COLOUR OF LICHENS 247 



In many cases, changes in the normal colouring 1 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 2 , 

 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 para- 

 physes. 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 



1 Knop 1872. 2 Bachmann 1890. 



