SECT. IT. 



HORIZONTAL LICHENS. 



299 



on their summit. The asci with their paraphyses in- 

 closed in vessels of various shapes, called perithecia, are 

 aggregated in discs or shields, which form projections 

 on the surface of the plant. Some of these discs are 

 closed, and give egress to the spores through a fracture 

 or pore on their surface ; others are open cups of various 

 forms, either with stalks or sessile on the frond, and 

 through these the spores have egress. Hence the whole 

 order of lichens is naturally divided into two groups, 

 according as their discs are open (Gymnocarpei), or 

 closed (Angiocarpei), the first being incomparably the 

 most important. Fig. 37 shows open, and sections of 

 closed cups, or perithecia. 



Fig. 37. Lichens : a, Trypethelium Sprengelii, pustules, with sporidia ; b, Verrncaria 

 variolosa, section of perithecium with sporidium ; c, Endocarpon lacteum, thallus 

 with section and fruit ; d, Stegobolus Berkeleianus, portion of plant with ascus, 

 and sporidia. 



The highest type of horizontal lichens, of which 

 fig. 37 b is a perpendicular section, has a firm, spread- 

 ing, superficial crust or surface, formed of oblong- 

 coloured cells, or of coloured filaments closely aggregated, 

 and which covers two distinct layers of cellular tissue. 

 In the layer immediately below the surface, the cells 

 are globular and of a paler colour ; the second layer, or 



