the mechanical tissue is arranged on the plan of a hollow cylinder, 

 with a secondary adaptation to resist longitudinal tensions. The 

 structure is comparatively simple and is essentially as follows : The 

 outer mechanical tissue consists of a continuous layer of closely ag- 

 glutinate hyphae extending in a longitudinal direction ; occasional 

 branches are irregularly interwoven between these ; the cells of 

 the hyphae situated toward the outer surface tend to become cortical 

 in structure, and are often more or less colored, even becoming 

 entirely black ; deposits of acid crystals are very plentiful through- 

 out the entire hyphal structure ; this mechanical layer is quite 

 thick, forming by far the greatest bulk of the substance ; the hyphal 

 walls are very thick, gelatinous and become brittle when dry ; the 

 algal layer which follows next is very deficient, consisting of 

 rather minute, sparingly scattered clusters of algae suspended in a 

 very loose and deficient medullary tissue. The interior is not hollow, 

 though the hyphal tissue is sparingly developed. 



In A. sarmentosa the entire thallus bears oval soredial patches 

 which apparantly serve a two-fold purpose, that of supplying a 

 means of vegetative propagation and of admitting air into the in- 

 terior. 



The color of the thallus varies from light to black. This varia- 

 tion may be observed in the same plant at different periods of its de- 

 velopment (A. sarmentosa}. 



The apothecia are few, of medium size, sessile upon the sides of 

 the thallus. The hypothecium is colorless and consists of two dis- 

 tinct layers of semicortical tissue ; sometimes a third intermediary 

 layer is noticeable, forming a sort of transition layer between the 

 upper and lower layers. The spore-sacs and simple paraphyses are 

 of medium size and length. 



The spores are simple, rather large, usually two in each spore- 

 sac. The spore-wall is divided into two distinct layers (exosporium 

 and endosporium) which are of considerable thickness. The size 

 and thickness of the wall at once distinguish these spores from those 

 of related groups. They are usually described as colorless, but 

 they become distinctly brown (smoky-brown) with age ; they do, 

 however, remain colorless for a considerable time. 



The Alcctorias are confined to the arctic and subarctic zones, 

 and to mountainous districts. Only a few species have so far been 

 reported from our territory. They occur upon trees and rocks. 



