140 



and several other species of algae occur in and upon the thallus, espec- 

 ially in the lower forms. The apothecia are discoid, from minute to 

 medium in size, with a flattened disk ; the epithecium and upper ends 

 of the simple paraphyses are dark ; the hypothecium is non-cortical and 

 usually colorless ; the spores are quite large, multilocular, oblong-el- 

 liptical or nearly spindle-shaped and usually colorless. It is, how- 

 ever, probable that the hypothecium as well as the spores are dark 

 colored in some species. 



The Gyalectas are quite generally distributed ; they occur upon 

 rocks and trees. 



PLATE 15. fig. 6. 

 Gyalecta. 



Paraphyses and spore-sac with spores. 



14. Psora Hoffm. Flora Deutsch. Crypt. 161. 1795. 



This is the first group of lichens belonging to the Lecideaceae in 

 in which the thallus is typically foliose. The upper cortical tissue is 

 comparatively thick and consists of two layers ; the upper, which 

 may be designated as the dermal, consists of hyphal cells lying 

 horizontally, with enormously thickened gelatinous cell-walls and 

 much reduced cell-lumina ; the layer below this consists of nearly 

 normal cortical cells extending vertically, differing from the normal 

 ones in that their cell-walls are thicker (more gelatinous), while the 

 cell-lumina are much reduced. 



The algal layer consists of the alga Cystococcus humicola en- 

 closed by a network of ordinary hyphae instead of typical haustoria 

 so that the hyphal tissue has the semblance of being cortical. The 

 medullary tissue consists of a comparatively close network of hyphae. 

 There is no lower cortical tissue, but the terminal hyphal branches 

 are perhaps somewhat more crowded and show a tendency to ar- 

 range themselves vertically to the surface. 



The color of the thallus is quite variable ; the upper surface may 

 vary from light grey to black with the lower surface lighter ; in some 

 species the margin is almost white. In the dark thallus the lower 

 surface is practically of the same color as the upper. The thallus- 

 lobes may be somewhat ascending or closely adnate to the sub- 

 stratum. 



The apothecia are small to medium, distributed over the thallus, 

 but sometimes near the margin ; they are usually disk-like but more 



