209 



5. Spores; a, gelatinous exosporium. 



6. Algae; a, impoverished chain from the upper portion of thallus; b, 

 normal free chain; c and d, colonies of algae enclosed by the common cov- 

 ering or membrane. 



9. Peltigera Willd. Fl. Berol. 347. 1787. 



Although the representatives of this genus are distinctly foliose, 

 yet they have characters not occurring in the great majority of foli- 

 ose lichens heretofore discussed. One of these characteristic fea- 

 tures is the total absence of the lower cortical layer; there is not 

 ■even any indication of a line of demarcation between the medullary 

 layer and the rhizoidal structure. The thallus itself is large, com- 

 paratively taick and rigid, but not brittle, as in Collema; it is typic- 

 ally dorsiventral in structure and lies flat upon the substratum, to 

 which it is rather loosely attached ; it is simple, with marginal lobes 

 upon which the apothecia are borne ; the color above is blue-green 

 which changes to brown ; the lower surface is light brown. 



The structural features are clearly defined ; the upper cortical 

 layer is well developed ; its surface is roughened by numerous 

 •short hyphae which are structurally and functionally comparable to 

 ■certain trichomes of higher plants ; they also serve to retain the sore- 

 ■dia of Peltigera and other lichens (St/ctinas)where these develop into 

 warty structures (P. afhthosa) . These trichomatic hyphal cells dif- 

 fer from the hyphae of the medullary layer in the greater thickness of 

 their walls and the shortness of the cells. The cells of the cortical 

 layer are closely united, the lumina large, the walls firm and non- 

 gelatinous. 



The algal layer is well developed, indicating a high assimilative 

 function; the algae are Poly coccus -punctiformis ; the colonies are 

 small and the chains scarcely discernible ; the individual cells are 

 ■considerably larger than the normal. 



The medullary layer consists of a rather loose network of hyphae 

 as in the majority of lichens ; it usually contains a large quantity of 

 air ; below the medullary layer is another hyphal layer in which the 

 majority of the hyphae extend horizontally in the direction of growth ; 

 the cells of this layer are larger and walls thicker than in the medul- 

 lary layer ; it contains but little air, and it no doubt forms a me- 

 chanical as well as a protective tissue. An additional mechanical 

 .support is furnished by the numerous hyphal bundles occurring 



