In _Coenogonium_, the alga is some species of _Trentepohlia_, 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 _Thamnolia vermicularis_[884]; but others are very brightly coloured. These latter belong to two chemical groups and are found in widely separated lichens[885]: 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_[886] 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 _Xanthoriae_ 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_.