^6 



the structural differences of the thallus may therefore be of signifi- 

 cance in separating them. 



The representatives of this genus are also mostly northern, oc- 

 curring upon trees, rocks and fences. Most of the American species 

 occur north and west of our territory. 



PLATE 49. 



' Bryopogon Fremontii (Tuck.) 



1. Portion of thallus natural size. 



2. Portion of branch with apothecium magnified. 



3. Section of apthecium. 



4. Radial transverse section of thallus. 



a, outer protective and mechanical tissue of longitudinal hyphae ; b r 

 medullary tissue; c, mechanical bundles of longitudinal hyphae. 



5. Paraphysis and spore-sac. 



6. Spores. 



12. Usnea Adans. FAm. PL 2: 7. 1763. 



The Usneas are at once recognized by the well developed central 

 mechanical bundle of hyphae ; they represent the acme of develop- 

 ment in the fruticose thalli, both as to size and functional as well as 

 morphological specialization. 



The fruticose thallus is frequently branched, cylindrical, without 

 being flattened at the points of branching. The branches usually 

 form an angle approaching 90 . A section reveals the following 

 structures which are quite constant in all the representatives of the 

 genus. An outer semicortical tissue, whose exact structure is rather 

 difficult to make out, owing to the fact that cell-walls are quite indis- 

 tinct ; it consists of agglutinate, somewhat cortical hyphal cells, 

 which for the most part extend vertical to the surface ; it is practically 

 impossible to determine the exact outline of the individual cells, and 

 the difficulty is increased by the presence of acid crystals deposited 

 upon the cell-walls, particularly toward the outer surface. This 

 layer is followed by the algal layer, which consists of groups of algae, 

 held together by the haustorial branches; as xnAJectoria and Bryo- 

 pogon the algae are comparatively few in number. The third layer 

 constitutes the medullary tissue which consists of a loose network of 

 hyphae. In the middle occurs the mechanical bundle of elongated 

 hyphae, which is sometimes separated from the medullary layer by 



