Poriki; — Attachment Organs of some Common Parmeliae. 207 



lower layers of epiphloeodic, crustaceous lichens i Lindau i. 1 The development 

 of a definite cortex is probably dependent on external conditions. Thus, if 

 the thallus is growing on a surface to which immediate and intimate 

 adherence is possible, there is no definite cortex, but the hyphae of the 

 medullary, or even the gouidial, layer turn downward, effecting an attachment 

 which is exceedingly difficult to destroy. On the other hand, if the thallus 

 cannot effect such adherence, a definite cortex is developed from which 

 numerous single hyphae grow later. 



Where a thalliue lobe bends upwards or outwards from the substratum, 

 the lowest layers of medullary hyphae resume their normal direction, and 

 form a tissue of interwoven threads running parallel to the lower surface. 

 These horizontal hyphae appear to the naked eye to be of a lighter colour 

 than the more nearly vertical ones, owing to the more limited accumulation 

 of detritus for which their youth and arrangement are responsible. 



Fig. 1 shows the general structure of the thallus after carefully removing 

 the moss on which it grew. 



Fig. 2 shows the same more highly magnified, and is taken from a 

 portion in which the gouidial and medullary tissues are not clearly delimited. 

 An interesting feature of this is the pyramidal arrangement of gonidia which 

 occurs more or less clearly in many lichens, recalling the shape of palisade 

 cells in some higher plants and the distribution of chlorophyll corpuscles in 

 those cells. 



Fig. 3 represents the details of a rhizoidal layer developed from a 

 definite cortical layer, showing that occasionally branching takes place and 

 suggesting a comparison with the branched, anastomosing hyphae of Anzia 

 eolpodts as described and figured by Eeinke. 2 



2. Parmelia conspersa [Ehrh.). 



This species has a brown under-surface, the rhizines being dotted all over 

 and reaching almost to the edge. The thallus is pressed very closely to the 

 substratum, and in consequence the rhizines are less well developed than in 

 the next species P. saxalUis, under which heading they will be more folly 

 described. In P. conspersa they appear as short, peg-like outgrowths of the 

 dark-coloured lower cortex. 



The cortex is comparatively thin, but has the power of form: _ gee or 



connecting masses of tissue of a dark colour and dotted appearance. This 



1 Lindau, G. Lichenologische TTntersuchunyeu 1. [jber Wachstum mid Aulu-t 

 weise der RLndenflechteu. Dresden, 1895. 

 : Reinke, Ivc. cit.. p. 402. 



■J. 1 -1 



