246 PHYSIOLOGY 



hue owing to the purple colour of the gonidial Nostoc cells. The dull-grey 

 crustaceous thallus of the Pannariaceae becomes more or less blue-green 

 when moistened, and the same change has been observed in the Hymeno- 

 lichens, Cora, etc. 



In Coenogonium, the alga is some species of Trentepoklia, a filamentous 

 genus mostly yellow, which often gives its colour to the slender lichen 

 filaments, the covering hyphae being very scanty. Other filamentous species, 

 such as Usnea barbata, etc., are persistently greenish from the bright-green 

 Protococcaceous cells lying near the surface of the thalline strands. Many 

 of the furfuraceous lichens are greenish from the same cause, especially when 

 moist, as are also the larger lichens, Physcia ciliaris, Stereocaulons, Cladonias 

 and others. 



b. Colour due to Lichen-Acids. These substances, so characteristic 

 of lichens, are excreted from the hyphae, and lie in crystals on the outer 

 walls; they are generally most plentiful on exposed tissues such as the 

 cortex of the upper surface or the discs of the apothecia. Many of these 

 crystals are colourless and are without visible effect, except in sometimes 

 whitening the surface, strikingly exemplified in Thawinolia vermicularis^ ; 

 but others are very brightly coloured. These latter belong to two chemical 

 groups and are found in widely separated lichens" : 



1 . Derivatives of pulvinic acid which are usually of a bright-yellow colour. 

 They are the colouring substance of Letharia vulpina, a northern species, not 

 found in our islands, of Cetraria pinastri and C. juniperina" which inhabit 

 mountainous or hilly regions. The crustaceous species, Lecidea lucida and 

 Rhizocarpon geographicum, owe their colour to rhizocarpic acid. 



The brilliant yellow of the crusts of some species of Caliciaceae is due to 

 the presence of the substance calycin, while coniocybic acid gives the greenish 

 sulphur-yellow hue to Coniocybe furfuracea. Epanorin colours the hyphae 

 and soredia of Lecanora epanora a citrine-yellow and stictaurin is the deep- 

 yellow substance found in the medulla and under surface of Sticta aurata 

 and S. crocata. 



2. The second series of yellow acids are derivatives of anthracene. They 

 include parietin, formerly described as chrysophanic acid, which gives the 

 conspicuous colour to Xantlioriae and to various wall lichens; solorinic acid, 

 the crystals of which cover the medullary hyphae and give a reddish-grey 

 tone to the upper cortex of Solorina crocea, and nephromin which similarly 

 colours the medulla of Nephromium lusitanicum a deep yellow, the colour of 

 the general thallus being, however, scarcely affected. In this group must 

 also be included the acids that cause the yellow colouring of the medulla in 

 Parmelia subaurifera and the yellowish thallus of some Pertusariae. 



1 Zopf 1893. 2 Zopf 1907. ' Zopf 1892. 



