128 GUIDE TO THE STUDY OF LICHENS. 



smooth. Apothecia linear, branching, partially en- 

 closed by the thallus. Spores colorless, eight-celled, 

 30/x X 8.5^. 



Jf.. Xylographa. 



The thallus begins its development below the sur- 

 face of the substratum, finally spreading over the sur- 

 face in the form of a thin whitish layer. The algae 

 0,re Chroolepus and Protococcus. The apothecia are 

 linear or irregular, extending parallel to the fibres of 

 the woody substratum. The disk is dark. The hypo- 

 thecium is nearly colorless. The spores are rather 

 small, simple, elliptical, colorless and usually sparingly 

 present. 



They occur upon rotten logs and bark of trees. 



1. Xylographa parallella. Thallus becomes epi- 

 phloeodal at an early period, smooth, uniform, light- 

 gray. Apothecia linear, extending parallel to the fibres 

 of the wood. Disk dark. Spores simple, colorless, 

 ovoid, 15/x X 6.5/x. 



This plant occurs upon rotten logs, and is character- 

 istic in appearance. The algae are Protococcus. 



2, Xylographa opegraphilla. Thallus soon becomes 

 epiphloeodal, distinct, areolate, areoles easily removed, 

 pale-gray. Apothecia short-linear, rarely branching. 

 Disk pale-brown to dark. Spores simple, ovoid, 

 colorless, 14/x X 6yu,. 



In this lichen the algae are Chroolepus. 



5. Arthonia. 

 This is in many respects a difficult group for study, 

 principally because its representatives are numerous 



