Porter — Attachment Organs of some Common Parmeliae. 211 



lacking the stimulus of the presence of a suitable substratum, have become 

 basally thickened and rugose, but have not developed apical ly. Whether this 

 stimulus is tactical, chemical, or physical, would be hard to decide. 



Conclusion. 



The attachment organs of the Parmeliae are, as a rule, rhizines, i.e., 

 strands of hyphae, usually of a dark colour, holding the thallus more or less 

 closely appressed to the substratum. The hyphae are derived from the tissues 

 of the medulla and lower cortex ; they penetrate and disintegrate bark or even 

 the wood of posts, and by the radial expansion of their apices they may form 

 a continuous layer on the surface of the substratum. Species which have 

 no rhizines are attached by the action of individual hyphae. 



R.I.A. PROC, VOL. XXXIV, SKCT. B. [2 F\ 



