ILLUMINATION OF LICHENS 241 



Wiesner 1 made a series of exact observations on what he has termed 

 the " light-use " of various plants. He took as his standard of unity for the 

 higher plants the amount of light required to darken photographic paper in 

 one second. When dealing with lichens he adopted a more arbitrary standard, 

 calculating as the unit the average amount of light that lichens would receive 

 in entirely unshaded positions. He does not take account of the strength or 

 duration of the light, and the conclusions he draws, though interesting and 

 instructive, are only comparative. 



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 | of 

 full illumination. Dermatocarpon miniatum, a rock lichen with a peltate 

 foliose thallus, is at its best from to ^ of illumination, but it grows well in 

 situations where the light varies in amount from I to -fa. Psora (Lecided) 

 lurida, with dark-coloured crowded squamules, grows on calcareous soil 

 among rocks well exposed to the sun and has an illumination from I to -fa, 

 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 I 

 to \\ in more shady situations, where the light had declined to -$, 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 i to y 1 ^, though they are also found not unfrequently in habitats 

 where the light cannot be more than ^. Among these light-loving plants 

 are the Northern Tundra species of Cladonia, Stereocaulon, Cetraria, Par- 

 melia, Umbilicaria, and Gyrophora, as also Xanthoria partetina, Placodtum 

 elegans, P. murorum, etc., with some cruslaceous species such as Lecanora 

 atra, Haematotnma ventosum, Diploschistes scruposus, many species of Leci- 

 deaceae, 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 islatidica for instance in an open 

 situation is darker than when growing in woods ; C. aculeata on bare sand- 



1 Wiesner 1895. 

 S. L. l6 



