242 PHYSIOLOGY 



dunes is a deeper shade of brown than when growing entangled among 

 heath plants. Parmelia saxatilis when growing on exposed rocks is fre- 

 quently a deep brown colour, while on shaded trees it is normally a light 

 bluish-grey. 



An example of colour-change due directly to light influences is given by 

 Bitter 1 . 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 2 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 muralts. He attributes a protective function to 

 the dark colour and observes that it seemingly spreads from centres of con- 

 tinued 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. 



Gallic 3 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 4 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 5 remarked the same difference of development in plants of Parmelia 

 physodes\ he found that the better lighted had a thicker cortex, about 20- 

 30 n in depth, as compared with 15-22/4 or even only 12/x 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 pariitina becomes a brilliant yellow in the sunlight: in the 



1 Bitter 1901, p. 465. * Fink 1909. 3 Gallic 1908. * Zukal 1896. B Bitter 1901. 



