148 



The apothecia are linear or irregular, extending parallel with the 

 fibres of the substratum ; their outline is not as clearly denned as 

 in Gracilis, they being more or less covered by the thallus and sub- 

 stratum. The upper ends of the indistinct simple paraphyses are dark 

 colored, likewise the epithecium. The hypothecium is nearly color- 

 less. The spores are rather small, elliptical, simple, colorless, and 

 usually sparingly produced. 



The species occur upon rotten logs in the north temperate zone. 



PLATE 30. 

 Xylographa paralella Fr. 



1. Natural size. 



2. Portion magnified. 



3. Section of apothecium. 



a, thecium ; b, hypothecium ; c, hypophloeodal hyphal tissue. 



4. Section of thallus. 



5. Paraphyses and spore-sac. 



6. Spores. 



7. Algae {Cystococcus humicola). 



5. Arthonia Ach. Lich. Univ. 25. 1810. 



This is one of the most difficult groups of lichens for several 

 reasons. It contains a large number of species, although only com- 

 paratively few occur within the territory. With few exceptions the 

 genus presents no marked characters; spores are very frequently 

 wanting, and the thallus is in all cases deficient and variable in its algal 

 characters. Only a critical study of good material will enable the 

 student to make an approximate identification of species. 



The thallus begins its development below the substratum ; in fact 

 it remains partly hypophloeodal in the majority of species. In the 

 higher species the thallus becomes distinctly crustaceous, even some- 

 what areolate ; no cortical tissue is ever present. Perhaps the most 

 remarkable feature about the entire group is the algal tissue. The 

 algae are numerous and almost entirely enclosed by ectotro- 

 phic haustoria, and endotrophic haustoria are also common. In 

 no group of lichens is the structural relationship between fungus and 

 alga so intimate, and this is true whether the alga is Cystococcus humi- 

 cola or Chroole^pus umbrina. Incidentally it may be remarked that 

 since both forms of algae are represented it is evident that the genus 

 might be subdivided according to these algal differences. 



