65 



As regards the epithecium, thecium and hypothecium there is 

 great uniformity in all apothecia, but as regards the remaining struc- 

 tures there is a great difference met with among the various lichen- 

 groups. The appearance also varies greatly according to the por- 

 tion of the apothecium examined. If we examine a vertical section 

 through the free portion of the cup of an apothecium (of Parmelia, 

 for example) we shall find the following additional tissue-layers to be 

 described : 



(d) Upper Algal Lav cr . 

 (c) Medullary Layer. 

 ( /") Lower Algal La\er. 

 (&) Cortical Layer. 



(d) The Upper Algal La ver. 



This layer occurs immediately below the hypothecium. In gen- 

 eral it corresponds to the algal layer of the thallus ; the algae are, 

 however, less numerous and decrease in number from the periphery 

 of the apothecium toward the middle ; they are usually entirely 

 wanting below the middle of the hypothecium ; this is explained by 

 the fact that at this point they are too far removed from sunlight. 



(e) The Medullary Layer. 



This corresponds in all respects to the medullary layer of the 

 thallus. It consists of a network of hyphae which usually extend 

 from the periphery to the middle of the point of attachment ; near 

 this point they extend downward toward the medullary layer of the 

 thallus. 



(_/") The Lower Algal Layer. 



This resembles the upper algal layer, but, as a rule, contains 

 more algae. THe relative number of algae in the two layers depends 

 upon the inclination of the sides of the cup to the surface of the thal- 

 lus : if this angle is large the lower algal layer will contain by far 

 the more algae ; if the angle is small the relative difference will be 

 greater. This layer is continuous with the algal layer of the thallus. 



(g') The Cortical Layer. 



This layer is a direct continuation of the upper cortical layer of 

 the thallus ; it may, however, differ considerably from this in thick- 



