204 



again they appear larger, colorless, three to five-septate with thin 

 septa and spore-wall. It is likely that further studies of the group 

 will separate it into several genera ; at present I have retained them 

 in the one genus, since only a few representatives occur in the terri- 

 tory. Even if all the undoubted and doubtful Omfhalarias are 

 grouped together they do not form a large genus. The difficulty of 

 studying the group is increased since the higher Omfhalarias are 

 frequently or quite constantly sterile. 



The species occur in the temperate and arctic zones ; they grow 

 upon rocks in much the same localities as the Lichinas. 



PLATE 67. 



Omphalaria umbeila Tuck. 



1 . Natural size. 



2. Terminal portion of thallus with apothecia, magnified. 



3. Section of apothecium. 



4. Longitudinal radial section of the thallus. 



5. Spore-sac and paraphyses. 



6. Spores. 



7. Algae and haustoria. 



8. Chain of Nostoc. 



5. Polychidium Ach. ; S. F. Gray, Brit. PI. 1 : 401. 182 1. 



This interesting genus is represented by only one species, P. 

 muscicolum, quite generally included under Leptogium, from which 

 it is, however, essentially different ; the casual observer is at once 

 struck by its resemblance to JEphebe fubescens ; its histology is, how- 

 ever, quite unlike. 



The thallus is minute, typically fruticose, consisting of cylindri- 

 cal branches of a dark color, in fact almost identical with Ejihebe 

 as far as external appearances are concerned ; usually, however, the 

 lobes are shorter. 



Upon examining carefully prepared sections the following struc- 

 ture is revealed : there is an outer tissue consisting of one layer, 

 which is typically cortical, very closely resembling the epidermal layer 

 of a leaf in higher plants ; the entire interior is occupied by a hyphal 

 tissue and the algae ; the algae (Rivularia nitida) are most numer- 

 ous toward the outer surface ; the central tissue is, however, never 

 totally devoid of algae ; since the thallus is typically fruticose the 



