the apical apparatus or entire ascus wall of an ascus, or be a macroscopic reaction on tissue where a drop of the reagent is applied. Negative reactions, called inamyloid or nonamyloid, are for structures that remain pale yellow-brown or clear. A reaction producing a deep reddish to reddish-brown staining is either termed a dextrinoid reaction (pseudoamyloid is a synonym) or a hemiamyloid reaction. Melzer's reagent reactions Hemiamyloidity Hemiamyloidity in mycology refers to a special case of cell wall amyloidity where the blue staining by iodine only occurs when the tissue was pretreated with potassium hydroxide solution (KOH) or other strong bases, whereas direct application of iodine causes a red reaction when using Lugol's solution, but no reaction when using Melzer's reagent. Hemiamyloidity is so far only known in Ascomycota, but here widespread and an important taxonomic distinction criterion. If cell walls stain blue by iodine reagents without pretreatment with KOH, this is called euamyloid. The term amyloid comprises both variants. Properties A hemiamyloid element of the cell wall does not directly stain blue with iodine reagents added to a water preparation, but only when it has been pretreated with potassium hydroxide solution (KOH). Without KOH pretreatment, the result depends much on the type of iodine reagent: with Lugol's solution (IKI), hemiamyloid structures react red to reddish-brown, whereas any reaction is suppressed when using Melzer's reagent (MLZ). This masking effect (false inamyloidity) is due to the high chloral hydrate concentration in MLZ. The alternative to hemiamyloid is called euamyloid. Euamyloid and KOH-pretreated hemiamyloid structures react blue regardless of the type of iodine reagent. Hemiamyloid and euamyloid reactions may occur at a time, either at spatially separated sites of the cell wall (e.g., ascus apical ring euamyloid, lateral wall hemiamyloid), or as an intermediate type of the same wall region. In the latter case, an overlay of blue and red can be observed in Lugol's solution without KOH