90 



MORPHOLOGY 



b. Lower Cortex. In some foHose lichens such as Peltigera there is 

 no special tissue developed on the under surface. In Lobaria pulmonaria 

 large patches of the under surface are bare, and the medulla is exposed to 

 the outer atmosphere, sheltered only by its position. In some other lichens 

 the lowermost hyphae lie closer together and a kind of felt of almost parallel 

 filaments is formed, generally darker in colour, as in Lecanora lentigera, and 

 in some species of Physcia. 



Most frequently however the tissues of the upper cortex are repeated on 

 the lower surface, though differing somewhat in detail. In all of the brown 

 Parmeliae, according to RosendahP, the structure is identical for both 

 cortices, though the upper develops now hairs, now isidia, breathing pores, 

 etc., while the lower produces rhizinae. The amorphous mucilaginous cuticle 

 so often present on the upper surface is absent from the lower, the walls 

 of the latter being often charged instead with dark-brown pigments. 



c. Hypothallic Structures. An unusual development of hyphae 

 from the lower cortex occurs in the genera Anzia and Pannoparmelia' — both 



closely related to Parmelia — whereby a 

 loose sponge-like hypothallus of anasto- 

 mosing reticulate strands is formed. In 

 one of the simpler types, Anzia colpodes, 

 a North American species, the hyphae 

 passing out from the lower medulla be- 

 come abruptly dark-brown in colour, and 

 are divided into short thick-walled cells. 

 Frequent branching and anastomosis of 

 these hyphae result in the formation of 

 a cushion-like structure about twice the 

 bulk of the thallus. In another species 

 from Australia {A. Japonica) there is a 

 lower cortex, distinct from the medulla, 

 consisting of septate colourless hyphae 

 with thick walls. From these branch out 

 free filaments, similar in structure but dark 

 in colour, which branch and anastomose 

 as in the previous species. 



In Pannoparmelia the lower cortex 

 and the outgrowths from it are several 

 cells thick; they may be thick-walled as 

 in Anzia, or they may be thin-walled as 

 described and figured by Darbishire^ in 



Fig. 51. Pannoparmelia anzioides Darb. 

 Vertical section of thallus and hypo- 

 thallus. a, cortex ; b, gonidial zone ; 

 c, medulla; d, lower cortex; e, hypo- 

 thallus. xca. 450 (after Darbishire). 



1 Rosendahl 1907. 



^ Darbishire 191 2. 



