In many cases, changes in the normal colouring[887] are caused by the breaking up of the acids on contact with atmospheric or soil ammonia. Alkaline salts are thus formed which may be oxidized by the oxygen in the air to yellow, red, brown, violet-brown or even to entirely black humus-like products which are insoluble in water. These latter substances are frequently to be found at the base of shrubby lichens or on the under surface of leafy forms that are closely appressed to the substratum. _c._ COLOUR DUE TO AMORPHOUS SUBSTANCES. These are the various pigments which are deposited in the cell-walls of the hyphae. The only instance, so far as is known, of colours within the cell occurs in _Baeomyces roseus_, in which species the apothecia owe their rose-colour to oil-drops in the cells of the paraphyses, and in _Lecidea coarctata_ where the spores are rose-coloured when young. In a few instances the colouring matter is excreted (_Arthonia gregaria_ and _Diploschistes ocellatus_); but Bachmann[888], who has made an extended study of this subject and has examined 120 widely diversified lichens, found that with few exceptions the pigment was in the membranes. Bachmann was unable to determine whether the pigments were laid down by the protoplasm or were due to changes in the cell-wall. The middle layer, he found, was generally more deeply coloured than the inner one, though that was not universal. In other cases the outer sheath was the darkest, especially in cortices one to two cells thick such as those of _Parmelia olivacea_, _P. fuliginosa_ and _P. revoluta_, and in the brown thick-walled spores of _Physcia stellaris_ and of _Rhizocarpon geographicum_. Still another variation occurs in _Parmelia tristis_ in which the dark cortical cells show an outer colourless membrane over the inner dark wall. The coloured pigments are mainly to be found in the superficial tissues, but if the thallus is split by areolation, as in crustaceous lichens, the internal hyphae may be coloured like those of the outer cortex wherever they are exposed. The hyphae of the gonidial layer are persistently colourless, but the lower surface and the rhizoids of many foliose lichens are frequently very deeply stained, as are the hypothalli of crustaceous species. The fruiting bodies in many different families of lichens have dark coloured discs owing to the abundance of dark-brown pigment in the paraphyses. In these the walls, as determined by Bachmann, are composed generally of an inner wall, a second outer wall, and the outermost sheath which forms the middle lamella between adjacent cells. In some species the second wall is pigmented, in others the middle lamella is the one deeply coloured. The hymenium of many apothecia and the hyphae forming the amphithecium are often deeply impregnated with colour. The wall hyphae of the pycnidia are also coloured in some forms; more frequently the cells round the opening pore are more or less brown.