GENERAL CHARACTERS OF BRITISH LICHENS. 55 



getate in a moist or aqueous medium ; and Fries, no mean 

 authority, defines Lichens to be "Algce born in the air, in- 

 terrupted in their development by the deficiency of water, 

 and stimulated into forming a nucleus by light." Excess 

 of moisture seems decidedly prejudicial to Lichen develop- 

 ment ; it produces a sterile state of the thallus, and favours 

 the production of a pulverulent or persistent hypothalline 

 type. A knowledge of this fact has enabled some authors 

 to cause sterile species to become fertile, by merely altering 

 their habitat. No species grows normally under water, or 

 in habitats devoid of light, though apparent exceptions have 

 occasionally been described. The nature of the habitat fre- 

 quently exercises a powerful influence, not only by means 

 of the organic or inorganic materials which the surface 

 whereon a Lichen grows supplies to the building up of 

 its thallus, that is, by means of its chemical characters ; but 

 sometimes on account of its physical structure, such as its 

 capability to absorb or retain heat and moisture. To this 

 subject we shall more fully recur under the head of Geogra- 

 phical Distribution. Season and the vicissitudes of climate 

 are not without their effect, if we may judge by the changes 

 which they appear to produce in the chemical composition 

 or products of certain species. It is well known that many 



