THE THALLUS 299 



are composed of compact strands of hyphae extending from the cells of 

 the lower cortex. 



In the genus Parmelia rhizinae are almost a constant character, though 

 in a few species, such as Parmelia physodes, they are scanty or practically 

 absent. It is not possible, however, to consider that these species form a 

 lower group, as in other respects they are highly evolved, and rhizinae may 

 be found at points on the lower surface where there is irritation by friction. 

 Soredia and isidia occur frequently and, in several species, almost entirely 

 replace reproduction by spores. In one or two northern or Alpine species, 

 P. stygia and P,pubescens, the lobes are linear or almost filamentous. They 

 are retained in Parmelia because the apothecia are superficial on the fronds 

 which are partly dorsiventral, and because rhizinae have occasionally been 

 found. Some of the Parmeliae attain .to a considerable size; growth is centri- 

 fugal and long continued. 



Two monotypic genera classified under Parmeliaceae, Physcidia and 

 Heterodea, are of considerable interest as they indicate the bases of parallel 

 development in Parmelia and Cetraria. The former, a small lichen, is corti- 

 cate only on the upper surface, and without rhizinae; and from the description, 

 the cortex is of a fastigiate character. The solitary species grows on bark 

 in Cuba; it is related to Parmelia, as the apothecia are superficial on the 

 lobes. The second, Heterodea Miilleri,a. soil-lichen from Australasia, is more 

 akin to Cetraria in that the apothecia are terminal. The upper surface is 

 corticate with marginal cilia, the lower surface naked or only protected by 

 a weft of brownish hyphae amongst which cyphellae are formed ; pseudo- 

 cyphellae appear in Cetraria. 



The genus Cetraria contains very highly developed thalline forms, either 

 horizontal (subgenus Platysma}, or upright (Eucetraria). Rhizinae are scanty 

 or absent, but marginal cilia in some upright species act as haptera. Cetraria 

 aculeata is truly fruticose with a radiate structure. 



An extraordinary development of the under cortex characterizes the 

 genera Anzial. and Pannoparmelia: rhizinae-like strands formed from the 

 cortical cells branch and anastomose with others till a wide mesh of a 

 spongy nature is formed. They are mostly tropical or subtropical or Austra- 

 lasian, and possibly the spongy mass may be of service in retaining moisture. 

 A species of Anzia has been recorded by Darbishire 2 from Tierra del Fuego. 



dd. USNEACEAE. As we have seen, the change to fruticose structure 

 has arisen as an ultimate development in a number of groups; it reaches 

 however its highest and most varied form in this family. Not only are there 

 strap-shaped thalli, but a new form, the filamentous and pendulous, appears ; 

 it attains to a great length, and is fitted to withstand severe strain. The 



1 See p. 90. ' l Darbishire 1912. 



