An example of colour-change due directly to light influences is given by Bitter[872]. He noted that the thallus of _Parmelia obscurata_ on pine trees, and therefore subject only to diffuse light, grew to a large size and was of a light greyish-green colour marked by lighter-coloured lines, the more exposed lobes being always the most deeply tinted. In a less shaded habitat or in full sunlight the lichen was distinguished by a much darker colour, and the lobes were seamed and marked by blackish lines and spots. Bruce Fink[873] noted a similar development of dark lines on the thallus of certain rock lichens growing in the desert, more especially on _Parmelia conspersa_, _Acarospora xanthophana_ and _Lecanora muralis_. He attributes a protective function to the dark colour and observes that it seemingly spreads from centres of continued exposure, and is thus more abundant in older parts of the thallus. He contrasts this colouration with the browning of the tips of the fronds of fruticose lichens by which the delicate growing hyphae are protected from intense light. Galløe[874] finds that protection against too strong illumination is afforded both by white and dark colourations, the latter because the pigments catch the light rays, the former because it throws them back. The white colour is also often due to interspaces filled with air which prevent the penetration of the heat rays. A deepening of colour due to light effect often visible on exposed rock lichens such as _Parmelia saxatilis_ is more pronounced still in Alpine and tropical species: the cortex becomes thicker and more opaque through the cuticularizing and browning of the hyphal membranes, and the massing of crystals on the lighted areas. The gonidial layer becomes, in consequence, more reduced, and may disappear altogether. Zukal[875] found instances of this in species of _Cladonia_, _Parmelia_, _Roccella_, etc. The thickened cortex acts also as a check to transpiration and is characteristic of desert species exposed to strong light and a dry atmosphere. Bitter[876] remarked the same difference of development in plants of _Parmelia physodes_: he found that the better lighted had a thicker cortex, about 20-30 µ in depth, as compared with 15-22 µ or even only 12 µ in the greener shade-plants, and also that there was a greater deposit of acids in the more highly illuminated cortices, thus giving rise to the deeper shades of colour. Many lichens owe their bright tints to the presence of coloured lichen-acids, the production of which is strongly influenced by light and by clear air. _Xanthoria parietina_ becomes a brilliant yellow in the sunlight: in the shade it assumes a grey-green hue and yields only small quantities of parietin. _Placodium elegans_, normally a brightly coloured yellow lichen, becomes, in the strong light of the high Alps, a deep orange-red. _Rhizocarpon geographicum_ is a vivid citrine-yellow on high mountains, but is almost green at lesser elevations.