GENERAL CHARACTERS OE BRITISH LICHENS. 47 



stitute the plant. An examination of the pulverulent or 

 rudimentary thallus of many common Cladonias and Le- 

 cideas will prove it to be composed of the same elements. 

 In this light also we must regard the old genus Zepraria as 

 a persistent hypothalline type or abortive condition of the 

 thallus of various familiar species of the genera just men- 

 tioned. In some foliaceous species, the hypothallus becomes 

 developed into fixum, which are composed of bundles of 

 filaments having the same characters as those described 

 under the head of the medullary layer of the thallus ; in some 

 crustaceous species it appears to form the adnate base be- 

 fore alluded to. 



The colour of the Lichen-thallus is as varied as its form, 

 and as subject to alteration by external circumstances, 

 terrestrial and aeriaL It is generally greenish, greyish, or 

 brownish; frequently also whitish, yellowish, reddish, and 

 blackish ; or it possesses various shades and combinations of 

 these colours. The colouring matters on which these depend 

 are confined to the cortical layer of the thallus. Of this 

 the student may convince himself by tearing across any 

 very dark-coloured thallus, such as the bronze-coloured or 

 almost pitchy thallus of Parmelia Fahlunensis, the external 

 or cortical layer of which is of a deep brown colour, while 



