196 



rhizoids also hinders the attacks of snails which feed upon lichens 

 (Zukal). 



As a rule the few representatives which occur in the territory are 

 sterile, while the soredia are very numerous ; soredioid or isidioid 

 branches are sometimes common on the upper surface of the thallus 

 {M. saturninum) ; in all probability propagation proceeds from so- 

 redia. The apothecia present the general characteristics of those of 

 Leptogium ; the hypothecium is, however, not cortical ; as far as I 

 have been able to observe, it consists of two layers of hyphae the 

 lower of which extends parallel to the surface of the disk. 



The habitat and distribution is the same as in Collema. 



PLATE 63. 



MALLOTIUM SATURNINUM (Dcks.). 



1 . Thallus natural size. 



2. Section of apothecium. 



3. Section of thallus, with long rhizoids. 



4. Paraphyses and spore-sac. 



5. Spores. 



4. HYDROTHYRIA Russ. Proc. Essex Inst. I : 188. 1856. 



The general macroscopic characters indicate such a close rela- 

 tionship to the foregoing genera of the Collemaceae that one is sur- 

 prised at the actual histological and physiological differences. In- 

 stead of Nostoc we find that Rivularia is the symbiotic alga, 

 but it may be possible that the original symbiont was Nostoc. The 

 changes in the fungal portion, due to the symbiotic association, may 

 have adapted it to become associated with Rivnlaria rather than 

 Nostoc ; a similar condition of affairs is met with in Stictina and 

 Sticta. The general morphological characters of the thallus very 

 closely resemble those of Leptogium and Mallotium. The corti- 

 cal layers are less clearly marked, in fact, the entire hyphal tissue 

 is semicortical ; the outer cortical tissue proper contains no algae ; 

 its cells are thin-walled and quite irregular in size, and it usually con- 

 sists of more than one layer of cells. The lobes of the thallus are 

 quite large and, in color, thickness and consistency, remind one of 

 Leptogium ; the lower surface is, however, distinctly marked with 

 veins extending from the central point of attachment toward the per- 

 iphery ; in appearance these strongly resemble the venation of a leaf 



