53 



tion to them. He described them as " white circular depressions," 

 which was about all that was known concerning these structures at 



O 



the time. Acharius was the first to introduce the term cyphellae. 

 In spite of the fact that these formations have been observed for 

 more than one hundred and twenty years, little is known of their 

 origin and development, and still less of their function. 



The cyphellae occur almost exclusively in the genera Sticta and 

 Stictina ; in position they are limited to the lower surface of the thal- 

 lus. Structurally they are circular breaks in the loAver cortical layer, 

 filled in by a secondary tissue formation. They begin their devel- 

 opment above a breathing pore in the lower cortical layer. That 

 is, the pore, which is simply an intercellular canal, is first formed 

 and becomes enlarged by the separation of the cells. While this 

 pore is yet very minute, the secondary tissue or cortex begins to 

 develop in its immediate vicinity. The hyphae of the medullary 

 layer give off a number of nearly spherical or short branches which 

 close the enlarging pore-opening ; the formation of the secondary 

 cortical tissue keeps pace with the enlargement of the opening in 

 the primary cortical tissue. 



As to structure, two forms of cyphellae may be recognized. 

 Form i is seen in Stictina damaecornis. The cyphellae appear as 

 circular depressions, concave inward, the edge of the primary cor- 

 tex forming a constricted outer margin, which projects somewhat be- 

 yond the surface of the cortical layer. The cells of the secondary 

 cortical formation are neaVly spherical, loosely united, leaving nu- 

 merous intercellular spaces ; they extend in a direction at right 

 angles to the outer surface. In thickness this scarcely equals the pri- 

 mary cortex. Its outer surface is usually smooth, devoid of rhizoids 

 and paler in color than the primary cortex. The cell-walls are quite 

 thin, usually more so than those of the medullary cells. (/Y. 2. 



/') 



Form 2 occurs in the majority of Stictas and Stictinas. It differs 



from the former in several respects. The secondary tissue consists 

 of a dense network of short branching hyphae whose longitudinal 

 axes extend at right angles to the surface of the cyphella. Instead 

 of a depression, as in the former case, there is usually a protrusion of 

 the secondary tissue. The margin of the primary coxtex also 

 projects somewhat, but its outline is less regularly circular. (/Y. 2, 



