56 THE THALLU^S OF THE GENUS PARHELIA, 



the hvphae forming the medullary layer. (Plate lA^-, 

 fig. 5.) 



An American lichen, P. cetrata v. sorediifera, Wain., 

 shows the pores that form such a marked feature of the 

 genr:si. There are .06 x .02 mm., usually cylindrical and 

 oblique tO' the thallus, and in several instances were seen, 

 as in P. limhata, Laxir., to be in communication with 

 the large hollow rhizoids. Occasionally they are conical, 

 and in this case the apex of the cone is always at the 

 upper cortex, while the broad base is in connection with 

 a rhizinai. The thallus of this lichen is frequently cover- 

 ed, on the upper side, by colonies of a Sirosiphon, and 

 h3Tphal threads are sieen toi project from the ooxtex into the 

 sheath of the Sirosiphon. (Plate IV., fig. 6.) 



The thallus of P. latissiina, Fee, strongly resembles 

 tnat of P. loerlaia, Ach., of which species it is by some 

 authors regarded as a variety ; but it differs essentially in 

 one respect, it isi much more freely supplied with absorp- 

 tion pores. These are .05 x .016 mm. in their cross 

 diameters, and are usually oblique to the surface. As in 

 p. iaxa, hyphae lining the inner surfa,ce of the canals 

 stain more deeply/ with Delafield's hsematoxylin than those 

 of the medulla, and thus aid in revealing the pores. In 

 searching for these canals it is better to use a high than 

 a low power, a ^ for choice, with an eyepiece that will 

 give a magnification of 500-700 diameters. The pores 

 are then out of focus, and appear as blotchesi when the rest 

 of tne thalline section is in good view. The pores sc'em 

 to be higher developments from the cyphellce of the geaera 

 Hotierodea and Sticta, and when an oblique section shows 

 only the lower opening of a pore it can hardly be dis- 

 tinguished from that for-m known as CyyhellK vera^, which 

 are lined by the hyphse, and appear under the lens as 

 minute urceolat© or thelotremoad depressions. The pores, 

 ho'weveir, are much more minute, and are not to be con- 

 fused with the perforations of the thallus of Parmelia 

 pertusa, Schaer., which are 1-2 mm. in diameter, or with 

 the perforations in the apothecia of P. iierfbrata, Ach., 

 wlaich are also visible to the unaided eye. As several 

 species of Stictaoeae are without cyphellcC, either true 

 cyphellse or pseudo-cyphellag, an examination was made of 

 the thalli of S. puhnniiaria, L., and 8. retigera, Ach., 

 which belong to the section Ecyphellatae, and it was found 

 that they also pcesess the absorption pores of the Par- 

 meliaceae. It seiems clear from this that whatever function 

 is performed for the genera Heterodea and Sticta by the 

 cyphellje is performed for the lichens of the gentis Par- 



