=(2) Substances with little or no chemical reaction.= There is only one such to be noted: “Parmelia-brown,” usually a very dark pigment, which is lodged in the outer membranes of the cells. It becomes a clearer colour with nitric acid, and if the reagent be sufficiently concentrated, some of the pigment is dissolved out. Some tissues, such as the lower cortex of some _Parmeliae_, may be so impregnated and hardened, that nothing short of boiling acid has any effect on the cells; membranes less deeply coloured and changed, such as the cortex of the _Gyrophorae_, become disintegrated with such drastic treatment. With potash the colour becomes darker, changing from a clear brown to olivaceous-brown or-green, or in some cases, as in a more faintly coloured epithecium, to a dirty-yellow, but the lighter colour produced there is largely due to the swelling up of the underlying tissues to which the potash penetrates readily between the paraphyses. “Parmelia-brown” is a colouring substance present in the dark epithecium and hypothecium of the fruits of many widely diverse lichens, and in the cortical cells and rhizoids of many thalli. In some plants the thallus is brown both above and below, in others, as in _Parmelia revoluta_, etc. only the under surface is dark-coloured. _e._ COLOUR DUE TO INFILTRATION. There are several crustaceous lichens that are rusty-red, the colour being due to the presence of iron. These lichens occur on siliceous rocks of gneiss, granite, etc., and more especially on rocks rich in iron. Iron as a constituent of lichens was first demonstrated by John[892] in _Ramalina fraxinea_ and _R. calicaris_. Grimbel[893] proved that the colour of rust lichens was due to an iron salt, and Molisch[894] by microscopic examination located minute granules of ferrous oxide as incrustations on the hyphae of the upper surface of the thallus. Molisch held that the rhizoids or penetrating hyphae dissolved the iron from the rocks by acid secretions. Rust lichens however grow on rocks that are frequently under water in which the iron is already present. Among “rusty” lichens are the British forms, _Lecanora lacustris_, the thallus of which is normally white, though generally more or less tinged with iron; it inhabits rocks liable to inundation. _L. Dicksonii_ owes its ferruginous colour to the same influences. _Lecidea contigua_ var. _flavicunda_ and _L. confluens_ f. _oxydata_ are rusty conditions of whitish-grey lichens. Nilson[895] found rusty lichens occurring frequently in the Sarak-Gebirge, more especially on glacier moraines where they were liable, even when uncovered by snow, to be flooded by water from the higher reaches. It is the thallus that is affected by the iron, rarely if ever are apothecia altered in colour. BACHMANN’S PIGMENT REACTIONS