STRUCTURES PECULIAR TO LICHENS 



Lecidea pelobotrya, with Stigonema or Nostoc; 



Pilophorus robustus, with Gloeocapsa, Stigonema, or Nostoc. 



137 



Fig. 77. Lecanora gelida Ach.. a, lobate cephalodia 

 X 11 (after Zopf). 



Riddle^ has employed cephalodia with their enclosed algae as diagnostic 

 characters in the genus Stereocaulon. When the alga is Stigonema, as in 

 v?. paschale, etc., the cephalodia are generally very conspicuous, grey in 

 colour, spherical, wrinkled or folded, though sometimes black and fibrillose 

 (S. denudatum). Those containing Nostoc are, on the contrary, minute and 

 are coloured verdigris-green (^S. tonientosum and 5'. alpinum). 



Instances are recorded of algal colonies adhering to, and even penetrating, 

 the thallus of lichens, but as they never enter into relationship with the 

 lichen hyphae, they are antagonistic rather than symbiotic and have no 

 relation to cephalodia. 



D. Development of Cephalodia 



a. ECTOTROPHIC. Among the most familiar examples of external cepha- 

 lodia are the small rather dark-coloured warts or swellings that are scattered 

 irregularly over the surface of Peltigera {Peltidea) aphthosa. This lichen has 

 a grey foliose thallus of rather large sparingly divided lobes; it spreads 

 about a hand-breadth or more over the surface of the ground in moist up- 

 land localities. The specific name " aphthosa " was given by Linnaeus to 



' Riddle 1910. 



