Porter — Attachment Organs of some Common Parmeliae. 209 



layer from which slender mycelial strands branch among the bark cells, 

 breaking up the tissue by their wedge-like action, and hastening the decom- 

 position of the individual cells. 



The lower cortex and rhizines are exemplified in fig. 9, representing a 

 strip removed from the under-surface and flattened beneath a cover glass. 



In figs. 10 and 11 the detailed structure of the rhizines is shown, the 

 former illustrating also the structure of the lower cortex, the latter one mode 

 of branching of the rhizines. In fig. 9, the more usual antler-like method 

 is seen. 



■1. Parmelia Borreri Turn. 



The brown under-surface becomes paler towards the edge, which is reached 

 by papillae, but not by fully developed rhizines (fig. 12). Corrugations occur 

 as in the last species, and the rhizines are similar in structure, but of a paler 

 colour. They consist of fewer hyphae which run together from a broader 

 base, as a rule. The core of medullary hyphae is more loosely woven than 

 in P. saxatilis; it is well seen in the younger stages of the rhizines, or as a 

 light-coloured mass more or less filling the dark-edged hypothalline cup. 



5. Parmelia omphalodes (L.). 



This species has a bronze or dark-brown upper-surface, and was formerly 

 regarded as a sub-species of P. saxatilis. Its under-surface resembles that of 

 this species, but the marginal rhizines are, as a rule, more prominent, even 

 giving a ciliated appearance to the thallus lobes (fig. 13). 



The general structure of the thallus (fig. 14) is seen to differ from that 

 figured for P. saxatilis; but as Eeinke 1 has remarked concerning the constant 

 anatomical structure of the genus, this variation is a question merely of the 

 relative and absolute depth of the individual layers. Thus, P. onvphalodes has 

 a comparatively greater development of the cortical layers which is associated 

 with its darker colour and its more exposed habitat. Bitter 2 has commented 

 on the darkening of exposed thalli and the variations in colour in the same 

 species exposed to different intensities of illumination. This species occurs 

 chiefly on exposed rocks in mountainous districts, whereas P. saxatilis prefers 

 more shaded rocks or trees and reaches its best development at lower levels. 



1 Reinke, loc. cit., p. 385. 



- Bitter, G. .Tjber die Variabilitat eiuiger Liuibtiecliten unci iibev den Einrluss 

 ausserer Bedingungen auf ihr Wacbstum. Pringaheim's Jahrbuchei' f. w. Bot. 36, 1901, 

 p. 464. 



