PLATE 34. 



RlNODINA SOPHODES (Ach.) N\l. 



1. Natural size. 



2. A small portion magnified. 



3. Section of apothecium. 



a, thecium ; b and c, the two layers of the hypothecium ; d, uppei' 

 algal layer; e, medullary layer; f, lower algal layer; g, cortical 

 layer. 



4. Section of thallus. 



5. Paraphyses and spore-sac. 



6. Spores. 



7. Paraphyses and spore-sac of If. constans. 



8. Spores of R. constans. 



2. PLACODIUM Ach. Lich. Suec. 100. 1798. 



This genus, which in many of its characters resembles Rinodina^ 

 is characterized by the orange color of the apothecial disk ; this rarely 

 changing from a rusty red to dark ( P. ferrugineum ) . The 

 thallus is usually yellow or orange, rarely grey to dark. The spores 

 are colorless, with terminal plasmic masses, otherwise they are char- 

 acteristic of the family ; they are rarely simple. 



The thallus varies from almost typically crustaceous (P. citrinum) 

 to typically foliose (P. mtirorunt) ; in general, it may be said that 

 it is more highly differentiated than in Rinodina ; as in Rinodina 

 lobation first shows itself near the margin, the inner portion of 

 the thallus becoming areolate ; usually both upper and lower 

 cortical layers are present. In P. elcgans and other species the 

 mutual adaptation of mechanical and physiological functions are 

 beautifully illustrated (see plate 35) ; the upper cortical layer is 

 not uniformly thick ; at certain areas the tissue is very thin, which 

 allows the algae to approach near the upper surface of the thallus 

 for the purpose of assimilation ; the thickened areas supply the nec- 

 essary mechanical support. The rhizoids are more specialized than 

 in Rinodina\ the hyphal cells are thicker and more rigid. 



In the lower representatives of the genus the thallus is closely 

 adherent to the substratum, but in its higher forms, as P. elegans, P. 

 murorum and a few others, the thallus-lobes are more or less ascend- 

 ing, approaching in this character the higher groups. 



The yellow color is due to the deposition of crystals of chryso- 



