_a._ SUN LICHENS. The illumination of the Tundra lichens is reckoned by Wiesner as representing his unit of standard illumination. In the same category as these are included many of our most familiar lichens, which grow on rocks subject to the direct incidence of the sun’s rays, such as, for instance, _Parmelia conspersa_, _P. prolixa_, etc. _Physcia tenella_ (_hispida_) is also extremely dependent on light, and was never found by Wiesner under 1/8 of full illumination. _Dermatocarpon miniatum_, a rock lichen with a peltate foliose thallus, is at its best from 1/3 to 1/8 of illumination, but it grows well in situations where the light varies in amount from 1 to 1/24. _Psora_ (_Lecidea_) _lurida_, with dark-coloured crowded squamules, grows on calcareous soil among rocks well exposed to the sun and has an illumination from 1 to 1/30, but with a poorer development at the lower figure. Many crustaceous rock lichens are also by preference sun-plants as, for instance, _Verrucaria calciseda_ which grows immersed in calcareous rocks but with an illumination of 1 to 1/3; in more shady situations, where the light had declined to 1/29, it was found to be less luxuriant and less healthy. Sun lichens continue to grow in the shade, but the thallus is then reduced and the plant is sterile. Zukal has made a list of those which grow best with a light-use of 1 to 1/10, though they are also found not unfrequently in habitats where the light cannot be more than 1/50. Among these light-loving plants are the Northern Tundra species of _Cladonia_, _Stereocaulon_, _Cetraria_, _Parmelia_, _Umbilicaria_, and _Gyrophora_, as also _Xanthoria parietina_, _Placodium elegans_, _P. murorum_, etc., with some crustaceous species such as _Lecanora atra_, _Haematomma ventosum_, _Diploschistes scruposus_, many species of Lecideaceae, some Collemaceae and some Pyrenolichens. Wiesner’s conclusion is that the need of light increases with the lowering of the temperature, and that full illumination is of still more importance in the life of the plants when they grow in cold regions and are deprived of warmth: sun lichens are, therefore, to be looked for in northern or Alpine regions rather than in the tropics. _b._ COLOUR-CHANGES DUE TO LIGHT. Lichens growing in full sunlight frequently take on a darker hue. _Cetraria islandica_ for instance in an open situation is darker than when growing in woods; _C. aculeata_ on bare sand-dunes is a deeper shade of brown than when growing entangled among heath plants. _Parmelia saxatilis_ when growing on exposed rocks is frequently a deep brown colour, while on shaded trees it is normally a light bluish-grey.