i57 



The color of the thallus in most species of Physcia is grey, 

 tinged with green above, but having a different color beneath, 

 usually lighter, rarely dark to black ; much of the dark colora- 

 tion of the lower surface is due to the black rhizoids. Growth of 

 the thallus is usually radial ; sometimes it becomes eccentrically ra- 

 dial, due to local influences. Branching is usually distinctly dichoto- 

 mous and very frequent ; in the forms with a fruticose tendency 

 the lobes become linear, with the margin lined with long simple or 

 branched cilia (P. ciliaris, P. leucomela). Some of the species are 

 quite constantly sterile, in which cases they usually bear numerous 

 soredia upon the upper surface of the thallus as well as along the 

 margin of its lobes. 



As compared with Parmelia, the apothecia are of medium size, 

 sessile, typically discoid with a highly developed thalloid exciple ; 

 the thalloid margin usually extends above the disk ; in some forms 

 the excipular margin becomes crenate or rugose, in others it remains 

 smooth. In the fruticose forms the margin of the thalloid exciple 

 bears thalloid fringes of considerable size. The disk is dark in 

 color ; the spores are in most respects similar to those of Rinodina ; 

 the color and size is, however, more variable. Some species are 

 said to have four-celled to eight-celled spores, but none of them have 

 come to my notice ; it is probable that these are not Physcias. 



In their distribution the Physcias, especially the higher forms, have 

 a southern range. They occur most frequently upon tree-trunks, less 

 commonly upon rocks. Like nearly all the Physciaceae, this group 

 is adapted to a comparatively dry substratum, but for their growth 

 they require a moist atmosphere, at least an atmosphere in which the 

 average percentage of moisture is high ; this accounts for the large 

 development of the highly hygroscopic tissue in the thallus (the cor- 

 tical tissue), which readily absorbs the atmospheric moisture and re- 

 tains it for the use of the plant. The algae {Cystococcus humicola) 

 are intimately enclosed by haustorial branches. Endotrophic haustoria 

 seem to be common in some species, especially in younger and more 

 actively growing plants. 



Another characteristic of manj' of the Physcias is the presence 

 of numerous spermagonia, which develop upon the younger por- 

 tion of the thallus, where they appear as minute black dots. I have 

 found it impossible to employ them in the determination of species, 

 but it is, however, noteworthy that they occur more frequently upon 

 some species than upon others. 



