54 DR. LAUDER LINDSAY ON CHEMICAL REACTION 



other exceptions to the general rule as regards the yellow Phy- 



scicB and Placodia, 



On the other hand, Nylander remarks on the absence of the 

 reaction in the yellow Platysmata, But to this generalization 

 also there are exceptions. In specimens of P. nivale from 

 Braemar (1855), I found the deep orange (natural) stains at the 

 base of the plant at once become red with potash, while in 

 Norwegian forms (1857) the normal yellow of the thallus exhi- 

 bited the same reaction. No effect, however, was produced on 

 the purple stains at the base of P. cucullatunij or on the normal 

 yellow of its thallus. If we are to trust the chrysophanlc test, 

 we must come to the conclusion that there are various yellow 

 colouring-matters in lichens, many of them having, however, the 

 same tint; for the reaction is absent in the beautiful yellow 

 thalli of Evernia vulpina^ Sticfa aurata, Lecanora cTiloropJiana 

 and oreina, X. vitellina (as a general rule), Lecidea galbulay 

 geograpTiica^ and citrinella. On the other hand, the test in ques- 

 tion associates PJiyscia parietina and other PTiyscice with Pla- 

 codium aureum, elegans and murorum, and other Placodia, Leca- 

 nora carina^ Jusco4utea, aurantiaca, and other Lecanorcdy Xe- 



4 



cidea ferruginea and other Lecidece. In this group the magni- 

 ficent crimson developed is equally intense on the apothecia and 

 thallus ; but the disk of the apothecium has sometimes a natural 

 crimson colour instead of its usual yellow, e.g. in PI. murorum or 

 X. aurantiaca. 



B. 



(f^ 



ally termed \A\e green-red sub-reaction) by its permanence [accord- 

 ingto Nylander]. — The typical beautiful lemon- (greenish) yellow is 

 best exhibited on thalli which are pale or white, and in proportion 

 to their whiteness. Thus the reaction is most vivid and con- 

 spicuous on the white thallus of various Physcice [stellaris, ccesia, 

 a8troided\y Lecanora [Seuteriy glaucoma^ tartar ea^^ Lecidece [ca- 

 nescens\'Contigtia]j Perticsaria, Phlyctis^ Lecanactis^ ArtTionia^ and 

 Stereocaulon. It is thus developed equally in the foliaceous and 

 fruticulose thallus, and in that which is crustaceous, especially 

 when it is thick and tartareous, and grey or white, in saxicolous 

 species. The intensity and character of the colour vary greatly. 

 Thus in Lecanactis illecebrosa it is less vivid or beautiful than in 

 Arthonia pruinosa j in the saxicolous Lecanorce and Lecidecd it is 

 frequently olive-green; in StereocauloUy where any reaction is 

 visible at all, it is brightest where the thallus is palest and most 



