REPRODUCTION IN DISCOLICHENS 171 



till a later stage. By continual growth in thickness of the overlying 

 thallus, the apothecia gradually become submerged and tend to degenerate; 

 meanwhile, however, a branch from the ascogonial hyphae at the base of 

 the hymenium pushes up along one side and forms a secondary ascogonial 

 cell-plexus over the top of the first-formed disc. A new apothecium thus 

 arises and remains sporiferous until it also comes to lie in too deep a position, 

 when the process is repeated. Sometimes the regenerating hypha travels to 

 the right or left away from the original apothecium, it may be to a distance 

 of 2 mm. or according to Fiinfstuck even considerably farther. Funfstiick 1 has 

 gathered indeed from his own investigations that such cases of regeneration 

 are by no means rare : ascogenous hyphae, several centimetres long, destined 

 to give rise to new apothecia are not unusual, and their activity can be recog- 



Fig. 97. Rhizocarpon petraeum Massal. Concentrically arranged apothecia, reduced 



(J. Adams, Photo.}. 



nized macroscopically by the linear arrangement of the apothecia in such 

 lichens as Rhizocarpon (petraeum} concentricum (Fig. 97). 



In Variolaria, a genus closely allied to or generally included in Per- 

 tusaria, Darbishire 2 has described the primordial tissue as taking rise almost 

 at the base of the crustaceous thallus: strands of delicate hyphae, staining 



1 Flinfstiick 1902. 2 Darbishire 1897. 



