48 



POPULAR HISTORY OP LICHENS. 



the centre consists of a white, cottony, spongy, medullary 

 tissue, between which and the cortical layer there may be 

 seen a thin layer of green gonidia. This colouring matter 

 is in many species easily extracted by boiling w 7 ater, and 

 other solvents, and has been abundantly applied by the 

 peasantry of Scotland, and other countries, in their house- 

 hold arts, and especially to the dyeing of home-spun vest- 

 ments. In the majority of Lichens, from the thinness and 

 comparative transparency of the cortical tissue, the bright 

 green colour of the subjacent gonidic layer plays an impor- 

 tant part in the production of the colour of the thallus. 

 When a Lichen is steeped in water or moistened, the trans- 

 parency of the cortical tissue is greatly increased, and if 

 previously of a pale colour, the green of the gonidic layer 

 now shines through it with almost undiminished intensity. 

 This sufficiently explains the circumstance that most Lichens, 

 when moistened by rain or when growing in damp situa- 

 tions, have much more of a green tint than under opposite 

 conditions. It may be laid down as a general proposition, 

 that whatever be the colour of the cortical layer, that of the 

 gonidic stratum is normally and always green. Changes in 

 colour are greatly under the influence of light, as well as 

 moisture. In the vertical thallus, which is equally exposed 



